JP2018170694A - Amplifier circuit and amplifier device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an amplifier that achieves low noise, maintains a frequency band and stabilizes operation, and stabilizes operation with low noise.SOLUTION: An amplifier circuit 2 includes: an amplifier 4 for amplifying an input signal; and a current source 6. A current source includes a capacitive element 66 and generates a cancellation current ifor reducing or canceling a feedback current iflowing from an output section 44 of the amplifier to an input section 43.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an amplifier circuit and an amplifier for amplifying an input signal.

  An output voltage of a sensor that detects a physical phenomenon as an electric signal may be a very small voltage (for example, several microvolts (μV)). In the field of such minute signal measurement, it is necessary to amplify the minute signal detected by the sensor without increasing the noise as much as possible in the detection and analysis of the electric signal.

  Conventionally, as a low noise technique for an amplifier circuit, a low noise technique by connecting amplifier circuits in parallel and averaging noise components is known (Patent Document 1).

In addition, a low noise / low distortion instrumentation amplifier (Low-Noise, Low-Distortion Instrumentation Amplifier) is known (Non-Patent Document 1). This low-noise / low-distortion instrumentation amplifier has a configuration that reduces circuit elements such as resistors as much as possible in order to suppress the generation of thermal noise. ing. In this low noise / low distortion instrumentation amplifier example, the operation tends to become unstable and oscillate easily at a low source impedance of less than 10Ω. In order to stabilize the operation of such a low-noise / low-distortion instrumentation amplifier, as shown in FIG. 2 (FIGURE 2) of Non-Patent Document 1, an impedance in which a resistor and an inductor are connected in parallel is an external input (V _{IN +} , V _IN- ) and the input of the low noise / low distortion instrumentation amplifier.

Japanese Patent Laid-Open No. 5-121977

“Low-Noise, Low-Distribution INSTRUMENTATION AMPLIFIER” (INA163 data sheet), [online], May 2005, Texas Instruments Incorporated, [March 9, 2017 search], Internet <URL: http: // www. tij.co.jp/jp/lit/ds/symlink/ina163.pdf>

  When the number of parallel connections of the amplifier circuits is increased in order to average the noise component, the number of parallel connections of the capacitance between the input unit and the output unit of each amplifier circuit increases as the number of parallel connections of the amplifier circuits increases. When the number of parallel connections of the capacitance between the input unit and the output unit increases, the capacitance between the input unit and the output unit of the entire amplifier circuit increases. For this reason, when it is going to reduce the noise of an amplifier circuit, the electrostatic capacitance between an input part and an output part increases. This capacitance reduces the operational stability of the amplifier circuit. This limits the number of amplifier circuits connected in parallel.

In order to stabilize the operation of the amplifier, if an impedance in which a resistor and an inductor are connected in parallel is placed between the external input and the input of the low-noise / low-distortion instrumentation amplifier, the resistance included in the impedance generates thermal noise. , Noise increases.
Further, if a capacitance is added between the input portion of the amplifier circuit and the common potential in order to ensure the stability of the operation of the amplifier circuit, the frequency band of the amplifier circuit is degraded.

  That is, in the conventional low noise technology, there is a problem that it is difficult to achieve both noise reduction of the amplifier circuit and maintenance of the frequency band and stable operation.

An object of the present invention is to perform amplification with low noise, maintaining a frequency band, and stable operation.

  In order to achieve the above object, according to one aspect of the amplifier circuit of the present invention, the amplifier circuit includes an amplifying unit that amplifies an input signal and a current source. It is sufficient to generate a canceling current that reduces or cancels the feedback current flowing through the section.

  In the amplifier circuit, the current source further includes a cancellation amplification unit, and the capacitive element is connected to an output unit of the cancellation amplification unit and an input unit of the cancellation amplification unit whose polarity is inverted from the output unit, A signal corresponding to the input signal may be input to the input unit of the cancellation amplification unit.

  In the amplifier circuit, the amplifying unit may include a plurality of amplifiers each having an output impedance added thereto, and the plurality of amplifiers each having the output impedance added thereto may be connected in parallel.

  In the amplifier circuit, the amplification unit may be a non-inverting amplification unit, and the cancellation amplification unit may be an inverting amplification unit.

  In the amplification circuit, the amplification unit and the cancellation amplification unit may include a differential input unit.

  In the amplification circuit, the amplification unit and the cancellation amplification unit may include a differential output unit.

  In the above amplifier circuit, the canceling amplifier section may be an amplifier circuit, an operational amplifier, or an instrumentation amplifier configured by a circuit including a discrete amplifier element.

  In the above amplifier circuit, the amplifier unit may be an amplifier circuit, an operational amplifier, or an instrumentation amplifier configured with a circuit including a discrete amplifier element.

  In the amplifier circuit, a resistor may be connected in series before the input section of the amplifier section.

  In the amplifier circuit, an inductor may be connected in parallel with the resistor.

In order to achieve the above object, according to one aspect of the amplifier of the present invention, the amplifier circuit may be provided.

  According to the present invention, any of the following effects can be obtained.

  (1) According to the amplifying circuit and the amplifying device of the present invention, it is possible to increase the operational stability while maintaining low noise.

  (2) According to the amplifying circuit and the amplifying device of the present invention, it is possible to maintain low frequency and maintain high operation stability while maintaining a frequency band.

Other objects, features, and advantages of the present invention will become clearer with reference to the accompanying drawings and each embodiment.

FIG. 1 is a diagram illustrating an example of an amplifier circuit according to the first embodiment. FIG. 2 is a diagram illustrating an example of an amplifier. FIG. 3 is a diagram illustrating an example of frequency characteristics of the amplifier. FIG. 4 is a diagram illustrating an example of an amplifier circuit in which an input impedance is connected to an amplifier. FIG. 5 is a diagram illustrating an example of a loop loop gain measurement circuit of the amplifier circuit. FIG. 6 is a diagram illustrating an example of simulation results of loop loop gain and phase. FIG. 7 is a diagram illustrating an example of a simulation result of the amplifier circuit according to the first embodiment. FIG. 8 is a diagram illustrating an example of a current source amplifier. FIG. 9 is a diagram illustrating an example of an amplifier circuit according to the second embodiment. FIG. 10 is a diagram illustrating an example of an amplifier circuit according to the third embodiment. FIG. 11 is a diagram illustrating another example of the amplifier circuit according to the third embodiment. FIG. 12 is a diagram illustrating an example of an amplifier circuit according to the fourth embodiment. FIG. 13 is a diagram illustrating an example of an amplifier circuit according to the fifth embodiment.

