JP6874976B2 - Amplifier circuit and amplifier - Google Patents

Description

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

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

Conventionally, regarding the noise reduction technology of an amplifier circuit, a noise reduction technology by connecting amplifier circuits in parallel and averaging noise components is known (Patent Document 1).

Further, 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 in which circuit elements such as resistors are reduced as much as possible in order to suppress the generation of thermal noise, and although it has the characteristic of low noise, it also has the disadvantage of being easy to oscillate. ing. In this example of a low noise / low distortion instrumentation amplifier, operation tends to be unstable and oscillation tends to occur 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, the impedance in which the resistor and the inductor are connected in parallel is the external input (V _{IN +} , _{It is placed between the VIN-} ) and the input section of the low noise / low distortion instrumentation amplifier.

Japanese Unexamined Patent Publication No. 5-121977

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

When the number of parallel connections of the amplifier circuit is increased in order to average the noise components, 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 circuit increases. As 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. Therefore, when trying to reduce the noise of the amplifier circuit, the capacitance between the input unit and the output unit increases. This capacitance reduces the operational stability of the amplifier circuit. Therefore, the number of parallel connections of the amplifier circuit is limited.

If an impedance in which a resistor and an inductor are connected in parallel is placed between the external input and the input section of a low-noise / low-distortion instrumentation amplifier in order to stabilize the operation of the amplifier, the resistance contained in the impedance will generate 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 deteriorates.

That is, the conventional low noise technology has a problem that it is difficult to reduce the noise of the amplifier circuit, maintain the frequency band, and achieve stable operation at the same time.

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

In order to achieve the above object, according to one aspect of the amplifier circuit of the present invention, the amplification unit for amplifying the input signal, the offset amplification unit, the output unit of the offset amplification unit, and the output unit are reversed in polarity. and a capacitive element connected to the input of the offset amplifier section and a including a current source, a signal corresponding to the input signal to the input of the cancellation amplifier unit is input, the current source of the amplifying section An offset current that reduces or cancels the feedback current flowing from the output unit to the input unit may be generated.

In the amplifier circuit, the amplifier unit may include a plurality of amplifiers to which output impedances are added, and a plurality of amplifiers to which output impedances may be added may be connected in parallel.

In the amplifier circuit, the amplifier may be a non-inverting amplifier and the offset amplifier may be an inverting amplifier.

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

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

In the amplifier circuit, the canceling amplifier may be an amplifier circuit, an operational amplifier, or an instrumentation amplifier composed of a circuit including a discrete amplifier element.

In the amplifier circuit, the amplifier unit may be an amplifier circuit, an operational amplifier, or an instrumentation amplifier composed of a circuit including a discrete amplifier element.

In the amplifier circuit, a resistor may be connected in series in front of the input unit of the amplifier unit.

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 device 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 amplifier circuit and the amplifier device of the present invention, it is possible to improve the stability of operation while maintaining low noise.

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

And other objects, features and advantages of the present invention will be further clarified by reference to the accompanying drawings and each embodiment.

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

Hereinafter, embodiments of the present invention will be described.

[First Embodiment]

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

The amplification unit 4 is an amplification unit that includes feedback due to capacitance (for example, stray capacitance), and is, for example, a positive feedback amplification unit that includes positive feedback that returns from the positive output unit of the amplification unit 4 to the positive input unit. is there. The amplification unit 4 is composed of an amplifier 42, an input-output capacitance 46, and an output impedance 48, and amplifies the input signal.

The amplifier 42 is a low-noise amplifier, 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 composed of an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit composed of a combination of a discrete amplification element and a passive element. The amplifier 42 is a non-inverting amplifier having the same polarity of the input unit 43 and the output unit 44. 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 also serves 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 can be obtained.

