JP3076160B2 - 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 an amplifier circuit, and more particularly to a capacitor-coupled input type feedback amplifier circuit.

[0002]

2. Description of the Related Art A conventional capacitor-coupled input type feedback amplifier circuit is shown in FIG. In FIG. 4, one end of an input coupling capacitor 2 is connected to an input terminal 1,
Is connected to the input of the feedback amplifier 100 and one end of the input bias resistor 3. The other end of the input bias resistor 3
The reference voltage terminal 4 is connected, and the output of the feedback amplifier 100 is connected to the output terminal 5.

FIG. 5 shows a feedback amplifier 100 shown in FIG.
FIG. 11 is a circuit diagram showing a conventional capacitor-coupled input feedback amplifier circuit using a non-inverting amplifier.

In FIG. 5, one end of an input coupling capacitor 2 is connected to an input terminal 1, and the other end of the input coupling capacitor 2 is connected to a differential amplifier 101 as an input of a feedback amplifier 100.
The output of the differential amplifier 101 is connected to the output terminal 5 and one end of a feedback resistor 104, and the other end of the feedback resistor 104 is connected to the inverting input (−) of the differential amplifier 101. ) And one end of the feedback resistor 105. The other end of the input bias resistor 3 and the other end of the feedback resistor 105 are connected to the reference voltage terminal 4 together with the high potential side of the reference voltage source 6, and the low potential side of the reference voltage source 6 is connected to the ground potential. The internal circuit of the differential amplifier 101 is configured such that the base of the NPN transistor 102 is connected to the non-inverting input, the base of the NPN transistor 103 is connected to the inverting input, and the NPN transistor 1
The emitters 02 and 103 are commonly connected.

Next, the voltage gain (Gv) of this amplifier circuit will be described. Here, it is assumed that the open-circuit voltage gain of the differential amplifier 101 is infinite, and the input impedance of the non-inverting input / inverting input is sufficiently larger than the resistance value of the input bias resistor 3 and can be ignored in the calculation. Therefore, the following equations (1), (2), and (3) are obtained.

[0006]

Here, α is the amount of attenuation of the low-frequency cutoff filter by the input coupling capacitor 2 and the input bias resistor 3, and G
VA is the voltage gain of the feedback amplifier 100, ω is 2πf, f is the signal frequency, CIN is the capacitance value of the input coupling capacitor 2,
RIN is the resistance value of the input bias resistor 3, RNF1 is the resistance value of the feedback resistor 104, and RNF2 is the resistance value of the feedback resistor 105.

The output offset voltage (ΔVO) due to the input bias current of the differential amplifier 101 of this amplifier circuit.
Is determined by the following equation.

[0009]

Here, IBQ102 is the base current of the NPN transistor 102, IBQ103 is the base current of the NPN transistor 103, RNF1 // RNF2 is the following, where the resistance of the feedback resistor 104 is RNF1 and the resistance of the feedback resistor 105 is RNF2. Parallel resistance value. From the above (4), to minimize the output offset voltage ΔVO, the following equation (5) is used.
What should be done is.

RIN = RNF1 // RNF2 (5)

Since this conventional amplifier circuit is a feedback amplifier circuit with a capacitor-coupled input, it is essential to provide an input bias resistor for biasing the non-inverting input terminal of the differential amplifier. A low-pass cutoff filter is formed by the input bias resistor and the input coupling capacitor. Therefore, when handling low-frequency signals in this amplifier circuit, the value of the input coupling capacitor and the input bias resistor must be increased in order to suppress the amount of attenuation in the low-frequency cutoff filter. In order to set the value to 0, the value of the feedback resistor needs to be large in accordance with the input bias resistance. For this reason,
There is a disadvantage that it is not suitable for integration into an integrated circuit.

As an example, when the signal frequency is 10 kHz, the amount of attenuation in the low-frequency cutoff filter by the input coupling capacitor and the input bias resistor is suppressed to within 1 dB.
From the above equation (2), if CIN = 50 μF, RIN
= 626 kΩ is required.

Further, in order to use this amplifier circuit with a voltage gain of 10 times and to make the output offset voltage zero,
From the above equations (3) and (5), CN = 50 pF, RIN =
When 626 kΩ, RNF1 = 6.26 MΩ, RNF2 =
696 kΩ is required.

SUMMARY OF THE INVENTION An object of the present invention is to provide an amplifier circuit which solves the above-mentioned disadvantages and does not need to increase the capacitance value of a capacitor to be used and the resistance values of an input resistor and a feedback resistor.