Embodiments of the present invention will be described below.

[First Embodiment]

  FIG. 1 shows an example of an amplifier circuit according to the first embodiment. In FIG. 1, a capacitance indicated by a broken line represents a stray capacitance and is distinguished from a capacitive element indicated by a solid line. In FIG. 1, triangles excluding the amplifiers 42 and 62 represent common potentials. The amplification circuit 2 includes an amplification unit 4, a current source 6 connected to the amplification unit 4, and an input impedance 8, and amplifies an input signal.

  The amplifying unit 4 is an amplifying unit including feedback by electrostatic capacitance (for example, stray capacitance), for example, a positive feedback amplifying unit including a positive feedback that feeds back from a positive output unit of the amplifying unit 4 to a positive input unit. is there. The amplifying unit 4 includes an amplifier 42, an input-output capacitor 46, and an output impedance 48, and amplifies the input signal.

  The amplifier 42 is an amplifier having low noise characteristics, and includes at least one input unit 43 and at least one output unit 44, and has an input signal amplification function. The amplifier 42 is, for example, a circuit constituted by an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit constituted by a combination of a discrete amplification element and a passive element. The amplifier 42 is a non-inverting amplifier in which the input unit 43 and the output unit 44 have the same polarity. The input unit 43 of the amplifier 42 also serves as the input unit of the amplification unit 4, and the output unit 44 of the amplifier 42 serves also as the output unit of the amplification unit 4. Due to the low noise property of the amplifier 42, the amplification unit 4 having low noise property is obtained.

The input-output capacitance 46 is an electrostatic capacitance between the input unit 43 and the output unit 44, for example, a stray capacitance of the amplifier 42. That is, the input-output capacitance 46 does not include a capacitive element as a component such as a capacitor, and indicates an inter-terminal capacitance that exists between the input unit 43 and the output unit 44. The input-output capacitance 46 may include a capacitive element as a component such as a capacitor. The input-output capacitance 46 is a positive input-positive output capacitance that exists between the positive input unit 43 and the positive output unit 44. A feedback current i _Cp flows from the output section 44 of the amplifier 42 to the input section 43 by the input-output capacitance 46. That is, in the amplification unit 4, the feedback current i _Cp flows from the output unit 44 to the input unit 43.

  The output impedance 48 includes, for example, a resistor, and adjusts the output impedance value of the amplifier circuit 2 to a value determined by this resistor (for example, 50Ω). This output impedance 48 may be omitted.

The current source 6 includes an amplifier 62 and a capacitive element 66, and generates a canceling current i _Cf that reduces or cancels the feedback current i _Cp of the amplifying unit 4. That is, the current source 6 is an example of a current canceling unit that reduces or cancels the feedback current i _Cp . Here, the current cancellation represents that a part or all of the feedback current i _Cp is subtracted by the cancellation current i _Cf , and this current cancellation includes a part or all of the positive charge and the negative charge. Charge neutralization that includes loss of charge in part or in whole is included. That is, the current source 6 is also an example of a current neutralization unit that neutralizes part or all of the feedback current i _Cp by the positive feedback with the cancellation current i _Cf.

The amplifier 62 is an example of a cancellation amplification unit used for generating the cancellation current i _Cf and is an amplifier that generates an output state opposite to that of the amplifier 42. That is, the amplifier 42 is a non-inverting amplifier, and the amplifier 62 is an inverting amplifier that inverts and amplifies the input signal. The amplifier 62 includes at least one input unit 63 and at least one output unit 64 and has an input signal amplification function. The amplifier 62 is, for example, a circuit constituted by an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit constituted by a combination of a discrete amplification element and a passive element.

  The capacitive element 66 is a capacitor, for example, and forms a capacitance between the input and output of the amplifier 62. The capacitive element 66 is connected to the input unit 63 and the output unit 64 of the amplifier 62. Since the amplifier 62 inverts the input signal, the capacitive element 66 is connected to the output unit 64 and the input unit 63 whose polarity is inverted from that of the output unit 64. In the amplifier circuit 2, the capacitive element 66 is used. However, for example, the stray capacitance of the amplifier 62 may be used, or a capacitive element may be used in combination.

The current source 6 is connected to the input unit 43 of the amplification unit 4. With this connection, a signal corresponding to or the same as the input signal input to the amplifying unit 4 is input to the amplifier 62 of the current source 6. An amplified and inverted input signal is output from the output section 64 of the amplifier 62. The output of the amplifier 62 is fed back to the input unit 63 via the capacitive element 66, and a canceling current i _Cf is generated. This canceling current i _Cf is supplied to the input unit 43 of the amplifying unit 4 to reduce or cancel the feedback current i _Cp of the amplifying unit 4. In other words, the feedback current i _Cp flows into the input portion 63 of the current source 6 as the canceling current i _Cf , so that the feedback current i _Cp is absorbed by the current source 6 and a part or all of the feedback current i _Cp due to the positive feedback is medium. To be summed.

The input impedance 8 includes the input unit 43 of the amplification unit 4 and an input resistor 82 and an input capacitor 84 connected to the common potential, and adjusts the input impedance of the amplification circuit 2. The input impedance 8 is a positive input impedance for adjusting the impedance on the positive input side of the amplifier circuit 2, for example, and the input resistance 82 and the input capacitance 84 are a positive input resistance and a positive input capacitance, respectively. The input resistor 82 and the input capacitor 84 are combined impedances composed of a resistance component and a capacitance component assuming the following (1), (2), (3), or (4).
(1) A resistor or capacitor as an electrical component intentionally placed in the input unit 43 of the amplifier unit 4 or a stray capacitance of the resistor or circuit in order to adjust the input impedance.
(2) The capacitance of the signal cable connected to the input unit 43 of the amplification unit 4.
(3) The output impedance of the signal sensor connected to the input unit 43 of the amplification unit 4.
(4) The input impedance of the amplification unit 4 itself.