The input-output capacitance 46 is the capacitance between the input unit 43 and the output unit 44, for example, the stray capacitance of the amplifier 42. That is, the input-output capacitance 46 does not include a capacitance element as a component such as a capacitor, and indicates the terminal capacitance existing between the input unit 43 and the output unit 44. The input-output capacitance 46 may include a capacitance element as a component such as a capacitor. The input-output capacitance 46 is a positive input-positive output capacitance existing between the positive input unit 43 and the positive output unit 44. _{Due to the input-output capacitance 46, the feedback current i Cp} flows from the output unit 44 of the amplifier 42 to the input unit 43. 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 capacitance element 66, and generates an offset current i _{Cf that reduces or cancels the feedback current i Cp of the amplification 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 offset means that a part or all of _{the feedback current i Cp is subtracted by the offset current i Cf} , and the offset of this current includes one part or all of the positive charge and the negative charge. It shall include the neutralization of charge, which in turn loses charge in part or in whole. That is, the current source 6 is also an example of a current neutralizing unit that neutralizes a part or all _{of the feedback current i Cp due to positive feedback by the canceling current i Cf.}

The amplifier 62 is _{an example of an offset amplification unit used to generate an offset current i Cf} , and is an amplifier that creates 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 composed of an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit composed of a combination of a discrete amplification element and a passive element.

The capacitance element 66 is, for example, a capacitor and forms the input-output capacitance 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. Although the capacitance element 66 is used in the amplifier circuit 2, for example, the stray capacitance of the amplifier 62 may be used or the capacitance element may be used in combination.

The current source 6 is connected to the input unit 43 of the amplification unit 4. By this connection, a signal corresponding to the input signal input to the amplification unit 4 or the same signal as the input signal is input to the amplifier 62 of the current source 6. An amplified and inverted input signal is output from the output unit 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 amplification unit 4 to reduce or cancel _{the feedback current i Cp of the amplification unit 4.} In other words, the feedback current i _Cp flows into the input unit 63 of the current source 6 as an offset 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 positive feedback is medium.} Be summed.

The input impedance 8 includes an input unit 43 of the amplifier unit 4, an input resistance 82 connected to a common potential, and an input capacitance 84, and adjusts the input impedance of the amplifier circuit 2. The input impedance 8 is, for example, a positive input impedance that adjusts the impedance on the positive input side of the amplifier circuit 2, and the input resistance 82 and the input capacitance 84 are the positive input resistance and the positive input capacitance, respectively. The input resistance 82 and the input capacitance 84 are composite 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 electric component intentionally placed in the input section 43 of the amplification section 4 in order to adjust the input impedance, or a stray capacitance of the resistor or circuit.
(2) 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) Input impedance of the amplification unit 4 itself.

This input impedance 8 may be omitted.

[Characteristics of amplifier 101]
FIG. 2 shows an example of an amplifier. The amplifier 101 shown in FIG. 2 can be used, for example, as the amplifier 42 of the amplification unit 4. The amplifier 101 has at least one input unit 102 and at least one output unit 103, and has an amplification function having a DC gain of Ao and a bandwidth of fc. The amplifier 101 is, for example, a circuit composed of an operational amplifier and a negative feedback circuit, an instrumentation amplifier, or a circuit composed of 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 in Equation 1.

