JP3080488B2 - Differential amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential amplifier, and more particularly to a differential amplifier suitable for reducing a common-mode signal component generated inside the differential amplifier and a noise component due to power supply fluctuation.

[0002]

2. Description of the Related Art Conventionally, many differential amplifiers having a cascode bootstrap circuit having a circuit diagram shown in FIG. 6 have been provided. In the drawing, reference numerals 1 and 2 denote input terminals of a differential amplifier. An input signal applied to the input terminal 1 is supplied to a base of a _first transistor Q1, and an input signal applied to an input terminal 2. Is the second transistor Q ₂
Supplied to the base. The resistors R ₃ and R ₄ are bias resistors.

The collectors of the _first and _second transistors Q ₁ and Q ₂ configured as a differential pair are connected to the third and fourth transistors Q ₃ and Q _{4, respectively.}
In is the bootstrap circuit 3 which is formed by being cascoded connected to the load resistor R _1, supplied via the R ₂ supply V _CC, the load resistance R _1, R ₂ and the third, fourth transistor
Q _3, was taken out to an output terminal 9 and 10 an output from the connection point between the collector of Q _4.

The above cascode bootstrap circuit 3
The bases of the third and fourth transistors Q ₃ and Q ₄ are directly connected to form a common base, and a resistor R ₂₀ and a level shift diode D ₃ are connected to the common base from a positive power supply V _CC , The bias circuit 5 of the cascode bootstrap circuit 3 is formed.

On the other hand, the common emitters of the first and second transistors Q ₁ and Q ₂ are connected to a negative power supply -V _CC via a _fifth transistor Q ₅ forming a constant current circuit 4 and a resistor R _21. ,
It is biased by a fifth transistor Q based ₅ of the level shift diode D ₂₀ connected from the negative power supply -V _CC constant current diode D ₂ and resistor R _22.

The input terminals 1 and 2 of the differential amplifier thus configured are
2 If the input signal to perform differential amplification operation is supplied, the first operating current of the transistor Q ₁ and I _1, the second operating current of the transistor Q ₂ and I _2, further cascode bootstrap circuit 3 of the third, in order to fourth transistors Q _3, Q ₄ bootstrap operation, third and fourth transistors Q _3, Q ₄ of the base bias the anode of the level shift diode D ₃ to form a voltage first , Connected to the common emitter of the second transistors Q ₁ and Q ₂ , and this bias current is
When I _3, the current I flowing through the constant current circuit 5 operates constant current becomes _{I = I 1 + I 2 +} I 3.

That is, the operation according to the input signal level of the first and second transistors Q ₁ and Q ₂ is such that the bias current I ₃ of the cascode bootstrap circuit 3 is biased by the constant current I of the constant current circuit 4 and linearly. Bootstrapped output signals can be obtained from output terminals 9 and 10.

Further, the load resistors R ₁ and R ₂ of the first and second transistors Q ₁ and Q ₂ of the differential amplifier and the boot of the third and fourth transistors Q ₃ and Q ₄ of the cascode bootstrap circuit 3. When the strap operation is performed under the same conditions, each input voltage applied to the input terminals 1 and 2 can be differentially amplified to obtain a difference output voltage, and a common mode component of each input voltage is removed. can do.

[0009]

[SUMMARY OF THE INVENTION] However, the conventional and the differential amplifier cascode bootstrap circuit resistance bias voltage necessary to 3 R ₂₀ and level shift diode
The bias current I ₃ flowing through D ₃ is generated by the bias current I ₃ as described above.
Since superimposed to the common emitter current I _1, I ₂ of Q _1, Q ₂ has the constant current I, first, the phase component of the second transistor Q _1, Q ₂ of the common emitter voltage of the input signal When varying the voltage bias circuit current I ₃ is generated across the varied resistance R ₂₀ also varies similarly, also, since the internal impedance of the level shift diode D ₃ is not zero, the variation of the bias current I ₃ There is a disadvantage that a constant current I is deteriorated in inherent common mode rejection characteristics of the differential amplifier fluctuates, and when the generated voltage of the resistor R ₂₀ varies greatly depending on the variation of the supply voltage V _CC, the same way In some cases, the constant current I fluctuated and the differential amplification characteristics were impaired.

Further, when there is an error in the collector load resistances R ₁ , R ₂ of the first and second transistors Q ₁ , Q ₂ and a difference in the input impedance at the subsequent stage of the differential amplifier, the differential amplification operation is similarly performed. There is also the problem that the output of the common-mode component of the output is not removed and appears in the output. To reduce the errors in the load resistors R ₁ and R ₂ and the input impedance in the subsequent stage, a highly accurate (up to 1% or less error) circuit There is a disadvantage that the element must be used and the cost is increased.

Now, for example, by using the input circuit shown in the circuit diagram of the measuring differential amplifier shown in FIG. 4 and configuring the differential amplifier circuit shown in FIG. 6 and applying an input signal to the input terminal 1, this input The signal is provided to the base of a _first transistor Q1,
On the other hand, the second base of the transistor Q ₂ resistors R ₁₀ = 10 [Omega
An input signal is supplied via a base input resistor R ₁₁ = 100 kΩ connected between the ground and the ground.

That is, the input signal level difference supplied to the bases of the first and second transistors Q ₁ and Q ₂ is a signal difference of −80 dB of the in-phase input level, and the output signal appearing at the output terminals 9 and 10 is Assuming that the antiphase component is a and the inphase component is b, the output voltage of the output terminal 9: V _O1 = a + b The output voltage of the output terminal 10: V _O2 = −a + b, and the output voltage V ₀₁ and the output voltage V _O2 By the addition, the negative-phase component a is removed, and only the in-phase component = 2b can be obtained. The in-phase component 2b becomes as shown in FIG.

FIG. 7 shows an output signal waveform measured by the oscilloscope 11 shown in FIG. 4. In the waveform diagram of FIG.
_Represents an in-phase signal waveform of the output voltage _V01 of the output terminal 9, 31 represents an in-phase signal waveform of the output voltage _VO2 of the output terminal 10, and 32 represents a waveform obtained by adding the in-phase signal waveforms 30 and 31. In this way the conventional differential amplifier, in practice the in-phase signal component is large and outputs the impact like cascode bootstrap circuit 3 of the bias current I ₃ of the variation and supply voltage variation, the inherent differential amplification characteristic It was difficult to secure.

The constant current circuit 4 includes a fifth transistor
Level shift diode D ₂₀ and constant current diode D ₂ with Q ₅ base bias connected to negative supply -V _CC and resistor
Although R ₂₂ is always formed to constant current constant,
When the collector potential fluctuates due to the in-phase component of the input signal, the fifth
By the Miller capacitance C ₁ of the transistor Q _5, the constant current characteristics are deteriorated, especially for high frequencies. Further, the collector output impedance of the fifth transistor Q ₅ is not an ideal constant current characteristic for not infinite, the fifth transistor
The collector of Q ₅ - current relative to the voltage variation between emitters a drawback in that change.

That is, the output of the output terminals 9 and 10 of the differential amplifier has a drawback that in-phase components and power supply fluctuations are output in addition to differential (opposite phase) component signals.

The present invention has been made in view of the above points, and has as its object to solve the disadvantages of the prior art and to provide a cascode bootstrap circuit of the third and fourth transistors Q ₃ and Q ₄ . A bias circuit is formed via an emitter follower circuit provided on a common emitter of the first and second transistors Q ₁ and Q ₂ , and a variation in bias current due to an input common mode component flows through the emitter follower circuit, To provide a differential amplifier capable of improving constant current characteristics by suppressing fluctuation of constant current without superimposing the bias current on the constant current of the common emitter, and capable of performing accurate differential amplification. It is in.