  This input impedance 8 may be omitted.

[Characteristics of the amplifier 101]
FIG. 2 shows an example of an amplifier. The amplifier 101 shown in FIG. 2 can be used as the amplifier 42 of the amplifying unit 4, for example. The amplifier 101 has at least one input unit 102 and at least one output unit 103, and has an amplifying function with a DC gain of Ao and a bandwidth of fc. The amplifier 101 is, for example, a circuit configured by an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit configured by a combination of a discrete amplification element and a passive element.

・・・・式１ An amplifier circuit is formed by combining the amplifier 101 with another circuit. The transfer function A (f) of the amplifier circuit can be expressed as Equation 1.

.... Formula 1

For example, the gain-frequency characteristic when the DC gain Ao of the amplifier 101 is 40 dB (= 100 times) and the bandwidth fc is 1 MHz is expressed as shown in FIG. For example, it is expressed as shown in FIG. The amplifier 101 having the gain-frequency characteristic shown in FIG. 3A and the phase-frequency characteristic shown in FIG. 3B exhibits the following characteristics.
(1) It has a gain of 40 dB at a frequency sufficiently lower than 1 MHz.
(2) The gain at a frequency of 1 MHz is a flat portion gain (eg, 40 dB) to -3 dB (eg, 37 dB) where the gain does not change even if the frequency changes, and the phase of the output signal at a frequency of 1 MHz is the input signal -45 deg (degree) with reference to.
(3) At a frequency sufficiently higher than the frequency of the bandwidth fc, the gain-frequency characteristic has a slope of -20 dB / dec and the phase gradually approaches -90 deg.

[Operation of Amplifier Circuit with Input Impedance 8 Connected to Amplifier 101]
In the amplifier circuit shown in FIG. 4A, an input-output capacitor 104 (eg, a positive input-positive output capacitor), an input resistor 82, and an input are connected to the amplifier 101 with respect to the amplifier 101 having the frequency characteristics described above. A capacitor 84 is connected. The input-output capacitor 104 is a capacitance between the input unit 102 and the output unit 103 of the amplifier 101, for example, a stray capacitance of the amplifier 101. That is, the input-output capacitance 104 does not include a capacitive element as a component, and represents an inter-terminal capacitance that exists between the input unit 102 and the output unit 103. The input-output capacitor 104 may be used in combination with a capacitive element as a component.

  The amplifier circuit shown in FIG. 4B is a circuit in which the arrangement of the components of the amplifier circuit shown in FIG. That is, the amplifier circuit illustrated in FIG. 4B is an equivalent circuit of the amplifier circuit illustrated in FIG. 4A and 4B, a positive feedback circuit including a voltage dividing circuit formed by the input-output capacitor 104 and the input impedance 8 is equivalently formed. The output signal of the output unit 103 of the amplifier circuit shown in FIGS. 4A and 4B is divided by this voltage dividing circuit and positively fed back to the input unit 102.

[Frequency characteristics of loop loop gain and phase]
FIG. 5 shows an example of an evaluation model for evaluating the stability of the amplifier circuit shown in FIGS. 4A and 4B, and shows an example of a loop circuit gain measurement circuit of the amplifier circuit. Yes. The evaluation model shown in FIG. 5 includes an oscillator 105 between the output unit 103 and the input-output capacitor 104 in order to evaluate the stability of the feedback system of the amplifier circuit. The evaluation model shown in FIG. 5 is used to evaluate the frequency characteristics of the gain (loop loop gain) and the phase frequency characteristics obtained through a loop formed by the positive feedback circuit and the amplifier 101 described above by simulation. .

  6A shows the frequency characteristics of the loop loop gain according to the first simulation, FIG. 6B shows the frequency characteristics of the phase according to the first simulation, and FIG. Shows the frequency characteristic of the loop loop gain according to the second simulation, and FIG. 6D shows the frequency characteristic of the phase according to the second simulation. In FIG. 6A, the upper numerical value represents the capacity of the input capacitor 84, the middle numerical value represents the frequency at phase 0 deg, and the lower numerical value represents the loop loop gain. In FIG. 6C, the upper numerical value represents the capacity of the input-output capacitor 104, and the lower numerical value represents the loop loop gain.

The conditions for the first simulation are as follows.
Capacitance value of the input-output capacitor 104: 0.35 pF
Resistance value of input resistance 82: 100 kΩ
Capacitance value of input capacitance 84: 10 pF, 30 pF, 100 pF

The conditions for the second simulation are as follows.
Capacitance value of the input-output capacitor 104: 0.1 pF, 0.35 pF, 1 pF
Resistance value of input resistance 82: 100 kΩ
Capacitance value of input capacitance 84: 30 pF

  In the evaluation model of FIG. 5, the loop loop gain can be obtained by the ratio of the voltage Vo of the output unit 103 based on the voltage Voo of the connection unit between the oscillator 105 and the input-output capacitor 104. The voltage Voo and the voltage Vo are each expressed as a vector having a directional component, and the loop loop gain is expressed by a vector ratio of the voltage Vo based on the voltage Voo. In the range where the loop loop gain at the frequency at which the phase is 0 deg in the phase-frequency characteristics is 0 dB (= 1 times) or more, the operation of the amplifier circuit is unstable, and the feedback current is amplified by the amplifier 101, and the amplifier circuit Oscillates. On the other hand, if the loop loop gain at the frequency at which the phase is 0 deg is approximately ½ (≈−6 dB) or less, the operation of the amplifier circuit is stable.

  In FIG. 6A, when the input capacitance 84 is 100 pF, the loop loop gain becomes −9.3 dB, the amplifier circuit does not oscillate, and the operation is stabilized. On the other hand, when the input capacitance 84 is 10 pF or 30 pF, the loop loop gain exceeds 0 dB, and the amplifier circuit oscillates. That is, when the input-output capacitor 104 is constant, the amplifier circuit is more likely to oscillate as the input capacitor 84 is smaller.