・ ・ ・ ・ Equation 1

For example, the gain-frequency characteristics when the DC gain Ao of the amplifier 101 is 40 dB (= 100 times) and the bandwidth fc is 1 MHz are expressed as shown in (A) of FIG. 3, and the phase-frequency characteristics are as shown in FIG. For example, it is represented as shown in FIG. 3B. 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 gain of a flat portion (for example, 40 dB) to a gain of -3 dB (for example, 37 dB) in which the gain does not change even if the frequency changes, and the phase of the output signal at the frequency of 1 MHz is the input signal. It is -45 deg (gain) 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 an amplifier circuit in which an input impedance 8 is connected to an amplifier 101]
In the amplifier circuit shown in FIG. 4A, for the amplifier 101 having the frequency characteristics described above, the input-output capacitance 104 (for example, the positive input-positive output capacitance), the input resistance 82, and the input are input to the amplifier 101. A capacity 84 is connected. The input-output capacitance 104 is the capacitance between the input unit 102 and the output unit 103 of the amplifier 101, for example, the stray capacitance of the amplifier 101. That is, the input-output capacitance 104 does not include a capacitance element as a component, and represents the terminal capacitance existing between the input unit 102 and the output unit 103. The input-output capacitance 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 each component of the amplifier circuit shown in FIG. 4A is equivalently changed. That is, the amplifier circuit shown in FIG. 4B is an equivalent circuit of the amplifier circuit shown in FIG. 4A. In the amplifier circuit of FIG. 4A and FIG. 4B, a positive feedback circuit including a voltage dividing circuit formed by the input-output capacitance 104 and the input impedance 8 is formed equivalently. The output signal of the output unit 103 of the amplifier circuit of FIG. 4A and FIG. 4B is divided by this voltage dividing circuit and positively fed back to the input unit 102.

[Loop cycle gain and phase frequency characteristics]
FIG. 5 is 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 loop gain measurement circuit of the amplifier circuit. There is. The evaluation model shown in FIG. 5 includes an oscillator 105 between the output unit 103 and the input-output capacitance 104 in order to evaluate the stability of the feedback system of the amplifier circuit. Using the evaluation model shown in FIG. 5, the frequency characteristics of the gain (loop-round gain) obtained by going around the loop formed by the positive feedback circuit and the amplifier 101 described above and the frequency characteristics of the phase are evaluated by simulation. ..

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

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

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

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 with reference to the voltage Voo of the connection portion between the oscillator 105 and the input-output capacitance 104. The voltage Vo and the voltage Vo are represented as vectors having directional components, respectively, and the loop loop gain is represented by the vector ratio of the voltage Vo with respect to the voltage Vo. In the range where the loop loop gain at the frequency where the phase becomes 0 deg in the phase-frequency characteristic is 0 dB (= 1 times) or more, the operation of the amplifier circuit is unstable, the feedback current is amplified by the amplifier 101, and the amplifier circuit. Is in an oscillating state. On the other hand, if the loop loop gain at the frequency at which the phase becomes 0 deg is about 1/2 (≈-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 is −9.3 dB, the amplifier circuit does not oscillate, and the operation is stable. 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 capacitance 104 is constant, the smaller the input capacitance 84, the easier it is for the amplifier circuit to oscillate.

In FIG. 6C, when the input-output capacitance 104 is 0.1 pF, the loop loop gain is -10.0 dB, the amplifier circuit does not oscillate, and the operation is stable. On the other hand, when the input-output capacitance 104 is 0.35 pF or 1 pF, the loop loop gain exceeds 0 dB, and the amplifier circuit oscillates. That is, when the input capacitance 84 is constant, the larger the input-output capacitance 104, the easier it is for the amplifier circuit to oscillate. In FIG. 6D, the phase-frequency characteristic is substantially constant regardless of the input-output capacitance 104.

Summarizing the above simulation results, the following correspondences (1) and (2) are effective in order to construct an amplifier circuit that is difficult to oscillate by focusing on the input-output capacitance 104 and the input capacitance 84.
(1) Reduce the capacity value of the input-output capacity 104.
(2) Increase the capacity value of the input capacity 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 to it. Therefore, the capacitance value of the input capacitance 84 is preferably small from the viewpoint of securing a band required for use, which is a condition contrary to the stability of the amplifier circuit. Further, the low noise amplifier circuit tends to be unstable in operation and easily oscillate in the circuit configuration of the amplifier circuit shown in FIG. 4A and FIG. 4B due to the reduction of circuit elements. Therefore, in the amplifier circuit 2, the current source 6 _{generates a canceling current i Cf} , and the canceling current i _Cf stabilizes the operation of the amplifier 42 and the amplification unit 4 having low noise.