[0017]

SUMMARY OF THE INVENTION A differential amplifier according to the present invention includes first and second transistors forming a differential pair, and a first cascode boot connected to respective collectors of the first and second transistors. A strap circuit, a load resistor provided between the first cascode bootstrap circuit and a power supply, and a constant current circuit provided between a common emitter connecting the emitters of the first and second transistors and a negative power supply. A bias circuit that supplies a bias voltage to the first cascode bootstrap circuit, an input unit that applies an input signal to each base of the first and second transistors, and the first cascode bootstrap circuit. Output means for extracting an output signal from a connection point with a load resistor, wherein the first cascode bootstrap circuit is 3, formed by the fourth transistor,
A constant current diode connected between a common base connecting the bases of the third and fourth transistors and a power supply,
An emitter follower circuit connected to a common emitter of the first and second transistors is provided, and the emitter follower circuit is formed by seventh and eighth transistors, and the emitter of the eighth transistor is connected to the third and third transistors. A level shift diode connected between a common base of the fourth transistor, a bias circuit comprising the constant current diode and the level shift diode via the emitter follower circuit, and And a second cascode bootstrap circuit provided between the collector of the fifth transistor and the negative power supply is formed by a sixth transistor, and the base bias voltage of the sixth transistor is controlled by the emitter follower. −
It is configured to be supplied by a level shift diode provided between the emitter of the seventh transistor of the circuit and the negative power supply.

[0018]

[0019]

According to the present invention, the emitter follower circuit 6 is connected to the common emitter of the first and second transistors Q ₁ and Q ₂ forming a differential pair.
Is formed by _seventh and _eighth transistors Q ₇ and Q ₈ ,
The first cascode bootstrap circuit 3 via a level shift diode D ₃ than the emitter of the transistor Q ₈
Of the third and fourth transistors Q ₃ and Q ₄ .

[0020] determines the bias voltage by the level shift diode D ₃ from the positive supply V _CC via the constant current diode D _1, the bias current I ₃ is the eighth transistor
Through the second cascode bootstrap circuit 7 connected from Q ₈ to the constant current circuit 4 flows in the negative supply -V _CC.

The above-mentioned constant current circuit 4 makes the operating currents I ₁ and I ₂ for differential amplification of the first and second transistors Q ₁ and Q ₂ constant current to form a second cascode bootstrap circuit 7. It flows in the negative power supply -V _CC through the transistor Q ₆ in the sixth. The sixth base bias voltage of the transistor Q ₆ of the second cascode bootstrap circuit 7 the emitter follower - seventh transistor Q ₇ level shift diode D ₄ connected to the circuit 6, D ₅ and the constant current diode D ₂ to perform a bootstrap operation.

That is, the bias current of the first cascode bootstrap circuit 3 is set to the constant current I = I ₁ + I ₂ of the differential amplification.
Since I ₃ is formed without being superimposed, even bias current I ₃ varies relative to variation in the common emitter voltage due to the common-mode component of the input signal, the constant current I is the emitter follower - circuit 6 buffers The effect can suppress the fluctuation, and the constant current characteristics with respect to the differential amplification can be set in a favorable state.

The constant current circuit 4 includes a sixth transistor
It is provided with the cascode bootstrap circuit 7 by Q _6, by the bootstrap operation of the constant current circuit 4, 5
Transistor Q can be eliminated for the Miller effect of _5, it is possible to improve the constant-current characteristic with respect to high frequency signals, it is broadband high frequency range. Further, the output impedance of the constant current circuit 4 can be increased, and ideal constant current characteristics can be obtained.

That is, the differential amplifier of the present invention has a circuit configuration close to an ideal differential amplifier in which the internal impedance of the bias voltage source of the first cascode bootstrap circuit 3 is zero and the internal impedance of the constant current source is infinite. Can work.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a differential amplifier according to the present invention will be described with reference to FIGS. The same parts as those in the conventional example are denoted by the same reference numerals, and description thereof will be omitted. FIG. 1 is a block diagram of a differential amplifier provided with a second cascode bootstrap circuit. FIG. 2 is a circuit diagram showing the embodiment of FIG.
FIG. 3 is a simplified circuit diagram showing another embodiment, FIG. 4 is a circuit diagram for measuring a common mode rejection characteristic of a differential amplifier, and FIG.
FIG. 6 is a waveform diagram showing common-mode rejection characteristics.

