JPH01125107A - Amplifier circuit - Google Patents

Abstract

PURPOSE:To realize an amplifier circuit whose midpoint voltage at its output terminal varies with a change in a power voltage by constituting the amplifier circuit by a differential amplifier circuit, an amplifier circuit, two feedback resistors and a current mirror circuit so as to eliminate a negative feedback capacitor. CONSTITUTION:The amplifier circuit is made up of a differential amplifier circuit 11 comprising two transistors(TR) 12, 13, amplifier circuits 19, 20 amplifying an output signal of the differential amplifier circuit 11, a 1st feedback resistor 22 connecting between its output and a 2nd TR 13, a 2nd feedback resistor 23 connecting between a base and a reference potential point of the 2nd TR 13, a constant current source 24 supplying a constant current to the 2nd feedback resistor and a current mirror circuit 25. Thus, a DC voltage equal to that fed to a base of a 1st TR 12 is applied to a base of the 2nd TR 13 and no feedback capacitor is required. Then the midpoint voltage at the output terminal is set to a half the power voltage by using an optional power voltage independently of the bias voltage of the 1st TR 12.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to an amplifier circuit that can eliminate a negative feedback capacitor. This invention relates to an amplifier circuit that can change the .

(b) Prior Art Generally, when a negative feedback type amplifier circuit is constructed using an amplifier circuit having a relatively low bare gain, the circuit is stabilized by feeding back 100% of the DC component. Figure 2 is a circuit diagram showing such a negative feedback type amplifier circuit, where the input terminal (1)
, an amplifying section (4) having a negative feedback terminal (2) and an output terminal (3), and a signal source (5) connected to the input terminal (1).
), a first resistor (6) connected between the output terminal (3) and the negative feedback terminal (2), and a first resistor (6) connected between the output terminal (3) and the negative feedback terminal (2);
) and the ground, the second resistor (7) and the feedback capacitor (8) are connected in series. Second
In the case of the figure, the DC feedback is 100Ω via the first resistor (6).
%. Further, the AC feedback is set to a desired value by the ratio of the first resistor (6) and the second resistor (7).

Therefore, by using the circuit shown in FIG. 2, it is possible to obtain an amplifier circuit that stably has a desired gain. The amplifier circuit shown in FIG. 2 is described on page 343 of "85 Sanyo Semiconductor Handbook" published by CQ Publishing on March 20, 1985.

(c) Problems to be solved by the invention However, in the case of the circuit shown in Figure 2, a feedback capacitor (8〉) is required to make the feedback rates of DC and AC different, so it is not possible to miniaturize the equipment. This was a problem, especially when implementing an IC (integrated circuit), which required external components and the number of pins.

there were. Furthermore, when the feedback capacitor (8) is provided, there is a problem in that the voltage gain decreases when the frequency of the input signal is low. In order to raise the low frequency range, the capacitance of the feedback capacitor (8) may be increased. However, doing so would require a large capacitor.

(d) Means for Solving Problems The present invention has been made in view of the above points, and includes a first transistor to which an input signal is applied to the base, and an emitter of the first transistor that is commonly connected. a differential amplifier circuit comprising a second transistor, an amplifier circuit that amplifies the output signal of the differential amplifier circuit, an output terminal from which the output signal of the amplifier circuit is obtained, and a circuit between the output terminal and the second transistor; a first feedback resistor connected between the base of the second transistor, a second feedback resistor connected between the base of the second transistor and a reference potential point, and a constant current that supplies a constant current to the second feedback resistor. and a current mirror circuit whose input is connected to the power supply via a resistor and whose output is connected to the base of the second transistor.

(f) Function According to the present invention, a constant current is supplied to the second resistor by a constant current source, and the upper end of the second resistor is connected to the base of the second transistor. A DC voltage equal to the DC voltage applied to the base of the first transistor is now applied to the base of the first transistor, making a feedback capacitor unnecessary.