・・・・式２ [Feedback current i _Cp and offset current i _Cf ]
The voltage of the input unit 43 of the amplification unit 4 is V _i , the voltage of the output unit 44 is V _{op , the capacitance of the input-output capacitance 46 is Cp,} and the feedback current flowing through the input-output capacitance 46 is i Cp. .. At this time, the feedback current i _Cp can be expressed as in Equation 2.

・ ・ ・ ・ Equation 2

・・・・式３ Similarly, assuming that the voltage of the output unit 64 of _{the amplifier 62 is V on} and the capacitance of the capacitance element 66 connected to the amplifier 62 is Cf, the canceling current i _Cf flowing from the output unit 64 of the amplifier 62 to the capacitance element 66 is given by Equation 3. It can be expressed as.

・ ・ ・ ・ Equation 3

When the positive and negative signs of the feedback current i _Cp and the canceling current i _{Cf are opposite, the feedback current i Cp} and the canceling current i _Cf cancel each other out. 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 stable. Further, the positive and negative signs of the feedback current i _Cp and the canceling current i _{Cf are opposite, and the closer the absolute values of the feedback current i Cp} and the canceling current i _Cf are, the more stable the direction is, and when they are equal, the most stable. That is, i _Cp / i _Cf = -1 ... Equation 4
By providing the current source 6 adjusted so as to be, it is possible to obtain an 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 amplification unit 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 Equation 4 is satisfied. The relationship between the capacitance Cp of the input-output capacitance 46 of the amplification unit 4 and the capacitance Cf of the capacitance element 66 of the current source 6 is as shown in Equation 5 from Equations 2 and 3. become.

・ ・ ・ ・ Equation 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 capacitance element 66 is approximately 0.98 times the capacitance Cp of the input-output capacitance 46, that is, The capacitance may be almost the same. By adjusting the capacitance Cf to a value substantially the same 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 amplification unit 4.}

_{FIG. 7 shows the result of a 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 resistance 82 is 100 kΩ, the value of the input capacitance is 30 pF, the capacitance Cp is 0.35 pF, and the gain of the amplifier 42 and the gain of the amplifier 62 are 40 dB. In FIG. 7A, the numerical value in the upper row represents the capacitance of the capacitance Cf of the capacitance element 66, and the numerical value in the lower row represents the loop loop gain. In the simulation result of the capacitance Cf = 0.35pF in FIG. 7, Cf = Cp is set. When Cf = Cp, i _Cp ≈ −i _Cf , the feedback current i _Cp and the canceling current i _Cf are offset, and the influence of positive feedback is mitigated. In other words, the feedback current i _Cp flows into the input unit 63 of the current source 6 as an offset current i _Cf almost as it is, so that the feedback current i _Cp is absorbed and the feedback current i _Cp due to 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 (≈1/2 times) at a frequency of 227 kHz at which the phase is 0 deg, and is an amplifier circuit. 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 is 0.8 dB (≈1.1 times) at a frequency of 227 kHz at which the phase is 0 deg, and the amplifier circuit 2 oscillates.
That is, by connecting the current source 6, the loop loop gain of the amplifier circuit 2 is reduced from 0 dB or more to less than 0 dB, 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 about 40.1 dB (= 101 times) and the gain An of the amplifier 62 is about 19.1 dB (= 9 times), in order to satisfy the condition of Equation 4, the condition of Equation 4 is satisfied. 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 capacitance element 66 for canceling, absorbing or neutralizing the positive feedback is the static electricity of the input-output capacitance 46. The capacitance can be made larger than Cp.

When the gain An of the amplifier 62 is about 19.1 dB (= 9 times), the amplifier 62 is an inverting amplifier having a gain of 9 times and an input impedance of high impedance. As the amplifier 62, for example, an instrumentation amplifier in which the inverting input is connected to a common potential can be used. As another specific example of the amplifier 62, as shown in FIG. 8, an inverting amplifier IA in which a buffer Bu is arranged in the input stage and the resistance value of the resistor Rf is 9 times the resistance value of the resistor Ri is placed in the subsequent stage of the buffer Bu. The arranged circuit configuration can be mentioned.

