Analog Circuit Basics Series 6: Feedback Amplifier Circuit

1. Establishment of the concept of feedback

* Open-loop gain --- A
* Closed-loop gain --- Af
* Feedback depth --- 1 + AF
* Loop gain --- AF:
1. When AF>0, Af decreases, and this feedback is called negative feedback.
2. When AF=0, it indicates that the feedback effect is zero.
3. When AF<0, Af increases, and this feedback is called positive feedback.
4. When AF=-1, Af→∞. The amplifier is in a "self-oscillation" state.
II. Feedback form and judgment
1. Feedback range --- this level or between levels.
2. Feedback nature --- AC, DC or AC/DC. If
there is feedback in the DC path, it is DC feedback, if there is feedback in the AC path, it is AC feedback, and if there is feedback in both the AC and DC paths, it is AC/DC feedback. 
3. Feedback sampling --- voltage feedback: the feedback amount is sampled from the output voltage; it has the function of stabilizing the output voltage. (Feedback disappears when the output is short-circuited)       
 Current feedback: the feedback amount is sampled from the output current. It has the function of stabilizing the output current. (The feedback does not disappear when the output is short-circuited)

4. Feedback method - parallel feedback: the feedback quantity and the original input quantity are superimposed in the input circuit in the form of
                          current . The larger the Rs, the better the feedback effect.
                          The feedback signal is fed back to the input terminal) Series feedback: the feedback quantity and the original input quantity                           are superimposed
                in the input circuit in the form of voltage . The smaller the Rs, the better the feedback effect.                           The feedback signal is fed back to the non-input terminal) 5. Feedback polarity - instantaneous polarity method: (1) Assume that the polarity of a certain input signal at a certain instant is positive (indicated by +), and the frequency of the signal is in the mid-frequency band. (2) Based on this polarity, infer the instantaneous polarity of each relevant point in the amplifier circuit step by step (increase is indicated by +, decrease is indicated by -). (3) Determine the polarity of the feedback signal. (4) According to the polarity of Xi and Xf, determine the size of the net input signal. Xid decreases for negative feedback; Xid increases for positive feedback. III. Description method of feedback form       A feedback element introduces inter-stage (this stage) DC negative feedback and AC voltage (current) series (parallel) negative feedback. IV. The influence of negative feedback on the performance of amplifier circuit 1. Improving the stability of amplification factor







 

2.

3. Extend the frequency band
4. Reduce nonlinear distortion and suppress interference and noise
5. Change the input and output resistance of the amplifier circuit
         * Series negative feedback increases the input resistance by 1+AF times
         * Parallel negative feedback reduces the input resistance by 1+AF times
         * Voltage negative feedback reduces the output resistance by 1+AF times
         * Current negative feedback increases the output resistance by 1+AF times
V. Causes and conditions of self-oscillation
1. Causes of self-oscillation
          Additional phase shift converts negative feedback into positive feedback.
2. Conditions of self-oscillation

          If expressed as amplitude and phase conditions, then: