Op amp feedback judgment method

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Op amp feedback judgment method [Copy link]

Negative feedback amplifiers can be combined into four types, namely: current series, current parallel, voltage series, and voltage parallel. Judgment of positive and negative feedback The judgment of positive and negative feedback uses the instantaneous polarity method. Instantaneous polarity is an assumed state, which assumes that an instantaneous increase signal is introduced at the input end of the amplifier circuit. This signal returns to the input end through the amplifier circuit and the feedback loop. If the feedback signal increases the introduced signal, it is positive feedback, otherwise it is negative feedback. (The instantaneous polarity of the output of the operational amplifier is the same as that of the non-inverting input, and opposite to that of the inverting input)

Common emitter amplifier: the collector and base potentials are in opposite phases;

Common base amplifier: the collector and emitter potentials are in phase;

Common collector amplifier: the emitter and base potentials are in phase;

Positive feedback: the input polarity and feedback polarity are the same

Negative feedback: the input polarity and feedback polarity are opposite

(If the corresponding op amp returns not the same input terminal but another input terminal, and the input polarity and feedback level are the same, it is negative feedback)

Judgement of series and parallel connection

The series and parallel type of feedback refers to the way the feedback signal affects the input signal, that is, the connection method at the input terminal. Series feedback means that the net input voltage and feedback voltage are connected in series in the input loop, that is, they are superimposed in the form of voltage series (the input signal and the feedback signal are not at the same electrode), while parallel feedback means that the net input current and feedback current are connected in parallel in the input loop, that is, they are superimposed in the form of voltage series (the input signal and the feedback signal are at the same electrode).

Series: The input signal and the feedback signal are not at the same electrode

Parallel: The input signal and the feedback signal are at the same electrode

Judgement of voltage and current

Voltage and current feedback means that the feedback signal is taken from the output signal (voltage or current).

Usually, the method of short-circuiting the load resistor is used to distinguish voltage feedback and current feedback. The specific method is: if the load resistor RL is short-circuited, if the feedback effect disappears, it is voltage feedback; if the feedback effect exists, it is current feedback.