In FIG. 8, a voltage follower using an operational amplifier is illustrated as the buffer Bu, but a non-feedback buffer circuit, a buffer IC, a source follower, an emitter follower, and the like can also be applied. 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 inverting amplifier IA as a whole may be increased by 9 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 resistance Ri, so that this can be used as a substitute for the input resistance 82 or as a part of the input resistance 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 can be changed to the capacitance of the capacitive element easily available on the market, for example, the capacitance equal to or higher than the picofarad (pF) order. It is possible to set the gain An of the amplifier 62 according to the capacitance Cf of the capacitance element 66. For example, the current source 6 can be configured by the value of a capacitor easily available on the market instead of the capacitance value on the order smaller than 1 pF, which is considered as the stray capacitance between the terminals of the operational amplifier, and the amplifier circuit 2 is practical. Very useful on top.

[Effect of the first embodiment]
(1) Since the amplification unit 4 includes the low-noise amplifier 42, the noise of the amplifier 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, so that the amplification unit 4 The operation of 4 is stable. That is, it is possible to obtain an amplifier circuit 2 having high operational stability while maintaining low noise.

(2) Since the canceling 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.

[Modified example of the first embodiment]
The amplifier unit 4 and the current source 6 may have a configuration using an operational amplifier or an instrumentation amplifier, or may be an amplifier circuit using a discrete amplifier element.

[Second Embodiment]

FIG. 9 shows an example of an amplifier circuit according to the second embodiment. In FIG. 9, the triangles excluding the amplifiers 42a and 62a represent the common potential. In FIG. 9, the same parts as those in FIG. 1 are designated 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. The 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 unit 63a of the amplifier 62a forms the differential input unit. Further, 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 Input impedances 8 and 9 are connected to each of the two input units. The input impedance 8 is connected to the non-inverting input unit of the amplification unit 4a at a common potential, and the input impedance 9 is connected to the inverting input unit of the amplification unit 4a at a common potential. The input impedances 8 and 9 have, for example, the same or substantially the same impedance.

In the amplification circuit 2a, the difference between the two input units of the input unit 43a forming the differential input unit, that is, the inverting input unit (negative side input unit) and the non-inverting input unit (positive side input unit) is the amplification unit 4a. A positive feedback path is formed between the output section 44 of the amplifier 42a and the non-inverting input section. The difference between the two input units of the input unit 63a forming the differential input unit, that is, the inverting input unit and the non-inverting input unit becomes the input signal of the current source 6a, and the amplified and inverted input signals are output from the amplifier 62a. .. The output of the amplifier 62a is fed back to the inverting input unit of the input unit 63a via the capacitive element 66. The inverting input section of the current source 6a is connected to the non-inverting input section of the amplification section 4a, the non-inverting input section of the current source 6a is connected to the inverting input section of the amplification section 4a, and the amplifier 62a outputs the opposite of the amplifier 42a. Creating a state. In addition, if it is 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 unit of the amplifier unit 4a is short-circuited to the common potential of the amplifier circuit 2a, the amplifier unit 4a equivalently constitutes a 1-input positive feedback amplifier unit, and the current source 6a is equivalent. It constitutes a one-input inverting amplification unit. That is, when the inverting input unit of the amplifier unit 4a is short-circuited to the common potential of the amplifier circuit 2a, the input signals to the inverting input unit of the amplifier unit 4a and the non-inverting input unit of the current source 6a become 0, which are equivalently shown in FIG. It has the same configuration as the amplifier circuit 2 of the first embodiment. The amplifiers 42a and 62a having a differential input unit are, for example, a circuit composed of an operational amplifier, an instrumentation amplifier, or a circuit composed of a combination of a discrete amplification element and a passive element.

