Negative Feedback Amplifier

In the basic theory of transistor amplifiers, it has been discussed that DC negative feedback can stabilize the static operating point and make the amplifier work stably in the linear region. In addition, the introduction of AC negative feedback in the amplifier circuit can also improve the stability of the gain, reduce nonlinear distortion, widen the bandwidth, and change the input impedance and output impedance as needed, so that the amplification performance is greatly improved. Therefore, although negative feedback will reduce the gain, it is almost always used in actual amplifier circuits.

1. Block diagram, basic feedback equation

There are many types of negative feedback circuits, but according to the different sampling methods (voltage or current) of the feedback network from the basic amplifier circuit output, they can be divided into voltage feedback and current feedback; and according to the different summing methods of the feedback signal back to the input end, they can be divided into series feedback and associated feedback. In summary, negative feedback amplifiers are divided into four types, as shown in Figure 5.2-8, and Table 5.2-8 shows their basic feedback equations.

Figure 5.2-8 Block diagram of four types of negative feedback amplification

A Voltage parallel negative feedback B Current series negative feedback C Voltage series negative feedback D Current related negative feedback

The basic relationship between the closed-loop gain A1, the closed-loop gain A and the feedback coefficient B of the negative feedback amplifier is called the basic relationship and the basic feedback equation.

Feedback depth is an important physical quantity that reflects the strength of feedback. The larger its value, the stronger the negative feedback. When the feedback is very deep, that is, |AB|>1, it is called deep negative feedback, then the closed-loop gain

2. The impact of negative feedback on amplifier performance

Negative feedback amplifier circuits can improve many performances at the cost of reducing gain. Table 5.2-9 shows the effect of negative feedback on input resistance and output resistance; Table 5.2-10 shows the effect of negative feedback on several other main performances of the amplifier; Table 5.2-10 shows the effect of negative feedback on several other main performances of the amplifier.

3. Emitter follower

The emitter follower is a typical single-harmonic voltage series negative feedback amplifier circuit, and its circuit diagram is shown in Figure 5.2-9.

Figure 5.2-9 Emitter follower circuit

(1) Emitter follower characteristics

1) The voltage gain is less than 1, usually very close to 1, and positive.

2) The input resistance is high and can reach tens of kilo-ohms.

Where H10 is the input internal resistance of the transistor.

3) The output resistance is small, which can be as small as tens of ohms. When the internal resistance of the signal source is considered, the output resistance is

4) The frequency band follower is a 100% voltage negative feedback circuit. For the frequency characteristics of the tube itself, the feedback has the effect of widening the frequency band. It is through the automatic adjustment of negative feedback that the output voltage decreases slower and smaller as the frequency increases, thus widening the frequency band. Analysis shows that negative feedback increases the upper limit frequency by one feedback depth. As shown in Figure 5.2-8, its upper limit frequency

Where CO is the distributed capacitance and load capacitance.

If the conditions are met

The upper frequency

(2) Emitter follower practical circuit

1) Compound tube emitter follower Figure 5.2-10 shows an actual circuit of a compound tube emitter follower. This circuit is a high-power amplifier. The first tube uses a low-power switch tube 3AK20C as the driver stage, and the second tube uses a high-power tube 3AA12C. The above is a compound tube composed of two tubes of the same type. In practice, a compound tube can also be composed of tubes of different shapes. The use of compound tubes is mainly to increase the equivalent B. The use of compound tubes in emitter followers helps to increase input resistance and also helps to reduce output resistance. The compound tube circuit is also called a "Darlington" circuit.

Figure 5.2-10 Compound tube follower

2) Bootstrap follower The bootstrap circuit is an effective method to increase the equivalent input resistance of the bias circuit. Figure 5.2-11 shows a bootstrap emitter follower. The bootstrap circuit is used to increase the input resistance of the emitter follower. The principle is that the potential at the lower end of RB8 increases as the potential at the upper end increases, so that the AC voltage drop across RB8 is zero, that is, RBA is equivalent to an open circuit for AC, thereby avoiding the reduction of input resistance due to the shunting effect of the bias circuit.

Figure 5.2-11 Bootstrap emitter follower

3) Complementary follower Figure 5.2-12 shows an improved complementary follower circuit, which is equivalent to a composite tube circuit composed of two pairs of NPN and PNP tubes. Its characteristics are that there will be no cross-distortion due to mutual compensation, the input resistance is very high, and the equivalent B is very large, so that the circuit gain is very close to 1. Its typical application is the output stage of the holding amplifier of the high-speed sample-and-hold circuit.