In FIGS. 1 and 2, reference numeral 3 denotes a first cascode bootstrap circuit connected to the collectors of the first and second transistors Q ₁ and Q ₂ for differential amplification.
Cascode bootstrap circuit 3 third and fourth transistors Q _3, composed of Q _4, the third, base of the fourth transistor Q _3, Q ₄ forms a respectively directly connected common base . Reference numeral 4 denotes a constant current circuit connected to the common emitter of the first and second transistors Q ₁ and Q _2. The constant current circuit 4 is connected from the common emitter to the emitter of the fifth transistor Q ₅ via the resistor R _5. Connect and form.

Reference numeral 5 denotes a bias circuit for the third and fourth transistors Q ₃ and Q _4.
Constant current diode connected to the _CC D ₁ level shift diode D ₃ of the connection point of the third and fourth transistors Q _3, and formed to connect to the common base of Q _4, the bias circuit 5 seventh, The common emitter is connected via an emitter follower circuit 6 formed by _eight transistors Q ₇ and Q ₈ .

[0028] 7 is a second cascode bootstrap circuit, the second cascode bootstrap circuit 7 is formed of transistors Q ₆ of the sixth, and the collector of the fifth transistor Q ₅ of the constant current circuit 4 first 6 transistors Q ₆
The emitter and form a cascode circuit connected, the base is the emitter-follower of the transistor Q ₆ of the sixth - seventh level shift connected from the emitter of the transistor Q ₇ to the negative power supply -V _CC of the circuit 6 The circuit 8 is biased by the level shift diodes D ₄ and D ₅ and the constant current diode D ₂ to perform a bootstrap operation.

The differential amplifier having the above-mentioned configuration is connected to the third and fourth transistors Q ₃ and Q ₄ of the first cascode bootstrap circuit 3 by the level shift diodes D ₃ connected to the common base. 4 transistors Q ₃ , Q ₄ V
_CE is determined, and the bias current I ₃ flowing through the level shift diode D ₃ flows through the emitter follower circuit 6 to the negative supply power supply -V _CC , so that the first and second transistors
The first cascode bootstrap circuit 3 is not superimposed on the constant current circuit 4 connected to the common emitter of Q ₁ and Q _2.
Can be performed.

The base bias of the fifth transistor Q ₅ of the constant-current circuit 5, as shown, the emitter follower - of the seventh transistor Q ₇ level shift diode D ₄ connected to the emitter of the circuit 6, D ₅ The sixth transistor of the second cascode bootstrap circuit 7 supplied from the contact point
Base bias of Q ₆ is supplied from the connection point between the level shift diode D ₅ and the constant current diode D ₂ performs the constant current operation and the bootstrap operation.

As described above, the first and second transistors Q ₁ and Q _1
2 of the operating current I ₁ of the differential _amplifier, I ₂ is controlled from the common emitter with a constant current circuit 4 becomes the constant current I, the constant current I =
Bias currents of the _third and _fourth transistors Q ₃ and Q _{4, which} are noise sources due to common-mode input signals and power supply voltage fluctuations in I ₁ + I ₂
I ₃ and other circuit current, an emitter follower connected to the common emitter - prevents the flow by the buffer circuit 6, it is possible to prevent the influence of the in-phase component included like in the input signal to the constant current I.

Further, the fifth transistor of the constant current circuit 4
Since the Q ₅ is suffering from a boot strap that is cascode by the transistor Q ₆ of the sixth, the fifth transistor
The V _CE of Q ₅ becomes constant and the effect of the Miller capacitance C ₁ of the fifth transistor Q ₅ is eliminated, and a stable constant current characteristic can be obtained even at a high frequency. Can be better.

