1. Approximate Estimation under Deep Negative Feedback Conditions
That is to say, as long as F is calculated, the value of A F is also determined.
Example 1 (P296 Example 7.4.1)
The circuit is shown in Figure 7.13. Try to approximately calculate the voltage gain of the amplifier circuit and qualitatively analyze the input and output resistance of the amplifier circuit.
Under deep negative feedback conditions, the gain of the amplifier circuit is:
According to the characteristics of voltage feedback, the output resistance R if of the feedback amplifier is much smaller than the output resistance r o of the operational amplifier itself .
2. Application of the concepts of false shortness and false interruption
Now let us use the concepts of virtual short and virtual break to re-solve Example 1.
The circuit of Example 2 is shown in Figure 7.14. It is a transistor emitter bias circuit. Its AC path is shown in Figure 7.15. Try to approximate the voltage gain of the circuit.
Comparing the above formula with the calculation results in Chapter 3 
, it can be seen that when the circuit is in deep negative feedback, for example, when the β value is very large, 
it can be simplified 
to its physical meaning is obvious.
According to the characteristics of current series negative feedback, the input resistance of the circuit shown in Figure 7.14 (Figure 7.15) is greatly increased; and due to the effect of current feedback, the constant current characteristic of BJT is improved, so its output resistance is greatly increased, but when it is used as a voltage amplifier circuit, its actual output resistance R of ≈R c .
Current series negative feedback circuits are generally used as voltage-current converters. Due to its high input resistance, it can sometimes be used as the input stage of a multi-stage amplifier circuit.
Example 3 The circuit is shown in Figure 7.16. Please determine the feedback configuration in the circuit and derive the voltage gain expression. Assume that the operational amplifier is ideal.
Solution (1) Using the instantaneous polarity method, we know that the feedback polarity of the circuit is negative feedback; As shown in the figure, op amps A1 and A2 form a basic amplifier circuit, the input voltage is Vi , and the output voltage is V O , which is a voltage amplifier circuit. The feedback network is composed of a voltage divider (R3 , R4 ) , a common-mode amplifier circuit composed of op amp A3 and (R5 , R6 ) , and a second voltage divider circuit composed of ( R7 , R8 ) , which is an active feedback network.
Solution (2) Find the closed-loop gain 
: As shown in the question, each op amp is ideal, and the circuit composed of op amps is in a state of deep negative feedback, and the inter-stage feedback is also in a state of deep negative feedback; according to the concepts of virtual short and virtual short:
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