Feedback in Amplifier Circuits !

Feedback in Amplifier Circuits !

Why is feedback in amplier circuits needed?

When designers analyse circuits, they are mainly concerned with gain and impedance. The gain of a transistor amplifier is controlled by providing significant feedback, either local (within a single stage) or overall (over several stages). When the feedback is significant, the gains and impedances are essentially independent of transistor parameters, and the equations needed for the design are greatly simplified. Significant feedback exists when the feedback is sufficient to make a significant change in the amplifier performance. A 4 to 1 change in gain is defined as significant.

For example, in a single stage amplifier with emitter feedback as the one shown in the illustration, the resistor ratio RB/RE is called the amplifier S-factor. In a stage with collector feedback, the stage S-factor is S = Rf/RL. In either example, if the feedback is adequate, the stage current gain is essentially independent of the transistor current gain and is equal to S. S may be used as the amplifier current gain with feedback.

Feedback in Amplifier Circuits !

The effect of feedback on the output impedance is sometimes of interest, as in a hi-fi amplifier in which a low impedance is desired in order to damp out speaker transients. Generally, feedback amplifiers can be designed without concern for the effect of feedback on the internal output impedance. However, the effect of the load impedance on the feedback must always be considered. A reactive load may affect the response or stability of the amplifier.

Feedback sacrifices some power gain for improved performance. Many performance characteristics cannot be obtained without feedback. Because feedback reduces the gain, amplifiers must be designed with additional stages.

A simple explanation of how the amplifier with collector feedback shown on the picture works follows here. βdc increases with temperature, which causes IC to increase. An increase in IC produces more voltage drop across RL1, lowering the collector voltage. This, in turn, reduces the IB, which tends to offset the original increase in IC. The result is that the circuit tends to maintain a stable value of collector current, keeping the Q-point fixed. The reverse action occurs when temperature decreases. This is why feedback has a stabilising action.

Glossary of feedback-related terms:

Here, you will find a glossary of simple definitions for terms related to feedback in amplifier circuits.

Current feedback

Current feedback returned from the output amplifier increases the output impedance.

Local feedback

Feedback within a single amplifier stage. Local feedback is easily appplied and understood, but it makes uneconomical use of gain.

Overall feedback  

Feedback over several amplifying stages. Overall feedback may reduce the gain of an amplifier and may be difficult to apply while keeping the amplifier stable.

Internal feedback

Internal feedback is feedback within a transistor that is often a source of high-frequency problems.

Loop feedback

This exists when a signal path or loop may be traced from the amplifier input through the amplifier, back throught the amplifier network and into the amplifier input. Loop feedback has the effect of making the output signal more like the input signal. With loop feedback, the characteristics of the amplifier may be improved, either to make the amplifier less sensitive to the ambient temperature and the supply voltage or to change the frequency response and the impedance characteristics, or both. Loop feedback is not significant unless the net gain around the loop is 3 or 10 dB. Adequate loop feedback requires a careful design of the loop amplitude-phase characteristics in order to control the transient response or the stability of the amplifier under closed-loop conditions.

Negative feedback

An amplifier is said to have negative feedback when a voltage derived from the amplifier output is added to the input signal in such a way as to oppose the input signal. Negative feedback amplifiers may be more complex but have the advantage of being less sensitive to environmental and component variations.

Open-loop feedback

Open-loop amplifiers are easier to understand and design but are sensitive to environmental and component variations.

Shunt feedback

Applied in parallel with the input. This feedback tends to lower the input impedance.

Series feedback

Applied in series with the input. It raises the input impedance.

Voltage feedback

Voltage feedback returned from the output amplifier reduces the output impedance. With voltage feedback, the load may shunt and thereby reduce or eliminate feedback intended to control gain, impedance or frequency response. 

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