Why is negative feedback introduced in the amplifier circuit?

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Why is negative feedback introduced in the amplifier circuit? [Copy link]

(1) Why do we usually introduce feedback networks in amplifier circuits?
The main purpose of introducing negative feedback is to make the amplifier circuit work in the linear region and make the output voltage not exceed the maximum output voltage. Without negative feedback, the voltage amplification factor is very large, ideally it can be considered infinite. After the introduction, it is reduced to a very small value, but it is enough. Then there are some effects on the performance of the amplifier circuit: First, the impact on the amplification factor and stability of the amplifier circuit: the amplifier circuit is stabilized at the expense of the amplification factor. For example, when negative feedback is not introduced, the amplification factor is Au, and the change rate is 10%. When a feedback circuit with 1+AF of 100 is introduced, the amplification factor is Au/(1+AF), but the change rate is 0.1%; secondly, the impact on the input resistance: series negative feedback increases the input resistance by 1+AF times, and parallel negative feedback reduces the input resistance to 1/(1+AF); the impact on the output resistance: current negative feedback increases the output resistance by 1+AF times, and voltage negative feedback reduces the output resistance to 1/(1+AF); it can also widen the bandwidth: the lower limit frequency drops to 1/(1+AF), and the upper limit frequency rises by 1+AF times, so the bandwidth is wider. Finally, it can reduce nonlinear distortion and other effects.
(2) How to judge
which type of feedback it is?
The introduction of negative feedback is mainly to make the amplifier circuit work in the linear region, so that the output voltage does not exceed the maximum output voltage. Because without the introduction of negative feedback, the voltage amplification factor is very large, ideally it can be considered infinite, and after the introduction, it is reduced to a very small value, but it is enough. Then there are some effects on the performance of the amplifier circuit: First, the effect on the amplification factor and stability of the amplifier circuit: the amplifier circuit is stabilized at the expense of the amplification factor. For example, when no negative feedback is introduced, the amplification factor is Au, and the rate of change is 10%. When a feedback circuit with 1+AF as 100 is introduced, the amplification factor is Au/(1+AF), but the rate of change is 0.1%; secondly, the effect on the input resistance: series negative feedback increases the input resistance by 1+AF times, and parallel negative feedback reduces the input resistance to 1/(1+AF); the effect on the output resistance: current negative feedback increases the output resistance by 1+AF times, and voltage negative feedback reduces the output resistance to 1/(1+AF); it can also widen the bandwidth: the lower limit frequency drops to 1/(1+AF), and the upper limit frequency rises by 1+AF times, so the bandwidth is wider. Finally, it can reduce nonlinear distortion and other effects. For example, inverting proportional operational amplifier, non-inverting proportional operational amplifier, voltage follower, etc.
(3) How to determine the balance resistance of the amplifier? Can it be added? Can it be added? What is the effect of adding it? What is the impact of not adding it on the circuit? (Wait for the experiment to summarize)
(4) The distinction between the three types of amplifier circuits, with the emitter as the reference point of the AC circuit is called the common emitter amplifier circuit, and the same applies. The common emitter amplifier circuit can amplify both voltage and current; it belongs to the inverting amplifier circuit, and the passband is the smallest among the three circuits. It is suitable for low-frequency circuits and is often used as a unit circuit of low-frequency amplifier circuits.
Common collector amplifier circuit: There is no voltage amplification factor, only current amplification factor. It belongs to the non-inverting amplifier. It is the circuit with the largest input impedance and the smallest output resistance among the three amplifier circuits. It has the characteristics of voltage following and good frequency characteristics. It is often used in the input stage, output stage and buffer stage of voltage amplifier circuits.
Common base amplifier circuit: no current amplification, only voltage amplification, with the characteristics of current following, under input resistance, voltage amplification factor, output resistance and common emitter amplifier circuit are equivalent, in-phase amplifier circuit, the circuit with the best high-frequency characteristics among the three circuits, often used in high-frequency or wide-band input impedance occasions.
(5) Determination of the static operating point of the amplifier circuit: no distortion, bias resistance, input signal frequency, frequency parameters of the transistor, etc. will affect the amplification performance, amplification factor, distortion of the amplifier circuit, why is the amplifier amplification different in the negative half cycle and the positive half cycle? Why is the voltage amplification factor less than 1 when the frequency is lower than a certain value?
(6) When the input signal amplitude of the amplifier reaches a certain value, the output signal amplitude reaches the maximum. When the input signal amplitude continues to increase, the output amplitude will be saturated and distorted.
(7) Saturation distortion and cutoff distortion of transistor amplifier circuit, definition, analysis method?
(8) Positive feedback and negative feedback: If the feedback signal is injected into the input signal through the addition point and enhances the original signal, causing the amplifier amplification factor to increase, it is positive feedback, otherwise it is negative feedback.
(9) Voltage feedback and current feedback: If the feedback component is connected in parallel to the output end, it is voltage feedback. If the feedback component is connected in series to the output end, it is current feedback.

(10) Parallel feedback and series feedback: If the feedback component is connected in parallel at the input end, it is parallel feedback, otherwise it is series feedback
(11) There are two methods that are usually used to determine what kind of feedback circuit it is, observation method and signal short circuit method; signal short circuit method: short circuit the feedback signal of the amplifier to the ground at the input end; short circuit the output signal to the ground at the output end. If the input signal cannot be added to the amplifier after the feedback signal is short-circuited to the ground at the input end, it can be determined as parallel feedback; otherwise it is series feedback; at the output end, if the output signal is short-circuited to the ground and the feedback signal disappears, it can be determined as voltage negative feedback, otherwise it is current negative feedback.
(12) Coupling of multi-stage amplifier circuits: Capacitor coupling. The capacitor has the function of passing AC and blocking DC. Therefore, capacitor coupling can effectively filter out the DC component, and ultimately make the static operating point of each stage of the amplifier circuit unaffected by the previous stage. However, when the frequency of the output signal of the first stage is low, the impedance of the capacitor to it is very large. At this time, the capacitive reactance of the coupling capacitor will cause a large attenuation of the signal, which is very unfavorable for the transmission of slowly changing DC signals. Usually, capacitor coupling is not used at low frequencies.
(13) Power amplifier: Power amplifiers are divided into Class A power amplifier (relatively low efficiency, actually a common emitter amplifier circuit), Class B power amplifier, one is a common collector amplifier circuit, and the other is a Class B push-pull circuit (which will produce crossover distortion at the positive and negative junctions).