  In FIG. 6C, when the input-output capacitance 104 is 0.1 pF, the loop loop gain becomes −10.0 dB, the amplifier circuit does not oscillate, and the operation is stabilized. On the other hand, when the input-output capacitance 104 is 0.35 pF or 1 pF, the loop circuit gain exceeds 0 dB, and the amplifier circuit oscillates. That is, when the input capacitance 84 is constant, the amplification circuit is more likely to oscillate as the input-output capacitance 104 increases. In FIG. 6D, the phase-frequency characteristics are substantially constant regardless of the input-output capacitance 104.

Summarizing the above simulation results, the following measures (1) and (2) are effective in order to configure an amplifier circuit that does not easily oscillate with attention paid to the input-output capacitor 104 and the input capacitor 84.
(1) Decrease the capacitance value of the input-output capacitor 104.
(2) Increase the capacitance value of the input capacitor 84.

Since the output resistance of the signal source and the input capacitance 84 form a time constant, if the input capacitance 84 is large, the frequency band of the amplifier circuit decreases in inverse proportion thereto. For this reason, the capacitance value of the input capacitor 84 is preferably small from the viewpoint of securing a band necessary for use, which is a condition contrary to the stability of the amplifier circuit. In addition, the circuit configuration of the amplifier circuit shown in FIGS. 4A and 4B tends to become unstable and oscillate easily due to the reduction of circuit elements in the low noise amplifier circuit. Therefore, in the amplifier circuit 2, a current source 6 generates a cancellation current i _Cf, stabilize the operation of the amplifier 42 and the amplifier section 4 having a low noise by the cancellation currents i _Cf.

・・・・式２ [Feedback current i _Cp and cancellation current i _Cf ]
The voltage of the input unit 43 of the amplifying unit 4 is V _i , the voltage of the output unit 44 is V _op , the capacitance of the input-output capacitor 46 is _Cp, and the feedback current flowing through the input-output capacitor 46 is i _Cp . . At this time, the feedback current i _Cp can be expressed as shown in Equation 2.

.... Formula 2

・・・・式３ Similarly, when the voltage of the output section 64 of the amplifier 62 is V _on and the capacitance of the capacitive element 66 connected to the amplifier 62 is Cf, the canceling current i _Cf flowing from the output section 64 of the amplifier 62 to the capacitive element 66 is expressed by the following equation (3). It can be expressed as

.... Formula 3

When the signs of the feedback current i _Cp and the cancellation current i _Cf are opposite, the feedback current i _Cp and the cancellation current i _Cf cancel each other. That is, since the feedback current i _Cp is reduced or canceled, the amount of the feedback current i _Cp flowing into the amplifier 42 is suppressed, and the amplification of the feedback current i _Cp by the amplifier 42 and the oscillation of the amplification unit 4 are suppressed. That is, the operation of the amplifier circuit 2 is stabilized. The sign of the positive and negative feedback current i _Cp and cancellation currents i _Cf is reversed, there is more toward the stable direction with a value the absolute value is close to the feedback current i _Cp and cancellation currents i _Cf, is most stable when equal. In other words, i _Cp / i _Cf = −1 Equation 4
By providing the current source 6 adjusted so as to satisfy the above, it is possible to obtain the amplifier circuit 2 having the most stable operation in which oscillation due to positive feedback does not occur.

・・・・式５
例えば、増幅器４２および増幅器６２の利得がともに４０ｄＢ（＝１００倍）の場合は、容量素子６６の静電容量Ｃｆを、入力−出力間容量４６の静電容量Ｃｐのおよそ０．９８倍、つまりほぼ同じ静電容量にすればよい。静電容量Ｃｆを静電容量Ｃｐとほぼ同じ値に調整することで、式４の条件を充足させることができ、電流源６は増幅部４を流れる帰還電流ｉ_Cpを相殺することができる。 When the gain of the amplifier 42 of the amplifier 4 is Ap (≧ 1) and the gain of the amplifier 62 of the current source 6 is An (> 0, but the output of the amplifier 62 is inverted), the condition of Expression 4 is satisfied. In order to achieve this, the relationship between the capacitance Cp of the input-output capacitance 46 of the amplifying unit 4 and the capacitance Cf of the capacitive element 66 of the current source 6 is expressed by Equation 5 from Equation 2 and Equation 3. become.

.... Formula 5
For example, when the gains of the amplifier 42 and the amplifier 62 are both 40 dB (= 100 times), the capacitance Cf of the capacitive element 66 is approximately 0.98 times the capacitance Cp of the input-output capacitance 46, that is, What is necessary is just to make it the substantially same electrostatic capacitance. By adjusting the capacitance Cf to substantially the same value as the capacitance Cp, the condition of Equation 4 can be satisfied, and the current source 6 can cancel the feedback current i _Cp flowing through the amplifying unit 4.

FIG. 7 shows the result of simulation for confirming that the feedback current i _Cp due to the input-output capacitance 46 of the amplifier 42 is canceled by the current source 6 in the circuit of FIG. In this simulation, the value of the input resistor 82 is 100 kΩ, the value of the input capacitance is 30 pF, the capacitance Cp is 0.35 pF, the gain of the amplifier 42 and the gain of the amplifier 62 are 40 dB. In FIG. 7A, the upper numerical value represents the capacitance Cf of the capacitive element 66, and the lower numerical value represents the loop loop gain. In the simulation result of electrostatic capacitance Cf = 0.35 pF in FIG. 7, Cf = Cp is set. When Cf = Cp, i _Cp ≈−i _Cf , the feedback current i _Cp and the cancellation current i _Cf are canceled, and the influence of the positive feedback is alleviated. In other words, the feedback current i _Cp flows almost directly into the input portion 63 of the current source 6 as the canceling current i _Cf , so that the feedback current i _Cp is absorbed and the feedback current i _Cp due to the positive feedback is neutralized. In the simulation of the capacitance Cf = 0.35 pF in FIG. 7, the loop loop gain shows a negative dB value close to −6 dB (≈½ times) at a frequency of 227 kHz where the phase becomes 0 deg. 2 is in a stable state.

On the other hand, the simulation result of the capacitance Cf = 0.001 pF in FIG. 7 shows the loop loop gain when the capacitance Cf is set to a very small value and the canceling current i _Cf is hardly generated. In the simulation of the capacitance Cf = 0.001 pF in FIG. 7, the loop loop gain becomes 0.8 dB (≈1.1 times) at a frequency of 227 kHz where the phase becomes 0 deg, and the amplifier circuit 2 oscillates.
That is, the loop circuit gain of the amplifier circuit 2 is reduced from 0 dB or more to less than 0 dB by the connection of the current source 6, and the stable state of the amplifier circuit 2 is maintained.

