JP3131074B2 - Feedback amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a feedback amplifier circuit provided with a differential amplifier and a feedback circuit for amplifying a minute AC signal including a DC signal.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional feedback amplifier circuit. This feedback amplification circuit includes a differential amplifier A11 and a feedback circuit F11. The differential amplifier A11 is, for convenience,
An ideal differential amplifier, ie, input impedance ∞,
It is assumed that the circuit operates with an output impedance of 0 and an open loop gain 、, and has a sufficient frequency band with respect to frequency characteristics. The feedback circuit F11 includes a differential amplifier A
11, two feedback resistors R12 and R11 connected in series between the output terminal of the first output terminal 11 and the negative (-) side input terminal, and a feedback capacitor C connected between the output terminal and the input terminal.
11 and an NPN transistor Q11 whose base terminal is connected to a connection point P11 between feedback resistors R11 and R12.
And a feedback resistor R connected between the emitter terminal of the transistor Q11 and the input terminal of the differential amplifier A11.
13 Note that the collector terminal of the transistor Q11 is connected to a power supply.

The impedance of the entire feedback circuit F11 is Z
Assuming that the input signal Iin is 11, the output Vo of the differential amplifier A11 is expressed by the following equation (1). Vo = Iin × Z11 (1)

[0004] Speaking qualitatively, when the DC current component included in the input Iin is small, the voltage V1 at the point P11 is small. Therefore, transistor Q11 does not operate, and feedback impedance Z11 is determined by R11, R12 and C11. Conversely, when the DC current component included in the input Iin is large, the voltage V1 increases and the transistor Q11 operates. Therefore, transistor Q1
1, the current is also fed back through the feedback resistor R13, and the feedback impedance Z11 is Q11, R11, R12, R13
And C11. That is, the transistor Q
11, the feedback resistor R13 functions to reduce the gain of this feedback amplifier circuit when the DC current component increases, thereby avoiding saturation of this circuit.

Next, the characteristics of the feedback amplifier circuit will be described quantitatively.

(A) First, the DC current component contained in the input Iin is small, and the voltage V1 at the point P11 is
It is assumed that the base-emitter voltage (hereinafter referred to as “threshold voltage”) VBE11 required for the operation of No. 1 is lower than VBE11. That is, it is assumed that V1 <VBE11 (2). As mentioned above, transistor Q
11 does not operate, and no current flows through the feedback resistor R13, so that the feedback impedance is R11, R12 and C11.
Depends on The feedback impedance at this time is Z1
Assuming 1a, Z11a = (R11 + R12) / {1 + jωC11 (R11 + R12)} (3) Here, ω is an angular frequency.

The absolute value of Z11a is | Z11a | = (R11 + R12) / (1+ (ωC11 (R11 + R12)) ² ) ^1/2 (4) Further, the cutoff frequency fc11a is as follows: fc11a = 1 / (2πC11 (R11 + R12)) (5)

(B) Next, it is assumed that the direct current component included in the input Iin is large and the voltage V1 at the point P11 is higher than the threshold voltage VBE11. That is, it is assumed that V1> VBE11 (6). Strictly speaking, the emitter resistance RE, the current amplification factor hfe, and the frequency characteristic of the transistor Q11 are also related to the circuit operation.
It is assumed that 3 >> RE, the current amplification factor is sufficiently large, and the frequency characteristics can be neglected. At this time, the DC feedback current If generated by the change of the output voltage Vo is If ≒ Vo / (R11 + R12) + VoR11 / (R11 + R12) / R13 (7) Here, assuming that a portion of the feedback impedance Z11 other than the feedback capacitance C11 is a feedback resistor Rf, Rf = Vo / If ≒ (R11 + R12) R13 / (R11 + R13) (8) The total feedback impedance at this time is Z11
b, Z11b and its absolute value | Z11b |
Are (R11 + R) in the above formulas (3) and (4), respectively.
By replacing the part 12) with Rf, Z11b = Rf / {1 + jωC11Rf} (9) | Z11b | = Rf / (1+ (ωC11Rf) ² ) ^1/2 (10) Further, the cutoff frequency fc11b is as follows: fc11b = 1 / (2πC11Rf) (11)

The values of the feedback resistors R11, R12, R13 and the feedback capacitance C11 are respectively represented by R11 = R12 = 10
0KΩ R13 = 20KΩ When C11 = 1pF, absolute value of feedback impedance | Z11a |, | Z
The frequency characteristic (calculated value) of 11b | is represented as shown in FIG. In the figure, the solid line is | Z11a | and the broken line is | Z11b |
Is shown. In this example, | Z11b |
Has been reduced to about 1/6 of | Z11a |.

[0010]

By the way, the input signal I
When the DC current component of “in” increases, the feedback impedance is reduced as shown in FIG. 6, but at the same time, the frequency bandwidth is widened. That is, the above equations (5) and (1)
As can be seen from 1), the feedback resistance component (R11 + R1
2) As Rf decreases, the frequency band becomes wider in inverse proportion thereto. For this reason, the conventional feedback amplifier circuit has a problem that when the DC current component of the input signal Iin increases, sufficient band limitation or phase compensation is not performed, and the circuit easily oscillates.

SUMMARY OF THE INVENTION An object of the present invention is to provide a feedback amplifying circuit which can suppress the expansion of the frequency band when the direct current component of the input signal increases, thereby preventing the oscillation of the circuit.

[0012]

According to a first aspect of the present invention, there is provided a feedback amplifier circuit for amplifying a signal received at an input terminal and outputting the amplified signal to an output terminal. A feedback circuit that feeds back the output of the amplifier to the input terminal so as to indicate an amplification factor, wherein the feedback circuit includes two second terminals connected in series between the output terminal and the input terminal of the amplifier. A first feedback resistor, a first feedback capacitor connected between the output terminal and the input terminal, and a base terminal connected to a connection point between the first feedback resistors;
A feedback amplifier circuit having a transistor having a collector terminal connected to a power supply or the output terminal, and a second feedback resistor connected between an emitter terminal of the transistor and the input terminal of the amplifier. A second feedback capacitor is connected between the emitter terminal of the transistor and the input terminal.

Further, the feedback amplifier circuit according to claim 2 is
2. The feedback amplifier circuit according to claim 1, wherein a time constant consisting of a product of a total value of said first feedback resistance and a value of said first feedback capacitance, a value of said second feedback resistance, and said second feedback. It is characterized in that a time constant consisting of a product of the capacitance value and the time constant is set substantially equal.

Further, the feedback amplifier circuit according to claim 3 is
An amplifier that amplifies a signal received at the input terminal and outputs the amplified signal to an output terminal; and a feedback circuit that feeds back the output of the amplifier to the input terminal so that the amplifier exhibits a predetermined amplification factor. , N are natural numbers, and (N + 1) first feedback resistors connected in series between the output terminal and the input terminal of the amplifier, and the first feedback resistor connected between the output terminal and the input terminal. A first feedback capacitor, N transistors each having a base terminal connected to a connection point between the first feedback resistors, and a collector terminal connected to a power supply or the output terminal; N second feedback resistors respectively connected between the emitter terminal of the transistor and the input terminal; and N second feedback resistors respectively connected between the emitter terminal of each of the transistors and the input terminal. capacity And characterized in that:

[0015]

According to the feedback amplifying circuit of the present invention, the DC current component of the input signal increases, and the voltage at the connection point between the first feedback resistors becomes larger than the threshold voltage of the transistor. At this time, the impedance of the feedback circuit decreases. Thereby, the amplification factor of the circuit is controlled. Moreover, as described in detail in the section of the embodiment, the function of the second feedback capacitor suppresses the spread of the frequency band. Therefore, sufficient phase compensation is performed, and oscillation of the circuit is effectively prevented.

Further, in the feedback amplifying circuit according to the present invention, a time constant consisting of a product of a total value of the first feedback resistance and a value of the first feedback capacitance, a value of the second feedback resistance and a value of the second feedback resistance. The time constant, which is the product of the feedback capacitance value and the value of the feedback capacitance of 2, is set substantially equal. In this case, the cutoff frequency is substantially equal between the case where the DC component of the input signal does not exceed the threshold value of the transistor and the case where the DC component exceeds the threshold value. Therefore, more sufficient phase compensation is performed, and oscillation of the circuit is effectively prevented.

Further, in the feedback amplifying circuit according to the present invention, when the voltage at the N nodes between the first feedback resistors exceeds the threshold voltage between the base and the emitter of each transistor, A current flows through the second feedback resistor and the second feedback capacitor connected to the emitter terminal of the transistor. As a result, similar to the feedback amplifier circuit of claim 1 or claim 2,
The gain of the circuit is controlled. In addition, the expansion of the frequency band is suppressed. Therefore, sufficient phase compensation is performed, and oscillation of the circuit is effectively prevented.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a feedback amplifier circuit according to the present invention will be described in detail with reference to embodiments.

FIG. 1 shows a feedback amplifier circuit according to the first embodiment. This feedback amplifier circuit amplifies a signal received at an input terminal and outputs the amplified signal to an output terminal, and a differential amplifier A1 such that the differential amplifier A1 exhibits a predetermined amplification factor.
Is provided to the input terminal. The differential amplifier A1 works with an ideal differential amplifier, that is, an input impedance ∞, an output impedance 0, and an open loop gain 同 様, similarly to the conventional example shown in FIG. have. The feedback circuit F1 includes two first feedback resistors R2 and R1 connected in series between an output terminal of the differential amplifier A1 and a negative (−) input terminal, and the output terminal and the input terminal. And a first feedback capacitor C1 connected between the first feedback capacitor C1 and the first feedback capacitor C1. Also, the base terminals are feedback resistors R1 and R2
Transistor Q connected to a connection point P1 between
1, a second feedback resistor R3 connected between the emitter terminal of the transistor Q1 and the input terminal, and a second feedback capacitor C2 connected in parallel to the feedback resistor R3. . In this example, the collector terminal of the transistor Q1 is connected to a power supply.

The basic operation of this feedback amplifier circuit is substantially the same as that of the conventional example, but the frequency characteristics when the transistor Q1 operates are different. Next, the characteristics of the feedback amplifier circuit will be described in detail.

The impedance of the entire feedback circuit F1 is Z1
Then, when the input signal Iin is given, the output Vo of the differential amplifier A1 is expressed by the following equation (12). Vo = Iin × Z1 (12)

(A) First, it is assumed that the DC current component included in the input Iin is small and the voltage V1 at the point P1 is lower than the threshold voltage VBE1 required for the operation of the transistor Q1. That is, V1 <VBE1 (13). In this case, since the transistor Q1 does not operate and no current flows through the feedback resistor R3, the feedback impedance is determined by R1, R2 and C1. Assuming that the feedback impedance at this time is Z1a, Z1a = (R1 + R2) / {1 + jωC1 (R1 + R2)} (14) Here, ω is an angular frequency.

The absolute value of Z1a is as follows: | Z1a | = (R1 + R2) / (1+ (ωC1 (R1 + R2)) ² ) ^1/2 (15) Further, the cutoff frequency fc1a is as follows: fc1a = 1 / (2πC1 (R1 + R2)) (16)

(B) Next, the DC current component contained in the input Iin is large, and the voltage V1 at the point P1 is
It is assumed to be larger than BE1. That is, V1> VBE1 (17).

Further, for simplicity, the cutoff frequency fc11b when there is no feedback capacitance C2 (in the case of the conventional example)
(See equation (11)) is set to be sufficiently higher than the cutoff frequency [1 / (2πC2R3)] of the impedance Zx of the portion formed by the feedback capacitance R3 and the feedback capacitance C2 (usually, such a design I do.) In this case, C1 can be substantially ignored. The feedback impedance at this time is Z1b
Then, Z1b is expressed by replacing R13 in the equation (8) with Zx. Z1b ≒ (R1 + R2) Zx / (R1 + Zx) (18) ≒ (R1 + R2) / (1 + R1 / Zx) (19) Becomes As can be seen from equation (19), the frequency characteristic of Z1b is determined according to the frequency characteristic of Zx. R1 /
If Zx is sufficiently larger than 1, the absolute value of Z1b is approximately proportional to Zx.