[0015]

According to the structure of the amplifier circuit of the present invention, the base of the first transistor is connected to the input terminal via a capacitor, and the input of the feedback amplifier is connected to the emitter of the first transistor. An input bias resistor for providing an input bias of the feedback amplifier and a current output terminal of the first constant current source are connected, and a base of the first transistor is a current mirror for further supplying a base current of the first transistor. An output of the circuit is connected, an input of the current mirror circuit is connected to a base of an NPN-type or PNP-type second transistor of the same type as the first transistor, and an emitter of the second transistor is connected to an emitter of the second transistor. The current output terminal of the second constant current source is connected, the other end of the input bias resistor is connected to a reference voltage terminal, and the output of the feedback amplifier is It is connected to the force terminal, further the feedback
A first feedback between the inverting input terminal of the amplifier and its output
A resistor is connected, and the inverting input terminal of the feedback amplifier and the
Second feedback resistor, characterized in Rukoto connected between a reference voltage terminal.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an amplifier circuit according to one embodiment of the present invention. In FIG. 1, in the present embodiment, the impedance of the input bias resistor 3 as viewed from the input coupling capacitor 2 is h.
Using an NPN transistor 104 for multiplying by FE,
Accordingly, the transistor 201 included in the base current supply circuit 200 of the transistor 401 also becomes an NPN type.

One end of an input coupling capacitor 2 is connected to the input terminal 1, the other end of the input coupling capacitor 2 is connected to the base of an NPN transistor 401, and the emitter of the NPN transistor 401 is connected to the input of the feedback amplifier 100 and the feedback. One end of the input bias resistor 3 of the amplifier 100 and the constant current source 4
02, the other end of the input bias resistor 3 is connected to the reference voltage terminal 4, and the output of the feedback amplifier 100 is
Connect to the output terminals 5 respectively. Also, as the base current supply circuit 200 of the NPN transistor 401,
The base of the transistor 201 is connected to the current mirror circuit 3
00, and the output of the current mirror circuit 300 is connected to the base of the NPN transistor 401. N
The constant current source 20 is connected to the emitter of the PN transistor 201.
2 current output terminals.

Next, FIG. 2 is a circuit diagram showing a first example of an amplifier circuit when a non-inverting amplifier is used as the feedback amplifier 100 of FIG. 2, a feedback amplifier 100 has a non-inverting input (+) of a differential amplifier 101 connected to an input, an output of the differential amplifier 101 connected to an output terminal 5 and one end of a feedback resistor 104, and The other end is a differential amplifier 10
The other end of the feedback resistor 105 is connected to the reference voltage terminal 4 together with one end of the input bias resistor 3 and the high potential side of the reference voltage source 6. The low potential side of the reference voltage source 6 is connected to the ground potential.

The collectors of the NPN transistors 201 and 401 are both connected to the higher power supply terminal 7. The current mirror circuit 300 includes PNP transistors 301 and 30
2 is connected to the high-side power supply terminal 7, and the PNP transistor 301 short-circuits the collector and base.
The emitter of the P transistor 303 is connected to the base of the PNP transistor 302, the collector of the PNP transistor 302 is connected to the emitter of the PNP transistor 304, and the collector and base of the PNP transistor 304 are short-circuited so that the base of the NPN transistor 201 is connected to the P
Connected to the base of NP transistor 303, the collector of PNP transistor 303 is NPN transistor 401
Connect to the base. Other connection relationships are the same as those in FIG.

Next, the operation of the amplifier circuit will be described. First, in the input impedance RIN ′ of the amplifier circuit as viewed from the side of the input coupling capacitor 2 to which the base of the NPN transistor 401 is connected, the output impedance of the current mirror circuit 300 is equal to the output of the collector of the common emitter PNP transistor 303. Therefore, the impedance is very high and can be ignored with respect to the impedance when the input bias resistor 3 is viewed from the base of the NPN transistor 401.

Therefore, the amount of attenuation α in the low-frequency cutoff filter by the input coupling capacitor 2 and the input bias resistor 3 is:
The following equation (6) is obtained.

[0022]

Re401 is an NPN transistor 4
01, output impedance from the emitter side, hFE
401 is the current amplification factor of the NPN transistor 401, k is the Boltzmann constant, T is the absolute temperature, q is the charge of electrons, I
402 is a current value of the constant current source 402, and ω is 2πf.

From the above equation (7), when compared with the circuit of the prior art, at the same signal frequency f and the same input coupling capacitor capacitance CIN, the value of the input bias resistor RIN becomes
It has the following values:

RIN = RIN ″ / hFE401 + re401 (9) Here, RIN ″ is the resistance value of the input bias resistor in the circuit of the conventional example, and this value is considerably smaller than that of the conventional example.

FIG. 3 is a circuit diagram showing a second example of the amplifier circuit of the embodiment of FIG. In FIG. 3, the main difference between the second example of the present embodiment and the first example is that the PNP transistor is replaced with an NPN transistor and the NPN transistor is replaced with a PNP transistor.

In FIG. 3, a second transistor in the case where a PNP transistor is used as a transistor for multiplying the impedance of the input bias resistor 3 from the input coupling capacitor 2 by hFE.
Is shown. One end of an input coupling capacitor 2 is connected to the input terminal 1, and the other end of the input coupling capacitor 2 is
The base of the PNP transistor 403 is connected, and the emitter of the PNP transistor 403 is connected to the input of the feedback amplifier 100, one end of the input bias resistor 3 of the feedback amplifier 100, and the current output terminal of the constant current source 404, as in FIG. And
The other end of the input bias resistor 3 is connected to a reference voltage terminal 4, and the output of the feedback amplifier 100 is connected to an output terminal 5.

The PNP transistor 203 serves as a base current supply circuit 200 for the PNP transistor 403.
Is connected to the input of the current mirror circuit 300,
The output of the current mirror circuit 300 is connected to the base of the PNP transistor 403. PNP transistor 20
The current output terminal of the constant current source 204 is connected to the third emitter. The current mirror circuit 300 includes NPN transistors 305, 306, 307, 308.
Collectors of PNP transistors 203 and 403 and NP
The emitters of the N transistors 307 and 308 are connected to the lower power supply terminal 8.

The operation of the second example and the amount of attenuation α in the low-pass cutoff filter by the input coupling capacitor 2 and the input bias resistor 3 are the same as in the first example of FIG.

[0030]

As described above, the present invention is particularly applicable to a capacitor-coupled input-type feedback amplifier circuit in which the resistance between the input bias capacitor and the input bias resistor is hFE times the value of the input bias resistor between the input coupling capacitor and the input bias resistor. And the current mirror circuit that supplies the base current of the transistor with high output impedance, the capacitance value of the input coupling capacitor and the resistance values of the input bias resistance and the feedback resistance can be reduced, and the integrated circuit This has the effect of making the circuit suitable for

As an example, similar to the conventional example, when the attenuation in the low-frequency cutoff filter due to the input coupling capacitor and the input bias resistor is suppressed to within 1 dB at the signal frequency of 10 kHz, the above-mentioned (6) and (7) From equation (8), FIG.
Assuming that hFE of the NPN transistor 401 is 100 and the output current value of the constant current source 402 is 10 μA, CIN = 2
0 pF and RIN = 13.0 kΩ. Further, when the feedback resistance is determined by setting the voltage gain to 10 times, from the above equation (5), RNF1 = 130 kΩ and RNF2 = 14.5 kΩ.
These CIN, RIN, RNF1 and RNF2 can be made much smaller than in the conventional example, and all of them are values that do not pose a problem for integration into an integrated circuit.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing a first example of the amplifier circuit shown in FIG.

FIG. 3 is a circuit diagram showing a second example of the amplifier circuit shown in FIG. 1;

FIG. 4 is a block diagram of a conventional capacitor-coupled input type feedback amplifier.

FIG. 5 is a circuit diagram showing a specific circuit of the conventional circuit shown in FIG.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 input terminal 2 input coupling capacitor 3 input bias resistor 4 reference power supply terminal 5 output terminal 6 reference voltage source 7 high power supply terminal 8 low power supply terminal 100 feedback amplifier 101 differential amplifier 102, 103, 201, 401, 305 to 308
NPN transistor 104, 105 Feedback resistor 200 Base current supply circuit 202, 204, 402, 404 Constant current source 300 Current mirror circuit 301-304, 203, 403 PNP transistor

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-54809 (JP, A) JP-A-61-288605 (JP, A) JP-A-59-211305 (JP, A) 121959 (JP, A) JP-A-6-139794 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03F 1/34 H03F 3/45

Claims (3)

(57) [Claims] An input terminal is connected to a base of a first transistor via a capacitor, and an input bias resistor for providing an input of a feedback amplifier and an input bias of the feedback amplifier to an emitter of the first transistor, and a first transistor. Is connected to a current output terminal of a constant current source, and a base of the first transistor is further connected to an output of a current mirror circuit that supplies a base current of the first transistor;
The input of the current mirror circuit is connected to the base of a second transistor of the same type as the first transistor, of NPN type or PNP type, and the emitter of the second transistor has the emitter of a second constant current source. A current output terminal is connected; the other end of the input bias resistor is connected to a reference voltage terminal; an output of the feedback amplifier is connected to an output terminal ;
Between the inverting input terminal of the feedback amplifier and its output terminal.
Is connected to a first feedback resistor, and the inverting input of the feedback amplifier is connected to the first feedback resistor.
A second feedback resistor is connected between the input terminal and the reference voltage terminal.
Amplifier circuit according to claim Rukoto is continued. 2. The amplifying circuit according to claim 1, wherein the current mirror circuit includes four NPN or PNP transistors of the opposite type to the first and second transistors. 3. The constant current source according to claim 1, wherein the first and second transistors have the same current amplification factor, and the first and second constant current sources have the same constant current value. Amplifier circuit.