  The gains of the amplifier 42 and the amplifier 62 may be different. For example, when the gain Ap of the amplifier 42 is approximately 40.1 dB (= 101 times) and the gain An of the amplifier 62 is approximately 19.1 dB (= 9 times), in order to satisfy the condition of Equation 4, Cf = 10 Cp. That is, by making the gain An of the amplifier 62 smaller than the gain Ap of the amplifier 42, the capacitance Cf of the capacitive element 66 for canceling, absorbing or neutralizing the positive feedback is the static capacitance of the input-output capacitance 46. It can be larger than the capacitance Cp.

  When the gain An of the amplifier 62 is approximately 19.1 dB (= 9 times), the amplifier 62 is an inverting amplifier in which the input impedance becomes high impedance with a gain of 9 times. As the amplifier 62, for example, an amplifier in which an inverting input of an instrumentation amplifier is connected to a common potential can be used. As another specific example of the amplifier 62, as shown in FIG. 8, a buffer Bu is arranged in the input stage, and an inverting amplifier IA in which the resistance value of the resistor Rf is nine times the resistance value of the resistor Ri is provided in the subsequent stage of the buffer Bu. Arranged circuit configurations are listed.

In FIG. 8, a voltage follower using an operational amplifier is illustrated as the buffer Bu. However, a non-feedback buffer circuit, a buffer IC, a source follower, an emitter follower, or the like is also applicable. When the gain of the buffer Bu is other than 1, the gain of the inverting amplifier IA may be adjusted by changing the values of the resistor Ri and the resistor Rf, and the gain of the buffer Bu and the entire inverting amplifier IA may be increased by nine times. (For example, the amplification factor of the source follower is close to 1, but smaller than 1.)
When the buffer Bu is not used, the input impedance of the inverting amplifier IA is equal to the resistor Ri. Therefore, it can be used as a substitute for the input resistor 82 or as a part of the input resistor 82.

  If the gain of the amplifier 62 is made smaller than the gain of the amplifier 42, the capacitance Cf of the capacitive element 66 is set to a capacitance of a capacitive element that is readily available on the market, for example, a capacitance greater than the picofarad (pF) order. And the gain An of the amplifier 62 can be set in accordance with the capacitance Cf of the capacitive element 66. For example, the current source 6 can be configured with a capacitor value that is easily available in the market, instead of a capacitance value on the order of less than 1 pF, which is considered due to the stray capacitance between the terminals of the operational amplifier. Very useful on top.

[Effect of the first embodiment]
(1) Since the amplifying unit 4 includes the low-noise amplifier 42, the noise of the amplifying circuit 2 can be suppressed, and the feedback current i _Cp is reduced or canceled by the canceling current i _Cf of the current source 6. The operation of 4 is stabilized. That is, it is possible to obtain the amplifier circuit 2 having high operation stability while maintaining low noise.

(2) Since the cancellation current i _Cf reduces or cancels the feedback current i _Cp , the input capacitance 84 can be reduced, and the frequency band of the amplifier circuit 2 can be maintained.

[Modification of First Embodiment]
The amplifying unit 4 and the current source 6 may be an amplifying circuit using a discrete amplifying element in addition to a configuration using an operational amplifier or an instrumentation amplifier.

[Second Embodiment]

  FIG. 9 shows an example of an amplifier circuit according to the second embodiment. In FIG. 9, triangles excluding the amplifiers 42a and 62a represent a common potential. 9, the same parts as those in FIG. 1 are denoted by the same reference numerals.

  The amplifier circuit 2a of the second embodiment includes an amplifier unit 4a in which the input unit 43a is a differential input unit. In the amplifier circuit 2 of the first embodiment, the input unit 43 of the amplifier unit 4 has one input, whereas in the amplifier circuit 2a of the second embodiment, one input is replaced with a differential input. An amplification unit 4a is formed. Corresponding to the replacement with the differential input, the amplifier 62a in the current source 6a of the amplifier circuit 2a is also replaced with the differential input. That is, the input part 63a of the amplifier 62a forms a differential input part. In the amplifier circuit 2 of the first embodiment, the input impedance 8 is connected to the input unit 43 of the amplifier unit 4, whereas in the amplifier circuit 2a of the second embodiment, the amplifier unit 4a. Are connected to input impedances 8 and 9, respectively. The input impedance 8 is connected to the common potential with the non-inverting input part of the amplifying part 4a, and the input impedance 9 is connected to the common potential with the inverting input part of the amplifying part 4a. The input impedances 8 and 9 have, for example, the same or almost the same impedance.

  In the amplifier circuit 2a, the difference between the two input parts of the input part 43a forming the differential input part, that is, the inverting input part (negative input part) and the non-inverting input part (positive input part) is the amplification part 4a. The positive feedback path is formed between the output section 44 and the non-inverting input section of the amplifier 42a. The difference between the two input parts of the input part 63a forming the differential input part, that is, the inverting input part and the non-inverting input part, becomes the input signal of the current source 6a, and the amplified and inverted input signal is output from the amplifier 62a. . The output of the amplifier 62a is fed back to the inverting input section of the input section 63a through the capacitive element 66. The inverting input portion of the current source 6a is connected to the non-inverting input portion of the amplifying portion 4a, the non-inverting input portion of the current source 6a is connected to the inverting input portion of the amplifying portion 4a, and the amplifier 62a is an output opposite to the amplifier 42a. Creating a state. In addition, when configured as shown in FIG. 9, the operation of the amplifier circuit 2a can be considered in the same manner as the amplifier circuit 2 of the first embodiment.