The cutoff frequency fc1b is as follows: fc1b ≒ 1 / (2πC2R3) (20)

The values of the feedback resistors R1, R2, R3 and the feedback capacitances C1, C2 are expressed as follows: R1 = R2 = 100 KΩ R3 = 20 KΩ C1 = 1 pF C2 = 10 pF (21) (Values of R1, R2, R3 and C1) Is the same as the conventional example), the absolute value of the feedback impedance | Z1a |, |
The frequency characteristic (calculated value) of Z1b | is represented as shown in FIG. In the figure, the solid line indicates | Z1a |, and the broken line indicates | Z1b |. In this example, | Z1b |
The point that is reduced to about 1/6 of a | is the same as the conventional one. However, they are different for frequency bands. That is,
Conventionally (FIG. 6), the frequency band of | Z11b | is | Z11a.
In this example, the frequency band of | Z1b | is substantially equal to the frequency band of | Z1a |, whereas the frequency band of | Z1a |

In the first embodiment, as can be seen from the equation (21), the value of the time constant C1 (R1 + R2) is set equal to the value of the time constant C2R3. Therefore,
As derived from the equations (16) and (20), the cutoff frequencies fc1a and fc1b become substantially equal.

As described above, in the feedback amplifier circuit, when the DC current component of the input signal increases, the expansion of the frequency band can be suppressed. Therefore, sufficient phase compensation can be performed, and oscillation of the circuit can be effectively prevented.

FIG. 2 shows a feedback amplifier circuit according to the second embodiment. This feedback amplifier circuit is different in that the NPN transistor Q1 shown in FIG. 1 is replaced with a PNP transistor Q2. That is, this feedback amplification circuit includes a differential amplifier A2 and a feedback circuit F2. Differential amplifier A
2 is an ideal differential amplifier, ie, input impedance ∞, output impedance 0, open loop gain ∞
And has a sufficient frequency band with respect to frequency characteristics. The feedback circuit F2 is connected in series between the output terminal of the differential amplifier A2 and the negative (−) input terminal.
A plurality of first feedback resistors R5 and R4, and a first feedback capacitor C3 connected between the output terminal and the input terminal. An NPN transistor Q2 having a base terminal connected to a connection point P2 between feedback resistors R5 and R4.
And a second feedback resistor R6 connected between the emitter terminal of the transistor Q2 and the input terminal, and a second feedback capacitor C4 connected in parallel to the feedback resistor R6.

This feedback amplifier operates in exactly the same way as that of the first embodiment. That is, the DC current component of the input signal increases, and the voltage V1 at the point P2 becomes the threshold voltage VBE.
When the value becomes larger than 2, the impedance Z2 of the feedback circuit F2 decreases and the amplification factor of the circuit can be controlled, and further, the expansion of the frequency band can be suppressed. Therefore, it is possible to perform sufficient phase compensation,
Circuit oscillation can be effectively prevented.

Although the collector terminal of the transistor Q2 is connected to the output terminal of the differential amplifier A2 in this example, the present invention is not limited to this. Transistor Q2
Output terminal has sufficiently low impedance and PN
What is necessary is just to connect to the terminal which supplies DC voltage so that P transistor Q2 can operate without being saturated.

FIG. 4 shows a feedback amplifier circuit according to the third embodiment. This feedback amplification circuit generally has a configuration in which N sets (N is a natural number) of the transistor Q1, the feedback resistor R3, and the feedback capacitance C2 of FIG. 1 are provided. That is, this feedback amplifier circuit includes a differential amplifier A001 and a feedback circuit F001.
001. When N is a natural number, the feedback circuit F001 has (N + 1) first feedback resistors R101,..., R10N connected in series between the output terminal and the input terminal of the differential amplifier A001. R001) and a first feedback capacitor C001 connected between the output terminal and the input terminal. Also, NPN transistors Q001,..., Whose base terminals are connected to connection points P1,.
QN, N second feedback resistors R002,..., RN + 1 connected between the emitter terminal of each of the transistors and the input terminal, respectively, between the emitter terminal of each of the transistors and the input terminal. N connected each
, CN + 1. The collector terminals of the transistors Q001,..., QN are all connected to the output terminal of the differential amplifier A001.

This feedback amplifier circuit has a second circuit connected to the emitter terminal of each transistor when the voltage at points P1,..., PN exceeds the threshold voltage between the base and emitter of each transistor Q001,. A current flows through the feedback resistor and the second feedback capacitance. As a result, similarly to the first and second embodiments, the amplification factor of the circuit can be controlled, and furthermore, the expansion of the frequency band can be suppressed. Therefore,
Sufficient phase compensation can be performed, and oscillation of the circuit can be effectively prevented.

