Negative and Positive Feedback

Understanding Negative and Positive Feedback

  • Feedback in electronics refers to the process of injecting a fraction of the output signal back into the input. This methodology is absolutely critical in most circuits ranging from simple amplifiers to complex oscillators.

  • Negative feedback involves feeding back part of the output to the input in an out-of-phase manner or in a way that opposes the input signal. It attenuates the original signal and stabilises the overall gain of the system.

  • The primary effects of negative feedback are to reduce the gain, increase the input impedance, decrease output impedance, improve bandwidth, and reduce distortion. Therefore, negative feedback is highly beneficial for improving the performance of amplifiers.

  • Conversely, positive feedback injects the portion of the output back in phase with the input or in a way that reinforces the input signal. This results in a greater output signal and can lead to instability, oscillation or ‘latching’ in digital circuits.

  • Positive feedback is uncommon in most electrical design due to its potential for instability but is essential in certain applications such as oscillators and latch circuits.

Impact of Feedback on Amplifier Stability and Performance

  • Negative feedback can improve amplifier stability by making the gain less sensitive to variations in the parameters of the components (resistors, capacitors, transistors etc.) within the circuit.

  • Negative feedback also has a key role in linearising the operation of the amplifier which helps in reducing distortion and improving signal fidelity.

  • Positive feedback can lead to the creation of oscillators, where the circuit is deliberately designed to be unstable and set into a sustained oscillation. The frequency of these oscillations is predominantly determined by the components in the feedback path.

Feedback Configurations in Amplifiers

  • There are four primary types of feedback in amplifier configurations: voltage series, voltage shunt, current series, and current shunt feedback, distinguished by whether the feedback signal is a voltage or a current and how it is combined with the input: in series or in parallel (‘shunt’).

  • In voltage series feedback, also known as series-series feedback, a portion of the output voltage is fed back into the input through a series connection. This increases input impedance and decreases output impedance.

  • In voltage shunt feedback (series-shunt), the feedback signal is a fraction of output voltage applied in parallel to the input. This configuration results in increased input impedance and increased output impedance.

  • In a current series (shunt-series) feedback, the current from output is fed back to the input in series. This configuration results in decreased input impedance and increased output impedance.

  • Current shunt (shunt-shunt) feedback applies a fraction of the output current back in parallel to the input. This feedback configuration decreases both input and output impedance.

Designing Feedback Amplifiers

  • Estimating the closed-loop gain (the gain with feedback in place) requires analysing the amplifier with the feedback network. Usually, the closed-loop gain of a negative feedback amplifier is less than the open-loop gain (gain without feedback).

  • Special care must be taken in the design of feedback circuits, as they can cause instability if not implemented correctly – particularly in regard to phase shifts introduced by amplification stages, which can convert intended negative feedback into positive feedback.

  • The Barkhausen stability criterion is a commonly-used rule that provides a condition for oscillation in feedback circuits: the loop gain must be equal to 1 (or 0dB) and the phase shift around the loop equal to an integer multiple of 360 degrees at the frequency of oscillation.

  • This criterion can be used in the design and analysis of feedback circuits to ensure stability (in amplifiers) or to promote controlled oscillation (in oscillators).