The voltage gain, input impedance, output impedance, band width etc. are few important characteristics of an amplifier. These parameters are more or less constant for a given amplifier. Quite often, the values of these parameters are required to be controlled. This can be conveniently achieved by employing a technique, called the feedback.
When a part or fraction of output is combined to the input, feedback is said to exist. Thus the process of combining a fraction of output energy back to the input is called the feedback.
When the feedback voltage (or current) is so applied as to increase the input voltage (i.e. when feedback is in phase with input voltage), it is called the positive or direct or regenerative feedback. Since positive feedback causes excessive distortion and instability, it is seldom used in amplifiers. However, because of its capability of increasing the power of the original signal it is used in oscillator circuits.
When the feedback voltage (or current) is so applied as to reduce the input voltage (i.e. when the feedback is in phase opposition to the input voltage), it is called the negative, inverse, or degenerative feedback. Negative feedback reduces the amplifier gain but it has numerous advantages explained below, and is frequently used in amplifier circuits.
Principle of Feedback
The amplifiers using feedback are called the feedback amplifiers. The feedback amplifier has two parts i.e. amplifier and feedback circuit. The feedback circuits usually consist of resistors and return a fraction (say ß) of the output voltage back to the input. Let A be the gain of the amplifier i.e. vo/vi = A
Now with feedback
Input voltage, Vi = Vs + Vf = Vs + ßVo in case of +ve feedback
Input voltage, Vi = Vs - Vf = Vs - ßVo in case of –ve feedback
The quantity ß = vf/vo is called the feedback ratio or feedback fraction voltage gain with feed back
Af = vo/vs = A/1-Aß in case of +ve feedback
Af = Vo/ Vs = A/1 + Aß in case of –ve feedback
Advantages of Negative Feedback
There are numerous advantages of negative feedback which outweigh its only drawback of reducing gain. Among the advantages are:
1. Gain Stability. The voltage gain of an amplifier with negative feedback is given as Af = A/(1+ αß). If αß >>1, then the expression becomes Af = 1/ß i.e. overall gain of feedback amplifier Af is independent of internal gain and depends only on feedback ratio ß, and ß in turn depends on the passive elements such as resistors. Resistors remain fairly constant and so the gain is stabilized.
2. Reduced Non-linear Distortion. A large signal stage has non-linear distortion which is reduced by a factor (1+ αß) when negative feedback is used.
3. Reduced Noise. There is always a noise voltage in the amplifier which is reduced by a factor (1 + αß) when negative feedback is used.
4. Increased Bandwidth (or Improved Frequency Response). The band width (BW) of an amplifier without feedback is equal to the separation between the 3-dB frequencies f1 and f2. If A is the gain then gain-bandwidth product is A BW. With the negative feedback the amplifier gain is reduced and since gain-band width product has to remain the same in both cases, so obviously the bandwidth will increase to compensate for the decrease in gain.
5. Increased Input Impedance. The input impedance of the amplifier with –ve feedback is increased by a factor (1+Aß).
6. Reduced output Impedance. The output impedance of the amplifier with negative feedback is reduced by a factor (1 + AB).
Types of Negative Feedback
The feedback can also be classified as voltage feedback or current feedback. In voltage feedback, the voltage feedback to the input terminals is proportional to the output voltage, whatever may be the load impedance. On the other hand, in current feedback, the voltage feedback to the input terminals is proportional to the current through the load. A combination of both the voltage and current feedback may exist in a circuit. Both voltage and current can be feedback to the input either in series or parallel. Series feedback connections tend to increase the input impedance of the amplifier, while shunt feedback connections tend to reduce the input impedance. Voltage feedback tends to reduce the output impedance, while current feedback tends to increase the output impedance, while current feedback tends to increase the output impedance. In most of the cascaded amplifiers, higher input impedance and lower output impedance is desired, both of these can be provided by using series voltage feedback.