Analog electronic circuit fundamentals-negative feedback

First, what is feedback

The voltage or current at the output of the circuit will be amplified and returned to the input end of the amplifier in a certain way, which is called feedback

Second, the general relationship of negative feedback

xf=a/(1+AF)

Iii. How to determine the type of feedback

Assuming the load short-circuit, if the feedback disappears, is the voltage feedback , otherwise is the current feedback .
Suppose the input short circuit, if the feedback disappears, is the parallel feedback , otherwise is the series feedback .  

Four, 4 kinds of negative feedback examples

4.1 Voltage Series

The amplifier input is supposed to be shorted (ui=0), and the voltage R4 from the UO can still work on the amplifier input, i.e. the feedback does not go away, so it is a series of feedback. The load is shorted, then the R4 voltage becomes 0 and feedback disappears, so it is voltage feedback.

4.2 Current Series

the load is assumed to be shorted, Io is still flowing, feedback is still present, so it is current feedback. The amplifier input end of the hypothetical short circuit, the voltage generated by IO on the Re1 can still affect the transistor be knot, feedback does not disappear, it is a series of feedback.

4.3 current shunt

the output end of the hypothetical short circuit, the output current flow, after R3 and R5 shunt, the current on the R3 to the amplifier input, it is the current feedback; the input is supposed to short-circuit, R3 the left side of the ground, the feedback function disappears, it is the parallel feedback.

4.4 Voltage Shunt

The output end of the hypothetical short circuit, the transmission of the feedback signal R1 the right end of the ground, the feedback function disappears, it is the voltage feedback; The input is supposed to be short-circuit, the feedback signal transmitted by R1 is shorted, the feedback function disappears, so it is the parallel feedback.

influence of negative feedback on performance of amplifiers

5.1 Improved the stability of magnification
5.2 Broadening the band
When negative feedback is introduced, the lower limit frequency of the amplifier is reduced from FL to fl/(1+AF) without negative feedback, while the upper limit frequency is raised to (1+AF) FH by FH without negative feedback. The bandwidth of the amplifier is broadened, and the broadened band is about (1+AF) times when negative feedback is not introduced.
5.3 Reducing non-linear distortion
5.4 Effects on input and output resistors of amplifiers
1. Series negative feedback increases the input resistance, rif= (1+AF) RI
2. Shunt negative feedback reduces the input resistance, rif=ri/(1+AF)
3. Voltage negative feedback reduces the output resistance, rof=ro/(1+AF)
4. Current negative feedback causes the output resistance to increase and the situation is related to the specific circuit.

Analog electronic circuit fundamentals-negative feedback