  In the amplifier circuit 2a shown in FIG. 9, when the inverting input section of the amplifier section 4a is short-circuited to the common potential of the amplifier circuit 2a, the amplifier section 4a equivalently constitutes a one-input positive feedback amplifier section, and the current source 6a is equivalent. Thus, a 1-input inverting amplification unit is configured. That is, when the inverting input portion of the amplifying portion 4a is short-circuited to the common potential of the amplifying circuit 2a, the input signals to the inverting input portion of the amplifying portion 4a and the non-inverting input portion of the current source 6a become 0, equivalently shown in FIG. The configuration is the same as that of the amplifier circuit 2 of the first embodiment. The amplifiers 42a and 62a having a differential input unit are, for example, a circuit configured by an operational amplifier, an instrumentation amplifier, or a circuit configured by a combination of a discrete amplification element and a passive element.

The amplifier circuit 2a can be operated in the same manner as the amplifier circuit 2 of the first embodiment. Since the feedback current i _Cp is reduced or canceled by the connection of the current source 6a, the operation of the amplifier unit 4a is stabilized. . That is, it is possible to obtain an amplifier circuit 2a that maintains low noise while maintaining high operational stability and maintaining a frequency band.

[Third Embodiment]

  FIG. 10 shows an example of an amplifier circuit according to the third embodiment. In FIG. 10, triangles excluding the amplifiers 42b and 62b represent a common potential. 10, the same parts as those in FIGS. 1 and 9 are denoted by the same reference numerals.

The amplifier circuit 2b-1 according to the third embodiment includes an amplifier unit 4b in which the input unit 43a is a differential input unit and the output unit 44a is a differential output unit. In the amplifier circuit 2a of the second embodiment, the output unit 44 of the amplifier unit 4a has one output, whereas in the amplifier circuit 2b-1 of the third embodiment, the output unit 44a of the amplifier unit 4b. Is the differential output. As the amplifying unit 4b becomes a differential output, a path through the negative input-output capacitor 47 is added to the positive feedback path in addition to the input-output capacitor 46 described above. The input-output capacitance 47 is a capacitance between the inverting input unit of the input unit 43a and the inverting output unit of the output unit 44a, and is, for example, a stray capacitance of the amplifier 42b. The input-output capacitance 47 is, for example, a negative input-negative output capacitance that exists between a negative input section (inverting input section) and a negative output section (inverting output section). Due to the input-output capacitance 47, a second feedback current different from the feedback current i _Cp (first feedback current) already described in the first embodiment flows from the inverting output portion of the amplifier 42b to the inverting input portion. .

In order to cancel the positive feedback due to the second feedback current, the capacitor 67 is provided in the current source 6b. The capacitive element 67 is connected to the inverting output unit and the non-inverting input unit of the amplifier 62b. The inverting output output from the inverting output unit of the amplifier 62b is fed back to the non-inverting input unit of the amplifier 62b via the capacitive element 67, and the cancellation current i _Cf (first cancellation) described in the first embodiment is applied. A second canceling current different from (current) is generated. This second canceling current reduces or cancels the second feedback current.

  The amplifier circuit 2b-1 has a differential output. In the non-inverted output, the first canceling current reduces or cancels the first feedback current, and in the inverting output, the second canceling current is the second. Reduce or cancel the feedback current. The operation of the amplifying unit 4b is stabilized with both differential outputs. That is, it is possible to obtain a two-output amplifier circuit 2b-1 that maintains low noise while maintaining high operational stability and maintaining a frequency band.

[Modification]
In the amplifier circuit 2b-1 shown in FIG. 10, the inverting input portion of the current source 6b is connected to the non-inverting input portion of the amplifying portion 4b, and the non-inverting input portion of the current source 6b is connected to the inverting input portion of the amplifying portion 4b. However, as shown in the amplifier circuit 2b-2 shown in FIG. 11, these may be connected in reverse. In the amplifier circuit 2 b-2 shown in FIG. 11, the first cancellation current fed back to the inverting input portion of the amplifier 62 b through the capacitive element 66 and the second feedback current to the path through the input-output capacitor 47. The second cancellation current fed back to the non-inverting input of the amplifier 62b through the capacitive element 67 reduces or cancels the first feedback current to the path through the input-output capacitance 46. To do. Even in such a configuration, the operation of the amplifying unit 4b is stable with both differential outputs, the operational stability is high while maintaining low noise, and the two-output amplifier circuit 2b- in which the frequency band is maintained. 2 can be obtained.

[Fourth Embodiment]

  The amplifying unit 4 of the amplifying circuit 2 of the first embodiment is composed of a single amplifying unit 4, but the amplifying unit 4c of the amplifying circuit 2c of the fourth embodiment is a natural number N connected in parallel. It is comprised by the one amplification part 4c-1, 4c-2, ..., 4c-N. The amplifying unit 4a of the amplifying circuit 2a of the second embodiment is configured by one amplifying unit 4a, but the amplifying unit 4d of the amplifying circuit 2d of the fourth embodiment is a natural number N connected in parallel. Each of the amplifying units 4d-1, 4d-2,..., 4d-N. The amplifying unit 4b of the amplifying circuits 2b-1 and 2b-2 of the third embodiment is configured by one amplifying unit 4b. However, the amplifying unit 4e of the amplifying circuit 2e of the fourth embodiment is configured in parallel. .., 4e-N, which are natural number N amplifiers 4e-1, 4e-2,. With such a configuration, it is possible to reduce the noise by averaging the outputs as described in Patent Document 1 described in the background art.

In the amplifier circuit 2c shown in FIG. 12A, the amplifier unit 4c is composed of natural number N amplifier units 4c-1, 4c-2,..., 4c-N connected in parallel. Each of the amplifying units 4c-1, 4c-2,..., 4c-N has the same configuration as that of the amplifying unit 4 described in the first embodiment. Note that the gain Ap, output voltage V _o , output impedance R _o , and capacitance Cp in each amplification unit 4c-1, 4c-2,..., 4c-N are included in the amplification units 4c-1, 4c−. As with 2,..., 4c-N, a hyphen (-) and a serial number are attached. The parameters with serial numbers are expressed using a variable k such as gain Ap-k, output voltage V _ok , output impedance R _ok , and capacitance Cp-k (k = 1 to N).

・・・・式６

・・・・式７

・・・・式８ Looking at the amplifying unit 4c externally, the value R _o of the output impedance of the amplification section 4c is represented by Formula 6, input - the capacitance Cp of the output capacitance is represented by Formula 7, the output voltage V _o is , Represented by Equation 8. That is, the amplifying unit 4c can be represented by a configuration equivalent to the amplifying unit 4 of the first embodiment, and can be handled as a configuration similar to that of the first embodiment.

