What Causes Amplifier Feedback?

A feedback amplifier is an amplifier circuit that sends part or all of the output signal back to the input to change the amplification performance.

Amplifier that returns part or all of the output signal to the input to change performance ^[1]

The function of the feedback network is to make the output and input of the system contact to form a loop called a feedback loop.

The feedback amplifier consists of a basic amplifier, a feedback network, a sampling circuit, and a hybrid circuit.

The feedback amplifier is composed of a basic amplifier and a feedback network. The basic amplifier is to amplify the input and send it to the output. The feedback network is to return some or all of the output in the output loop back to the input loop. Therefore, the feedback amplifier judgment method is as follows:

Feedback amplifier

First determine whether there is a feedback branch, if there is, the amplifier has feedback; if not, the amplifier has no feedback. After judging that the amplifier has feedback, it is further judged that this branch is AC feedback if it only works on AC signals; if it only works on DC, it is DC feedback; if it works on both AC and DC, then AC and DC feedback.

Feedback amplifier feedback type

To judge whether the feedback amplifier is voltage feedback or current feedback, whether it is series feedback or parallel feedback, it is generally judged by the short circuit method.

Shorting the load RL of the feedback amplifier causes the output voltage Vo = 0. If no feedback signal is added to the input circuit of the feedback amplifier via the feedback branch, it is voltage feedback; otherwise, if a feedback signal is added to the input circuit of the feedback amplifier via the feedback branch, it is current feedback.

Short-circuit the input of the feedback amplifier to ground, so that the input voltage Vi = 0. If the feedback signal can be added to the input of the basic amplifier, it is serial feedback; otherwise, if the feedback signal cannot be added to the input of the basic amplifier, it is parallel feedback.

Feedback amplifier positive and negative feedback

Use the instantaneous polarity of voltage (called instantaneous polarity method) to judge positive and negative feedback

In the process of judging the manifestation of feedback polarity in the circuit, the most effective and basic method is the instantaneous polarity method ^[2] .

Set the phase of the input voltage Vi at the instant of the feedback amplifier to be positive (indicated by +). After the phase change of the basic amplifier step by step, the voltage returned from the output loop through the feedback branch back to the input of the amplifier is positive (using (+) (Representation), that is, positive feedback; conversely, if the voltage returned to the input of the amplifier via the feedback branch is negative (indicated by (-)), it is negative feedback.