Feedback of amplification circuit
Time:2023-05-22
Views:1012
(1) Why is feedback network generally introduced in amplification circuits
Introducing negative feedback is mainly to make the amplification circuit work in the linear region, so that the output voltage does not exceed the maximum output voltage. Because without negative feedback, the voltage amplification factor is very large, which can be idealized as infinite. After introducing it, it decreases to a very small value, but that‘s enough. Then there are some effects on the performance of the amplification circuit: firstly, the impact on the amplification factor and stability of the amplification circuit: sacrificing the amplification factor to stabilize the amplification circuit, for example, without introducing negative feedback, the amplification factor is Au with a rate of change of 10%, and introducing a feedback circuit with 1+AF of 100, the amplification factor is Au/(1+AF), but the rate of change is 0.1%; Secondly, the impact on the input resistance: the series negative feedback increases the input resistance by 1+AF times, while the 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, while voltage negative feedback reduces the output resistance to 1/(1+AF); It can also broaden the frequency band: the lower limit frequency decreases to 1/(1+AF), and the upper limit frequency increases by 1+AF times, resulting in a wider frequency band. Finally, it can reduce nonlinear distortion and other effects.
(2) How to determine which type of feedback amplification circuit is used?
Introducing negative feedback is mainly to make the amplification circuit work in the linear region, so that the output voltage does not exceed the maximum output voltage. Because without negative feedback, the voltage amplification factor is very large, which can be idealized as infinite. After introducing it, it decreases to a very small value, but that‘s enough. Then there are some effects on the performance of the amplification circuit: firstly, the impact on the amplification factor and stability of the amplification circuit: sacrificing the amplification factor to stabilize the amplification circuit, for example, without introducing negative feedback, the amplification factor is Au with a rate of change of 10%, and introducing a feedback circuit with 1+AF of 100, the amplification factor is Au/(1+AF), but the rate of change is 0.1%; Secondly, the impact on the input resistance: the series negative feedback increases the input resistance by 1+AF times, while the 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, while voltage negative feedback reduces the output resistance to 1/(1+AF); It can also broaden the frequency band: the lower limit frequency decreases to 1/(1+AF), and the upper limit frequency increases by 1+AF times, resulting in a wider frequency band. Finally, it can reduce nonlinear distortion and other effects. For example, inverse proportional operational amplifiers, in phase proportional operational amplifiers, voltage followers, etc
(3) How to determine the balance resistance of the amplifier and add it? Can we add it? What is the effect of adding it? What is the impact of not adding it on the circuit?
(4) The distinction between the three types of amplification circuits, with the emitter as the reference point for AC circuits, is called a common emitter amplification circuit, and so on. A common beam amplifier circuit can amplify both voltage and current; It belongs to the inverse amplification circuit. The passband is the smallest of the three circuits. It is suitable for low-frequency circuits and is often used as the unit circuit of low-frequency amplification circuits
Common collector amplifier circuit: There is no voltage amplification factor, only current amplification factor, belonging to the same phase amplifier. It is the circuit with the highest input impedance and the lowest output resistance among the three types of amplifier circuits. It has the characteristic of voltage following and good frequency characteristics, and is commonly used in the input stage, output stage, and buffer stage of voltage amplification circuits.
Common base amplification circuit: There is no current amplification effect, only voltage amplification effect, with the characteristic of current following. Under input resistance, the voltage amplification factor and output resistance are equivalent to common beam amplification circuit. In phase amplification circuit, the circuit with the best high-frequency characteristics among the three types of circuits is commonly used in high-frequency or broadband input impedance situations.
(5) The determination of the static working point of the amplification circuit: no distortion, bias resistance, frequency of the input signal, frequency parameters of the transistor, etc. can all affect the amplification performance of the amplification circuit, amplification factor, distortion. Why is the amplification of the amplifier different in negative and positive half cycles? Why is the voltage amplification factor less than 1 when the frequency is below a certain value?
(6) When the input signal amplitude of the amplifier reaches a certain value, the output signal amplitude reaches its maximum. When the input signal amplitude continues to increase, the output amplitude undergoes saturation distortion,
(7) Saturation distortion and cutoff distortion of transistor amplification circuits, definition, analysis methods?
(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 amplification of the amplifier to increase, it is positive feedback, otherwise it is negative feedback.
(9) Voltage feedback and current feedback: If the feedback components are connected in parallel at the output end, it is voltage feedback, and if the feedback components are connected in series at the output end, it is current feedback.
(10) Parallel feedback and series feedback: If the feedback components are connected in parallel at the input end, it means parallel feedback; otherwise, series feedback
(11) There are two methods commonly used to determine what feedback circuit is, observation method and signal short circuit method; Signal shorting method: Short circuit the feedback signal of the amplifier to ground at the input end; Short circuit the output signal to ground at the output end. If the feedback signal at the input end is short circuited to ground, and the input signal cannot be added to the amplifier, it can be determined as parallel feedback; Otherwise, it is a series feedback; At the output end, if the output signal is shorted to 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 amplification circuits: Capacitor coupling, which has the function of passing AC and isolating DC, can effectively filter out DC components, ultimately making the static working points of each stage of the amplification circuit unaffected by the previous stage. However, when the output signal frequency of the first stage is low, the capacitance has a large impedance on it, and at this time, the capacitive impedance of the coupling capacitor has a significant attenuation on the signal, The transmission of DC signals with slow changes is very unfavorable, and capacitance coupling is usually not used at low frequencies.
(13) Power amplifier: power amplifier is divided into Class A power amplifier (low efficiency, actually common emitter amplifier circuit) and Class B power amplifier. One is common collector amplifier circuit, the other is Class B push-pull circuit (which will produce Crossover distortion at the junction of positive and negative), and Class A and Class B power amplifier
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