.... Formula 6

.... Formula 7

.... Formula 8

When the amplifying units 4c-1, 4c-2,..., 4c-N have the same characteristics, the amplifying unit 4c has an output impedance R _o represented by Expression 9 and an input-output represented by Expression 10. amplifying portion unitary with the output voltage V _o which is represented by the capacitance Cp and formula 11 between capacity replaced equivalently to.
R _o = R _o-1 / N (9)
Cp = N · Cp−1 Equation 10
V _o = V _o-1 ... Formula 11

  Since the amplification units 4c-1, 4c-2,..., 4c-N are connected in parallel, the amplification units 4c-1, 4c-2,. The noise voltage of the amplifying unit 4c is a reciprocal of the square root of N (1 / N of root) times the noise voltage of N. That is, the noise of the amplifier 4c can be reduced by the parallel connection of the amplifiers 4c-1, 4c-2,..., 4c-N.

  According to Equation 10, the capacitance Cp increases with parallelization. As shown in the “loop loop gain and phase frequency characteristics” in the first embodiment, the increase in the capacitance Cp is a factor in which the stability of the amplifier circuit is likely to be impaired. However, since the amplifying unit 4c is equivalently replaced with a single amplifying unit, the current source 6 may be set as in the first embodiment. The amplifying circuit 2c can reduce or cancel the feedback current due to the positive feedback of the amplifying unit 4c by the canceling current of the current source 6, and the capacitance Cp is increased by the parallel connection of the amplifying units as in the amplifying circuit 2c. However, it is possible to reduce the noise and maintain the frequency band while stabilizing the operation.

  In the amplification circuit 2d shown in FIG. 12B, the amplification unit 4d is composed of natural number N amplification units 4d-1, 4d-2,..., 4d-N connected in parallel. Each of the amplifying units 4d-1, 4d-2,..., 4d-N has the same configuration as that of the amplifying unit 4a described in the second embodiment. The amplifying unit 4d can be represented by a configuration equivalent to the amplifying unit 4a of the second embodiment, and can be handled as a configuration similar to that of the second embodiment. Since the amplification units 4d-1, 4d-2,..., 4d-N are connected in parallel, the amplification units 4d-1, 4d-2,. The noise voltage of the amplification unit 4d is a reciprocal of the square root of N with respect to the noise voltage of N. That is, the noise of the amplifier 4d can be reduced by the parallel connection of the amplifiers 4d-1, 4d-2,..., 4d-N.

  Since the amplifying unit 4d is equivalently replaced with a single amplifying unit, the current source 6a may be set as in the second embodiment. The amplifying circuit 2d can reduce or cancel the feedback current due to the positive feedback of the amplifying unit 4d by the canceling current of the current source 6a, and the capacitance Cp is increased by the parallel connection of the amplifying units as in the amplifying circuit 2d. However, it is possible to reduce the noise and maintain the frequency band while stabilizing the operation.

  In the amplification circuit 2e shown in FIG. 12C, the amplification unit 4e is composed of natural number N amplification units 4e-1, 4e-2,..., 4e-N connected in parallel. Each of the amplification units 4e-1, 4e-2,..., 4e-N has the same configuration as the amplification unit 4b described in the third embodiment. The amplifying unit 4e can be represented by a configuration equivalent to the amplifying unit 4b of the third embodiment, and can be handled as a configuration similar to that of the third embodiment. Since the amplification units 4e-1, 4e-2,..., 4e-N are connected in parallel, the amplification units 4e-1, 4e-2,. The noise voltage of the amplifier 4e is a reciprocal of the square root of N with respect to the noise voltage of N. That is, the noise of the amplifier 4e can be reduced by the parallel connection of the amplifiers 4e-1, 4e-2,..., 4e-N.

Since the amplification unit 4e is equivalently replaced with a single amplification unit, the current source 6b may be set as in the third embodiment. The amplifying circuit 2e can reduce or cancel the feedback current due to the positive feedback of the amplifying unit 4e by the canceling current of the current source 6b, and the capacitances Cpa and Cpb are increased by the parallel connection of the amplifying units as in the amplifying circuit 2e. In this case, it is possible to reduce noise and maintain the frequency band while stabilizing the operation.

[Fifth Embodiment]

  FIG. 13 shows an amplifier circuit according to the fifth embodiment. In FIG. 13, the same parts as those in FIG.

  In the amplifier circuit 2f of the fifth embodiment, resistors 10 and 11 are connected to the positive and negative input sections 43a of the amplifier circuit 2a of the second embodiment. The resistor 10 is connected to the non-inverting input part of the input part 43a of the amplifier circuit 2a, and the resistor 11 is connected to the inverting input part of the input part 43a.

  In a state where the amplifier circuit 2a is oscillating, a signal with a large oscillation frequency is output from the amplifier 42a, and the input side also oscillates in voltage according to the magnitude of the signal and the gain at the oscillation frequency of the amplifier 42a. It becomes a state. When the voltage oscillation on the input side of the amplifier 42a is larger than the amplitude of the signal input to the amplifier circuit 2a of FIG. 9, the signal source when the voltage of the signal source supplying the signal to the amplifier circuit 2a is positive The current flows into the side, and the current is discharged from the signal source when the voltage is negative. In this situation, when the amplifier 42a is viewed as a resistor from the signal source, the sign of the applied voltage and the flowing current is opposite, and can be regarded as a negative resistance equivalent to the signal source, that is, a negative resistance. It becomes a state.

  In order to cancel this negative resistance, the resistors 10 and 11 shown in FIG. 13 are connected to the input unit 43a of the amplifying unit 4a. The impedance viewed from the signal source side can be obtained by measuring the frequency characteristic of the S parameter on the input side of the amplifier 42a with a network analyzer.

  In the loop loop gain and phase frequency characteristics (FIG. 6) already described in the first embodiment, the resistance value measured by the network analyzer is not so large in the frequency region including the frequency where the phase is 0 deg. The resistance values of the resistors 10 and 11 may be determined so as to be a positive value, for example, from 10Ω to 100Ω.