FIG. 4 shows that a common base of the third and fourth transistors Q ₃ and Q ₄ is biased by a DC voltage source E having no internal resistance, and a constant current source I having an infinite internal resistance is connected to the common emitter. FIG. 2 is a circuit diagram of the ideal differential amplifier. Reference numeral 11 denotes an oscilloscope for measuring a common-mode rejection characteristic of an output signal.

This ideal differential amplifier is actually constituted by the circuit of FIG. 1 of the present invention, and the input signals of the first and second transistors Q ₁ and Q ₂ of the differential amplifier are different from those of the conventional example. input level of the second transistor Q ₂ with respect to the input level in a manner similar to that described first transistor Q ₁ is, resistor R ₁₀ = 10 [Omega
And R ₁₁ = 100 kΩ, resulting in a common-mode input level difference of -80 dB. This input condition is equivalent to the input condition of the first-stage amplification of a feedback amplifier generally used in an audio signal amplifier.

The output signals appearing at the output terminals 9 and 10 of the measuring differential amplifier thus configured can be measured by the oscilloscope 11. As in the measurement waveform diagram of FIG. 7 of the conventional example described above, the output signals appearing at the output terminals 9 and 10 are:
Assuming that the negative-phase component is a and the common-mode component is b, the output voltage at the output terminal 9: V _O1 = a + b The output voltage at the output terminal 10: V _O2 = −a + b, and the output voltage V ₀₁ and the output voltage V _O2 are added. By doing so, the in-phase component a can be removed and only the in-phase component 2b can be obtained, and the in-phase component 2b has a waveform as shown in FIG.

In FIG. 5, reference numeral 20 denotes the output voltage of the output terminal 9.
_{V01 is} the output signal waveform, and 21 is the output voltage of output terminal 10.
5 is an output signal waveform of V _O2 . _Reference numeral 22 denotes a composite waveform obtained by adding the output voltage _V01 waveform 20 and the output voltage _V02 waveform 21, and as shown in the figure, the output voltage _V01 waveform 20 and the output voltage _V02 waveform 21 have only an inverted phase component and are added signals. The waveform 22 has no in-phase component output. That is, if the outputs are added and taken out between the output terminals 9 and 10 of the differential amplifier, the common-mode component appearing in the output can be ignored, and the performance of the differential amplification operation close to the ideal differential amplifier shown in FIG. Can be put out.

FIG. 3 is a circuit diagram showing another embodiment, in which the differential amplifier of FIG. 1 is simplified. In the figure, an emitter follower - circuit 6 is formed only in the transistor Q ₈ of the eighth, as shown, the level shift diode D ₃ from the emitter of the transistor Q ₈ of the eighth
, The common base of the third and fourth transistors Q ₃ and Q ₄ is biased, and the cascode bootstrap circuit 3
Is a bootstrap operation.

The constant current circuit 4 from the common emitter of the first and second transistors Q ₁ and Q ₂ is connected to a constant current diode D ₆
And simplified.

[0039] Thus, the same way, an emitter follower - using circuit 6 to form a bias circuit of the cascode bootstrap circuit 3, the operating bias current I ₃ of the bias circuit emitter follower - bias flowed in the circuit 6 is operated, since without bias current I ₃ is superposed on the constant current diode D ₆ of the constant current circuit 6, it is possible to prevent the constant current I at the time of the differential amplifier and the like bias current I ₃ is a noise source.

The above constant current I characteristic is represented by a constant current diode D ₆
Although the current is made constant by the internal resistance of the circuit and is simplified as compared with the circuit diagram of FIG. The simplified circuit of FIG. 3 is generally suitable for a circuit having a low common-mode input signal level, and is preferably used for a post-amplifier such as a D / A conversion circuit.

As described above, the amplifying elements of the differential amplifier are the first and second
Of constituted by transistors Q _1, Q _2, other circuit elements is also described with reference to transistors, other devices in place of the transistor amplifier element, for example may be constituted by replacing the like FET.