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

[Third Embodiment]

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

The amplifier circuit 2b-1 of 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 amplification unit 4b becomes a differential output, a path via the negative input-output capacitance 47 is added as a positive feedback path in addition to the input-output capacitance 46 described above. The input-output capacitance 47 is the capacitance between the inverting input unit of the input unit 43a and the inverting output unit of the output unit 44a, for example, the stray capacitance of the amplifier 42b. The input-output capacitance 47 is, for example, a negative input-negative output capacitance existing between a negative input unit (inverting input unit) and a negative output unit (inverting output unit). Due to the input-output capacitance 47, a _{second feedback current different from the feedback current i Cp} (first feedback current) described in the first embodiment flows from the inverting output section of the amplifier 42b to the inverting input section. ..

A capacitive element 67 is provided in the current source 6b to offset the positive feedback by the second feedback current. The capacitive element 67 is connected to the inverting output section and the non-inverting input section of the amplifier 62b. The inverting output output from the inverting output section of the amplifier 62b is fed back to the non-inverting input section of the amplifier 62b via the capacitive element 67, and the canceling current i _Cf (first canceling) described in the first embodiment is performed. A second canceling current different from the current) is generated. This second offset current reduces or cancels the second feedback current.

The amplifier circuit 2b-1 has a differential output, and at the non-inverting output, the first canceling current reduces or cancels the first feedback current, and at the inverting output, the second canceling current is the second. Reduces or offsets the feedback current of. The operation of the amplification unit 4b is stable at both differential outputs. That is, it is possible to obtain a two-output amplifier circuit 2b-1 that maintains low noise, has high operational stability, and maintains a frequency band.

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

[Fourth Embodiment]

The amplifier unit 4 of the amplifier circuit 2 of the first embodiment is composed of one amplifier unit 4, but the amplifier unit 4c of the amplifier circuit 2c of the fourth embodiment is a natural number N connected in parallel. It is composed of four amplification units 4c-1, 4c-2, ..., 4c-N. The amplifier unit 4a of the amplifier circuit 2a of the second embodiment is composed of one amplifier unit 4a, but the amplifier unit 4d of the amplifier circuit 2d of the fourth embodiment is a natural number N connected in parallel. It is composed of four amplification units 4d-1, 4d-2, ..., 4d-N. The amplifier 4b of the amplifier circuits 2b-1 and 2b-2 of the third embodiment is composed of one amplifier 4b, but the amplifier 4e of the amplifier circuit 2e of the fourth embodiment is in parallel. It is composed of amplification units 4e-1, 4e-2, ..., 4e-N having N natural numbers connected to. With such a configuration, it is possible to reduce noise by averaging the outputs as described in Patent Document 1 described in the background art.

The amplifier circuit 2c shown in FIG. 12A is composed of N natural number amplifier units 4c-1, 4c-2, ..., 4c-N in which the amplifier units 4c are connected in parallel. Each amplification unit 4c-1, 4c-2, ..., 4c-N has the same configuration as the amplification unit 4 described above in the first embodiment. Each amplifier section 4c-1,4c-2, ···, gain Ap in 4c-N, the output voltage V _o, output impedance R _o, the electrostatic capacitance Cp, the amplification unit 4c-1,4c- Like 2, ..., 4c-N, a hyphen (-) and a serial number are attached. The serially numbered parameters are expressed using the variable k, for example, gain Ap−k, output voltage V _ok , output impedance R _ok , 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 amplification unit 4c can be represented by a configuration equivalent to the amplification unit 4 of the first embodiment, and can be treated as the same configuration as that of the first embodiment.

・ ・ ・ ・ Equation 6

・ ・ ・ ・ Equation 7

・ ・ ・ ・ Equation 8

If amplifying unit 4c-1, 4C-2, · · ·, are 4c-N have the same characteristics, the amplification unit 4c, the output impedance R _o of the formula 9, formula 10 Input - Output amplifying portion unitary with the output voltage V _o which is represented by the capacitance Cp and formula 11 between capacity replaced equivalently to.
_Ro = _Ro-1 / N ・ ・ ・ ・ Equation 9
Cp = N ・ Cp-1 ・ ・ ・ ・ Equation 10
V _o = V _o-1 ... Equation 11

Since the amplification units 4c-1, 4c-2, ..., 4c-N are connected in parallel, the amplification units 4c-1, 4c-2, ..., 4c- The noise voltage of the amplification unit 4c is the reciprocal of the square root of N (1/1 of the route N) with respect to the noise voltage of N. That is, the noise of the amplification unit 4c can be reduced by connecting the amplification units 4c-1, 4c-2, ..., 4c-N in parallel.

