Electronic Circuits 1: Feedback Circuits

Feedback Circuits

• A feedback amplifier consists of two parts: an amplifier and a feedback circuit.
• The feedback signal can be either negative (out of phase with the input signal) or positive (in phase with the input signal).

Feedback Types

• Negative feedback: used in amplifier circuits, the feedback signal is out of phase with the input signal.
• Positive feedback: used in oscillator circuits, the feedback signal is in phase with the input signal.

Feedback Amplifier Concepts

• A simple block diagram of a feedback amplifier consists of a mixer circuit, an amplifier block, and a feedback network block.
• The gain of the feedback system can be estimated as: A_f = A / (1 + AB).
• When AB is very large, the gain of the feedback amplifier is almost determined by the feedback network.

Advantages of Negative Feedback

• Reduces frequency distortion: the gain with feedback is approximately B, which is purely resistive and not dependent on frequency.
• Reduces noise and nonlinear distortion: the noise signal gain is reduced by (1+AB).
• Increases bandwidth (BW): the bandwidth BW increases for an amplifier with negative feedback.
• Gain is stable with feedback: the change in the closed-loop gain A_f is less than the corresponding percent change in the open-loop gain A by the factor (1+AB).

Feedback Connection Types

• There are four basic ways of connecting the feedback signal:
  • Voltage-series feedback
  • Voltage-shunt feedback
  • Current-series feedback
  • Current-shunt feedback

Example Problems

• Example 1: Calculate the voltage gain of an amplifier with negative feedback.
• Example 2: Calculate the fraction of the output feedback and the percentage fall in gain with feedback due to ageing of components.
• Example 3: Calculate the change in gain of the feedback amplifier due to temperature.
• Example 4: Calculate the feedback fraction, overall voltage gain, and output voltage for a negative voltage feedback amplifier.