Furthermore, according to the present invention, since a current mirror circuit is arranged in which the input is connected to the power supply and the output is connected to the base of the second transistor, the output can be made at any power supply voltage regardless of the bias voltage of the first transistor. The midpoint voltage of the terminal can be set to the voltage of the power supply.

Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (9) is an input signal source, (10) is a DC bias source in the form of a battery, and (11) is a circuit diagram showing an embodiment of the present invention. A first transistor (12) to which an input signal from the input signal source (9) is applied to the base, a second transistor (13) whose emitters of the first transistor (12) are commonly connected, and a constant current source. (14
), a diode-connected third transistor (16) having a collector connected to the collector of the second transistor (13), and a third transistor (16) having a collector connected to the collector of the first transistor (12); a first current mirror circuit consisting of a fourth transistor (17) connected to the third transistor (16) and 'W!, and a fourth transistor (17) connected in a current mirror relationship; Drive transistors (19> and (20) connected to the collector of the drive transistor (18) push-pull amplify the output signal of the drive transistor (18) and output the output terminal (
the first and second output transistors occurring in (21);
2) is the first feedback resistor connected between the output terminal (21) and the base of the second transistor (13), and (23) is connected between the base of the second transistor (13) and ground. The second feedback resistor (24) is the second feedback resistor (24).
3) a constant current source connected to one end, and (25) a diode-connected fifth transistor (28) whose collector is connected to the power supply terminal (27) via a third resistor (26).
and a collector connected to the base of the second transistor (13), and the fifth transistor (28)
and a sixth transistor (29
) is the second current mirror circuit.

Next, the operation will be explained. Since the resistance value R8 of the first feedback resistor (22) is small, the DC voltage at the base of the second transistor (13) is constant between the constant current value of the current source (24) and the second feedback resistor (
23) is determined by the resistance value R of the DC via power supply (1
0) is set equal to the output DC voltage.

Therefore, when there is no signal, the collector currents of the first and second transistors (12) and (13) are equal. On the other hand, the collector current of the fourth transistor (17) is
The base current (2im) is smaller than the collector current of the third transistor (16). Therefore, a current corresponding to the base current is supplied from the base of the drive transistor (18), and a current obtained by amplifying the base current flows through the drive transistor (18). Then, the current flows through the first and second output transistors 19) and (2).
0), and the amplified output signal is sent to the output terminal (
21) occurs.

Then, the output signal is transmitted to the first feedback resistor (22) and the second feedback resistor (22).
The feedback resistor (23) performs a feedback operation by feeding back to the base of the second transistor (13).

By the way, in general, in a circuit as shown in FIG. 1, the voltage of the DC bias source (10) is set at a level equal to the power supply voltage in order to obtain the maximum dynamic range.

Then, depending on the feedback operation of the amplifier circuit, the output terminal (21)
The midpoint voltage and the voltage at point A can be fixed at Vcc/2,
It becomes unrelated to the power supply voltage. Therefore, even if the power supply voltage is changed, the midpoint voltage of the output terminal (21) cannot be changed, resulting in a problem that the dynamic range of the output signal is significantly narrowed.

Therefore, in this application, in order to ensure the dynamic range of the output signal in the negative feedback type amplifier circuit as described above, an output midpoint voltage changing circuit consisting of a third resistor (26) and a second current mirror circuit (sub) is provided. The output midpoint voltage is set to the optimum value in response to changes in the power supply voltage.