[0042]

As described above, in the differential amplifier according to the present invention, only the operating current of the first and second transistors Q ₁ and Q ₂ for differential amplification is converted into a constant current by the constant current circuit 6, and The noise current due to component and power supply voltage fluctuations is buffered by the emitter follower circuit connected to the common emitter, and the noise current works without being superimposed on the constant current circuit. It can be operated in a state close to an amplifier, and has an effect that a more precise differential amplifier can be constructed.

In the circuit for measuring the common-mode rejection characteristic of the differential amplifier shown in FIG. 4, the output waveform level of the common-mode component shown in the conventional example (addition waveform 32 in FIG. 7) and the circuit shown in FIG. The level ratio of the output waveform 22 of the in-phase component of the embodiment (the waveform 22 in FIG. 5) is 40 dB or more. In order to improve the in-phase component by -40 dB in the conventional circuit (the circuit diagram in FIG. 6), it is necessary to combine differential amplifiers. Although the load impedance error must be reduced to 1% or less at the maximum, which is extremely difficult, the circuit of the present invention can be easily realized.

Further, since the cascode bootstrap circuit of the constant current circuit improves the frequency characteristic, the differential amplifier can operate up to a high frequency band, and the differential amplifier can have a wider band. is there.

In addition, it has excellent features such as simple structure and easy implementation because it can be constructed at low cost.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a differential amplifier according to the present invention.

FIG. 2 is a circuit diagram showing an embodiment of the block diagram of FIG. 1;

FIG. 3 is a circuit diagram showing another simplified embodiment.

FIG. 4 is a circuit diagram for measuring a common-mode rejection characteristic of a differential amplifier.

FIG. 5 is an output waveform diagram showing common-mode rejection characteristics measured by the circuit of FIG. 4;

FIG. 6 is a circuit diagram showing a conventional example.

7 is an output waveform diagram measured by the conventional circuit of FIG. 6 in the measurement circuit of FIG. 4;

[Explanation of symbols]

 1,2 Input terminal of differential amplifier 3 First cascode bootstrap circuit 4 Constant current circuit 5 Bias circuit 6 Emitter follower circuit 7 Second cascode bootstrap circuit 8 Level shift circuit 9,10 Output terminal 11 Oscilloscope 12 Bias Voltage source E 13 Constant current source I 20 Output voltage waveform at output terminal 9 21 Output voltage waveform at output terminal 10 22 Composite output waveform obtained by adding both output voltages

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03F 3/45 H03F 1/22

Claims (1)

(57) [Claims] A first cascode bootstrap circuit connected to each collector of the first and second transistors; a first cascode bootstrap circuit connected to each collector of the first and second transistors; A load resistor provided between the circuit and a power supply, a constant current circuit provided between a common emitter connecting the emitters of the first and second transistors and a negative power supply, and a first cascode bootstrap circuit. A bias circuit for supplying a bias voltage; input means for applying an input signal to each base of the first and second transistors; and an output signal from a connection point between the first cascode bootstrap circuit and a load resistor. A differential amplifier comprising output means, wherein the first cascode bootstrap circuit is formed by third and fourth transistors. A constant current diode connected between a common base connecting the bases of the third and fourth transistors and a power supply, and an emitter follower circuit connected to a common emitter of the first and second transistors; A follower circuit is formed by seventh and eighth transistors, and an emitter of the eighth transistor and a level shift diode connected between a common base of the third and fourth transistors are provided. A bias circuit is formed by the constant current diode and the level shift diode via the first transistor, the constant current circuit is formed by a fifth transistor, and a second circuit is provided between the collector of the fifth transistor and the negative power supply. Cascode bootstrap circuit is formed by a sixth transistor, and a base via of the sixth transistor is formed. Wherein the voltage emitter follower - circuit of the seventh transistor of the emitter and the differential amplifier, characterized by being configured to be supplied by the level shift diode provided between the negative power supply.