negative feedback amplifier

1. Positive feedback and negative feedback Sending certain quantities from the output end of the amplifier back to the input end is called feedback. If the phase of the feedback quantity is the same as the input quantity, and adding feedback increases the amplification factor, it is called positive feedback; if the feedback quantity is in opposite phase to the input quantity, the feedback is used to generate oscillation in the circuit, and negative feedback is used to improve the performance of the amplifier. . 2. Feedback method and feedback effect The feedback effect is closely related to the nature and method of the feedback amount. If the negative feedback amount is proportional to the output voltage, the feedback effect can stabilize the output voltage and reduce the output resistance, which is called voltage negative feedback. ; If the amount of negative feedback is proportional to the output current, and its feedback effect can stabilize the output current and increase the output resistance, it is called current feedback. According to the input form of feedback amount, it is divided into series feedback and parallel feedback. The feedback amount of series feedback is connected in series to the input loop, and the feedback amount of parallel feedback is connected in parallel to the input loop. Series negative feedback can increase the input resistance, while parallel negative feedback reduces the input resistance. 3. Feedback judgment method Positive feedback or negative feedback is judged by the instantaneous polarity method. First, assume a certain instantaneous polarity of the input signal and then gradually derive the input and output terminals according to the principle of the input and output inversion of the single-stage common emitter amplifier. Instantaneous polarity, finally look at the polarity of the feedback amount fed back to the input terminal and the assumed polarity of the initial input signal: if the two polarities are the same, it is positive feedback, if the two polarities are opposite, it is negative feedback. Whether voltage feedback or current feedback is determined by the imaginary AC short circuit method at the output end. After short-circuiting the effective current at the output end, if the feedback amount disappears, it is voltage feedback. If there is still feedback amount, it is current feedback. Whether series feedback or parallel feedback is judged by examining the connection method of the input loop. If the feedback quantity and the input signal are the sum of voltages, it is series feedback, and if the feedback quantity and input signal are the sum of currents, it is parallel feedback. The four forms of negative feedback circuits are listed in Table 1, and their relevant calculation formulas are listed in Table 2. Table 1. Three forms of feedback current, parallel negative feedback voltage, parallel negative feedback current, series negative feedback. Suppose the input current Ii points to node B to increase, The feedback current is If, and the feedback process is: If↑→Uc1↓→Uf↓→If↓. Because Ib and If are in opposite phases, they are negative feedback. If the output terminal is short-circuited, the feedback quantity Uf still exists, so it belongs to current negative feedback. Since the feedback quantity If is connected to the input terminal in parallel, it belongs to current parallel negative feedback. Assume that the input voltage Ii points to node b to increase, and the fed current If The feedback process Ii↑→Uc↓→If↓ (deviating from node b) is negative feedback because Ib and If are inverse phase. If the output terminal is short-circuited, If=0, so it is feedback; in addition, the feedback amount is connected in parallel. Input terminal, so it belongs to voltage parallel negative feedback. Suppose the input voltage polarity is Ui↑, the feedback amount is Uf, the feedback process: Ui↑→Ie↑→Uf↑. If the input loop direction is used as the reference direction, Ui and Uf have opposite polarities, so It is negative feedback. Because Ui and Uf are connected in series. So it is a series of negative feedback. If the output terminal is short-circuited and Uf still exists, it is a current negative feedback. Reduce the input resistance to stabilize the output current. Increase the output resistance. Reduce the input resistance to stabilize the output voltage. Reduce the output resistance. Increase the input resistance to stabilize the output current. Increase the output resistance. Table 2. Four forms Negative feedback parameter calculation voltage negative feedback current negative feedback series parallel series parallel Kuu=Uo/Ube Fuu=Uf/UO=Rel/(Rel+Rf) rif=(1+Kuu+Fuu)ri rof=ro/(1+ KuuoF) Kui=Uo/Ib Fiu=If/Uo=-(1)/Rf rif=ri/(1+KuiFiu) rof=ro/1+KuioFiu Kiu=Uo/Ube Fui=(Uf/Io)=-Rf rif=(1+KiuFui)ri rof=(1+KiuoFui)ro Kii=Io/Ib Fii=(If/Ie2)=Re2/(Re2+Rf) rif=ri/(1+KiiFii) rof=(1+ KiioFii)ro