Next, we will explain the operation for setting the output midpoint voltage to the optimum value.The voltage at point A is set to be equal to the voltage of the DC bias source (10) as described above. For example, the voltage of (10) is V! If it is 1c,
The voltage at point A also becomes VOC. On the other hand, the current flowing through the sixth transistor (29) of the second current mirror circuit (sub) flows through the first feedback resistor (22), so that the current is reduced to 1
1. If the resistance value of the first feedback resistor (22) is R1, then the DC voltage VODC at the output terminal (21) is V. =Vo
c+i+R+・・・・・・・・・・・・
...(1). Here, the voltage v in equation (1)
I). Since the resistance value R1 is a constant value, the current i,
DC voltage V at the output terminal (21) depending on the value of . DC can be adjusted. The current iI is a fifth transistor (28)
The current i is equal to the current flowing through i+-(Vcc V+t)/Rs:Vcc/Rm
......(2). Expression (2) is converted into equation (1)
), the DC voltage VODC becomes. Equation (3) expresses the DC voltage V using the power supply voltage VCC and the voltage vDc. . . is determined. Therefore, regardless of the value of the DC voltage of the DC bias source (10), the DC voltage V. DC can be set to Vcc/2. For example, if power supply voltage Vcc>voltage vDc, equation (3) becomes, and DC voltage V(ID(:) is proportional to power supply voltage VCC. Therefore, the ratio of resistor R3 to resistor R8 is 1: 2, the DC voltage V.De becomes Vce/2.

Further, if the voltage VDe cannot be ignored with respect to the power supply voltage vec, the ratio between the resistors RI and R2 may be set appropriately. For example, the power supply voltage Vcc is 15 V, the voltage Voc is 2 V, and the resistor R is 300Ω,
If the voltage Vat is -0.65V, the current 11 flowing through the sixth transistor (29) is as follows, and the current flowing through the fifth transistor (28) is as follows. Therefore, by setting equation (5) - equation (6), the resistance R3 is R*=7B4 Ω ・・・・・・・・・・・・
......(7) Therefore, the resistance R,
By setting the ratio of the resistor R1 and the resistor R1 to approximately 1:2.61, it is possible to obtain an output midpoint voltage of VeC/2, that is, 7.5V.

In the case of the circuit shown in FIG. 1, the DC feedback rate is equal to the AC feedback rate. If you want to obtain a high gain, you can set the AC feedback rate (R*/(Rt + Rt)) to a low value, and if you do so, the DC feedback rate will also decrease.
If the gains of the differential amplifier circuit (11), drive transistor (18), first and second transistors (19) and (20) are set high so that the total gain is about 80 dB, it will be able to withstand slight changes in DC voltage. Even if it is, it will be able to follow it well.

(g) Effects of the Invention As described above, according to the present invention, it is possible to provide a negative feedback type amplifier circuit in which a negative feedback capacitor is omitted. Further, according to the present invention, the 'wL current flowing through the first feedback resistor is changed in accordance with a change in the power supply voltage, and the DC voltage at the output terminal is approximately reduced to Va.
Since it is maintained at c/2, the maximum dynamic range can be obtained with any power supply voltage, regardless of the bias voltage of the first transistor to which the input signal can be applied.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing an embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
FIG. 1 is a circuit diagram showing a conventional negative feedback type amplifier circuit. (10)...Input signal source, (11)...Differential amplifier circuit, (12)...First transistor, (13)...
・Second transistor, (21)...output terminal, (
22)...First feedback resistor, (23)...Second feedback resistor, (24)...Constant current source, (translation)...Second current mirror circuit, (26)...Second current mirror circuit 3 resistance.

Claims (1)

[Claims] (1) A differential amplifier circuit comprising a first transistor to which an input signal is applied to the base and a second transistor to which the emitters of the first transistor and the emitter are commonly connected;
an amplifier circuit that amplifies the output signal of the differential amplifier circuit; an output terminal from which the output signal of the amplifier circuit is obtained; and a first transistor connected between the output terminal and the base of the second transistor.
a feedback resistor; a second feedback resistor connected between the base of the second transistor and a reference potential point; a constant current source that supplies a constant current to the second feedback resistor; and a current mirror circuit whose output is connected to the base of the second transistor, and when the power supply voltage of the power supply is changed, the current flowing through the current mirror circuit is changed, and the midpoint voltage of the output terminal is changed. An amplifier circuit characterized in that the amplifier circuit can be changed.