According to Equation 10, the capacitance Cp increases with parallelization. As shown in "Frequency characteristics of loop round gain and phase" in the first embodiment, an increase in this capacitance Cp becomes a factor that tends to impair the stability of the amplifier circuit. However, since the amplification unit 4c is equivalently replaced with a single amplification unit, the current source 6 may be set as in the first embodiment. The amplifier circuit 2c can reduce or cancel the feedback current due to the positive feedback of the amplifier unit 4c by the canceling current of the current source 6, and the capacitance Cp becomes large due to the parallel connection of the amplifier units as in the amplifier circuit 2c. It is possible to reduce noise and maintain the frequency band while stabilizing the operation.

The amplifier circuit 2d shown in FIG. 12B is composed of N natural number amplifier units 4d-1, 4d-2, ..., 4d-N in which the amplification units 4d are connected in parallel. Each amplification unit 4d-1, 4d-2, ..., 4d-N has the same configuration as the amplification unit 4a described above in the second embodiment. The amplification unit 4d can be represented by a configuration equivalent to the amplification unit 4a of the second embodiment, and can be treated as the same configuration as 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, ..., 4d- The noise voltage of the amplification unit 4d is the inverse of the square root of N with respect to the noise voltage of N. That is, the noise of the amplification unit 4d can be reduced by connecting the amplification units 4d-1, 4d-2, ..., 4d-N in parallel.

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

The amplifier circuit 2e shown in FIG. 12C is composed of N natural number amplifier units 4e-1, 4e-2, ..., 4e-N in which the amplifier units 4e are connected in parallel. Each amplification unit 4e-1, 4e-2, ..., 4e-N has the same configuration as the amplification unit 4b described above in the third embodiment. The amplification unit 4e can be represented by a configuration equivalent to the amplification unit 4b of the third embodiment, and can be treated as the same configuration as 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, ..., 4e- The noise voltage of the amplification unit 4e is the inverse of the square root of N with respect to the noise voltage of N. That is, the noise of the amplification unit 4e can be reduced by connecting the amplification units 4e-1, 4e-2, ..., 4e-N in parallel.

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 amplifier circuit 2e can reduce or cancel the feedback current due to the positive feedback of the amplifier unit 4e by the canceling current of the current source 6b, and the capacitance Cpa and Cpb are large due to the parallel connection of the amplifier units as in the amplifier circuit 2e. Even so, 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 a fifth embodiment. In FIG. 13, the same parts as those in FIG. 9 are designated by the same reference numerals.

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

In the state where the amplifier circuit 2a is oscillating, a signal having 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 vibration on the input side of the amplifier 42a is larger than the amplitude of the signal input to the amplifier circuit 2a in FIG. 9, the signal source when the voltage of the signal source supplying the signal to the amplifier circuit 2a is positive. The situation is that a current flows to 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 are opposite, and it can be regarded as a resistor having an equivalent negative value when viewed from the signal source, that is, a negative resistance. Become in 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 amplification unit 4a. The impedance seen 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 round gain and phase frequency characteristics (FIG. 6) described in the first embodiment, the resistance value measured by the network analyzer is not so large in the frequency region including the frequency at which 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 resistors 10 and 11 on the input portion side of the amplifier circuit 2a, the operational stability of the amplifier circuit 2a is further enhanced. 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 by the resistors 10 and 11 is suppressed, and the low noise of the amplifier circuit 2f is maintained. To.