[0035]

As is apparent from the above description, in the feedback amplifying circuit according to the first aspect, the direct current component of the input signal is increased and the connection point between the first feedback resistors is increased as in the conventional example. Is higher than the threshold voltage (base-emitter voltage) of the transistor, the impedance of the feedback circuit can be reduced to control the amplification factor of the circuit. Moreover, the expansion of the frequency band can be suppressed by the function of the second feedback capacitance. Therefore, sufficient phase compensation can be performed, and oscillation of the circuit can be effectively prevented.

Further, in the feedback amplifier circuit according to the second aspect, a time constant consisting of a product of a total value of the first feedback resistance and a value of the first feedback capacitance, a value of the second feedback resistance and a value of the second feedback resistance. Since the time constant, which is the product of the feedback capacitance value and the value of the feedback capacitance of 2, is set to be substantially equal, the cutoff frequency differs depending on whether the DC component of the input signal does not exceed the threshold value of the transistor. Become approximately equal. Therefore, more sufficient phase compensation can be performed, and oscillation of the circuit can be effectively prevented.

Further, in the feedback amplifying circuit according to the third aspect, when the voltage at the N connection points between the first feedback resistors exceeds the threshold voltage between the base and emitter of each transistor, A current flows through the second feedback resistor and the second feedback capacitance connected to the emitter terminal of each transistor. As a result, the gain of the circuit can be controlled as in the case of the feedback amplifier circuit of the first or second aspect. Moreover, the spread of the frequency band can be suppressed. Therefore, sufficient phase compensation can be performed, and oscillation of the circuit can be effectively prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a feedback amplifier circuit according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a feedback amplifier circuit according to a second embodiment of the present invention.

FIG. 3 is a diagram showing characteristics of the feedback amplifier circuit of the first embodiment.

FIG. 4 is a circuit diagram showing a feedback amplifier circuit according to a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing a conventional feedback amplifier circuit.

FIG. 6 is a diagram showing characteristics of the conventional feedback amplifier circuit.

[Explanation of symbols]

A1, A2, A001 Differential amplifier C1, C3, C001 First feedback capacitance C2, C4, C002, ..., CN + 1 Second feedback capacitance F1, F2, F001 Feedback circuit Q1, Q002, ..., QN NPN transistor Q2 PNP Transistors R1, R2, R4, R5, R101, ..., R10N First feedback resistors R3, R6, R002, ..., RN + 1 Second feedback resistors

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03F 1/00-3/45

Claims (3)

(57) [Claims] 1. An amplifier for amplifying a signal received at an input terminal and outputting the amplified signal to an output terminal, and a feedback circuit for feeding back the output of the amplifier to the input terminal so that the amplifier exhibits a predetermined amplification factor. , The feedback circuit includes two first feedback resistors connected in series between the output terminal and the input terminal of the amplifier, and a first feedback resistor connected between the output terminal and the input terminal. A transistor having a feedback capacitor, a base terminal connected to a connection point between the first feedback resistor, and a collector terminal connected to a power supply or the output terminal; an emitter terminal of the transistor and the input terminal of the amplifier; A feedback amplifier circuit having a second feedback resistor connected between the input terminal and a second feedback resistor, wherein a second feedback capacitor is connected between the emitter terminal of the transistor and the input terminal; Instead amplifier circuit. 2. The first feedback resistor according to claim 1, wherein
And a time constant consisting of a product of the value of the second feedback resistor and the value of the second feedback capacitance is set to be substantially equal. The feedback amplifier circuit according to claim 1, wherein 3. An amplifier for amplifying a signal received at an input terminal and outputting the amplified signal to an output terminal, and a feedback circuit for feeding back the output of the amplifier to the input terminal so that the amplifier exhibits a predetermined amplification factor. The feedback circuit includes: (N + 1) first feedback resistors connected in series between the output terminal and the input terminal of the amplifier, where N is a natural number; A first feedback capacitor connected between the first feedback resistor and a base terminal connected to a connection point between the first feedback resistors, and a collector terminal connected to a power supply or the output terminal. N second feedback resistors respectively connected between the emitter terminal of each of the transistors and the input terminal; and N second feedback resistors respectively connected between the emitter terminal of each of the transistors and the input terminal And a second feedback capacitor.