  By providing the resistors 10 and 11 on the input portion side of the amplifier circuit 2a, the operation stability of the amplifier circuit 2a is further improved. Since the resistors 10 and 11 are used as an auxiliary to the current source 6a, the resistance values of the resistors 10 and 11 are small, the amount of thermal noise generated in the resistors 10 and 11 is suppressed, and the low noise of the amplifier circuit 2f is maintained. The

  The inductors 12 and 13 may be connected in parallel with the resistors 10 and 11. The combined impedance due to the parallel connection of the resistors 10 and 11 and the inductors 12 and 13 is close to a short circuit in the low frequency region and asymptotic to the resistor in the high frequency region, at the frequency fd where the resistance value and the reactance value by the inductor are equal. Shows frequency characteristics. In the high frequency region including the frequency where the phase is 0 deg in the frequency characteristics of the loop loop gain and the phase described above, this combined impedance becomes a resistance. On the other hand, the inductances of the inductors 12 and 13 are set according to the resistance so that the input impedance is sufficiently small in the low frequency region. With such inductance setting, the combined impedance of the resistors 10 and 11 and the inductors 12 and 13 connected in parallel is affected by the superimposition of thermal noise caused by the resistors 10 and 11 at the input portion of the amplifier circuit 2f in the low frequency region. It can be made not to receive.

  As the resistance values of the resistors 10 and 11 used as an auxiliary to the current source 6a become smaller, the inductance values of the inductors 12 and 13 can also be made smaller if the frequency fd is the same. Therefore, the inductors 12 and 13 can be inductors having a smaller inductance value, which is advantageous in terms of cost.

In this embodiment, the resistors 10 and 11 are connected to the positive and negative input portions of the second embodiment, or the combined impedance by the parallel connection of the resistors 10 and 11 and the inductors 12 and 13 is connected. Even if the resistors 10 and 11 are connected to the input portions of the amplifier circuits 2, 2b-1, 2b-2, and 2c to 2e of the other embodiments, or the combined impedance is connected, the same effect can be obtained. it can.

  With respect to the above-described embodiment, modifications and the like are listed.

  (1) Although the example of the amplifier circuit has been described in the above embodiment, the present invention is not limited to the amplifier circuit, and may be, for example, an amplifier device including the amplifier circuit described above. With the amplification circuit, the amplification device can obtain the same effect as the amplification circuit.

  (2) In the above embodiment, the current sources 6 and 6a include the capacitive element 66, the current source 6b includes the capacitive elements 66 and 67, and the current source 6c includes the capacitive element 66a. The capacitance between the input and output of the amplifier 62, the amplifier 62a of the current source 6a, the amplifier 62b of the current source 6b, and the amplifier 42a of the current source 6c may be used as a capacitive element.

  (3) In the above embodiment, an example in which the input-output capacitance 46 of the amplifiers 42, 42a and the input-output capacitances 46, 47 of the amplifier 42b are stray capacitances has been described. 46 and 47 may include a capacitive element as a component such as a capacitor.

As described above, the most preferable embodiment of the present invention has been described. However, the present invention is not limited to the above description, and is described in the claims or disclosed in the specification. It goes without saying that various modifications and changes can be made by those skilled in the art based on the gist, and such modifications and changes are included in the scope of the present invention.

The low noise amplifier circuit of the present invention can be used for various applications in addition to the measurement of minute signals detected by a sensor. For example, it can be widely used for the following applications (1) to (3).
(1) Measurement in physics / engineering signal amplification of superconducting devices in quantum computers, MCT (Mercury Cadmium Tellurium) sensor for infrared detection, SQUID (Superducting Quantum Interference Device) sensor for micro magnetic detection, microwave detection Photodetection elements such as high-temperature superconducting Josephson elements, photomultiplier tubes, and phototransistors.
(2) Biological signal measurement in the biological field CT (computer tomography) scanner, electromagnetic sensor for MRI (Magnetic Resonance Imaging).
(3) Others Fields that require low noise and high-speed response, such as various sensors, elements, and analytical instruments.
Improve lock-in amplifier sensitivity and remove common mode noise.

2 Amplifying circuit 4 Amplifying section 6 Current source 8, 9 Input impedance 10, 11 Resistance 12, 13 Inductor 42 Amplifier 43 Input section 44 Output section 46, 47 Input-output capacitance 48 Output impedance 62 Amplifier 63 Input section 64 Output section 66 , 67 Capacitance element 82 Input resistance 84 Input capacitance

Claims (11)

An amplifier for amplifying the input signal;
A current source that includes a capacitive element and generates a cancellation current that reduces or cancels a feedback current flowing from the output unit to the input unit of the amplification unit;
An amplifier circuit comprising:
The current source further includes a cancellation amplifier;
The capacitive element is connected to an output section of the cancellation amplification section and an input section of the cancellation amplification section whose polarity is inverted from the output section,
The amplifier circuit according to claim 1, wherein a signal corresponding to the input signal is input to an input section of the cancellation amplifier section.
3. The amplifier circuit according to claim 1, wherein the amplifying unit includes a plurality of amplifiers each having an output impedance added thereto, and the plurality of amplifiers each having the output impedance added are connected in parallel. .
The amplification unit is a non-inverting amplification unit;
The amplifier circuit according to claim 1, wherein the canceling amplification unit is an inverting amplification unit.
The amplifier circuit according to claim 1, wherein the amplifying unit and the cancellation amplifying unit include a differential input unit.
6. The amplifier circuit according to claim 5, wherein the amplifying unit and the cancellation amplifying unit include a differential output unit.
The amplifier circuit according to claim 1, wherein the canceling amplifier unit is an amplifier circuit, an operational amplifier, or an instrumentation amplifier configured by a circuit including a discrete amplifier element.
The amplifier circuit according to any one of claims 1 to 7, wherein the amplifier unit is an amplifier circuit, an operational amplifier, or an instrumentation amplifier configured by a circuit including a discrete amplifier element.
9. The amplifier circuit according to claim 1, wherein a resistor is connected in series before the input unit of the amplifier unit. 10.
The amplifier circuit according to claim 9, wherein an inductor is connected in parallel with the resistor.
An amplifying apparatus comprising the amplifying circuit according to any one of claims 1 to 10.

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