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 gradually approaches the resistance in the high frequency region with the frequency fd at which the resistance value and the reactance value of the inductor are equal. Shows frequency characteristics. In the high frequency region including the frequency at which the phase becomes 0 deg in the loop round gain and the frequency characteristics of 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. Due to such an inductance setting, the combined impedance due to the parallel connection of the resistors 10 and 11 and the inductors 12 and 13 is affected by the superposition of thermal noise by the resistors 10 and 11 in the input section of the amplifier circuit 2f in the low frequency region. You can avoid receiving it.

As the resistance values of the resistors 10 and 11 used as an auxiliary of 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 connecting the resistors 10 and 11 and the inductors 12 and 13 in parallel is connected. , The same effect can be obtained by connecting resistors 10 and 11 to the input portions of the amplifier circuits 2, 2b-1, 2b-2, 2c to 2e of other embodiments, or by connecting the combined impedance. it can.

Modifications and the like of the above-described embodiments are listed.

(1) In the above-described embodiment, an example of an amplifier circuit has been described, but the present invention is not limited to the amplifier circuit, and for example, an amplifier device including the above-mentioned amplifier circuit may be used. By including an amplifier circuit, the amplifier device can obtain the same effect as the amplifier 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 input-output capacitance 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 the capacitive element.

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

As described above, the most preferable embodiments of the present invention have been described, but the present invention is not limited to the above description, and the invention 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 purposes other than measurement of minute signals detected by a sensor. For example, it can be widely used for the following purposes (1) to (3).
(1) Measurements in the fields of physics and engineering MCT (Mercury Cadmium Tellurium) sensor for signal amplification and infrared detection of superconducting devices in quantum computers, SQUID (Superconducting Quantum Interface) sensor for micromagnetic detection, Microwave detection Photodetector elements such as high-temperature superconducting Josephson elements, photoelectron multipliers, and phototransistors.
(2) Measurement of biological signals in the field of biology Electromagnetic wave sensors for CT (Computed Tomography) scanners and MRI (Magnetic Resonance Imaging).
(3) Other fields where low noise and high-speed response are required for various sensors, elements, analytical instruments, etc.
Improving the sensitivity of lock-in amplifiers and removing common mode noise.

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

Claims (10)

An amplification unit that amplifies the input signal and
And offsetting amplifier section, and including a current source and a capacitor element connected to the input of inverted the canceling amplification of polarity to the output section and the output section of the phase killing amplification unit,
Equipped with a,
A signal corresponding to the input signal is input to the input unit of the offset amplification unit, and the signal corresponding to the input signal is input.
It said current source, an amplifier circuit, characterized that you generate cancellation current to reduce or cancel the feedback current flowing through the input section from the output of the amplification unit. The amplifier circuit according to claim 1, wherein the amplifier unit includes a plurality of amplifiers to which output impedances are added, and a plurality of amplifiers to which the output impedances are added are connected in parallel. The amplification unit is a non-inverting amplification unit.
The amplifier circuit according to any one of claims 1 or 2 , wherein the canceling amplifier is an inverting amplifier. The amplifier circuit according to any one of claims 1 to 3 , wherein the amplifier unit and the offset amplification unit include a differential input unit. The amplifier circuit according to claim 4, wherein the amplifier unit and the offset amplification unit include a differential output unit. The amplifier circuit according to any one of claims 1 to 5 , wherein the canceling amplifier is an amplifier circuit, an operational amplifier, or an instrumentation amplifier composed of a circuit including a discrete amplifier element. The amplifier circuit according to any one of claims 1 to 6 , wherein the amplifier unit is an amplifier circuit, an operational amplifier, or an instrumentation amplifier composed of a circuit including a discrete amplifier element. The amplifier circuit according to any one of claims 1 to 7 , wherein a resistor is connected in series to the front stage of the input unit of the amplifier unit. The amplifier circuit according to claim 8, wherein an inductor is connected in parallel with the resistor. An amplifier device comprising the amplifier circuit according to any one of claims 1 to 9.

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