Figure 1: Negative Feedback Amplifier.
The concept of feedback plays a significant role in electronic components. The basic parameters namely input impedance, output impedance, bandwidth, current or voltage gain can be varied accordingly with the help of feedback, figure 1 illustrates the block diagram of an amplifier with feedback.
Here,
A – Gain of the basic amplifier
β – Feedback ratio
Af – Gain of the feedback amplifier
Xs – signal in the input side (current or voltage)
Xf – Feedback signal (current or voltage).
Initially, the signal Xs from the signal source is fed as input to the mixer network. The output (either voltage or current) signal is sampled by an appropriate sampling network (voltage sampler or current sampler) and then applied to the feedback network. The output of feedback signal Xf combines with the source signal Xs in the mixer network i.e., Xs ± Xf = Xi and is given as input to the basic amplifier.
When the feedback signal (Xf) is out of phase with the source signal (Xs) then such type of feedback is referred to as negative feedback or degenerative feedback amplifier.
Closed Loop Gain of Negative Feedback Amplifier
In negative feedback amplifier, the total effect of feedback decreases the input signal of the amplifier (i.e., Xi = Xs – Xf) which in turn decreases the input voltage of the amplifier. As a result, the output voltage also decreases. The gain of the amplifier with negative feedback is,
\[{{A}_{f}}=\frac{{{X}_{o}}}{{{X}_{s}}}\]
\[=\frac{{{X}_{o}}}{{{X}_{i}}+{{X}_{f}}}\text{ }\left[ {{X}_{i}}={{X}_{s}}-{{X}_{f}}\right]\]
\[{{A}_{f}}=\frac{1}{\frac{{{X}_{i}}}{{{X}_{o}}}+\frac{{{X}_{f}}}{{{X}_{o}}}}\]
Since,
\[A=\frac{{{X}_{o}}}{{{X}_{s}}}\]
And
\[\beta =\frac{{{X}_{f}}}{{{X}_{o}}}\]
\[{{A}_{f}}=\frac{1}{\frac{1}{A}+\beta}=\frac{A}{1+A\beta }\]
\[{{A}_{f}}=\frac{A}{1+A\beta }\]
Characteristics of Negative Feedback Amplifier
The amplifier general characteristics and effects of negative feedback are as follows,
Stabilization of Gain: The gain of a negative voltage feedback amplifier is given by,
\[{{A}_{V}}=\frac{A}{1+\beta A}\]
Where,
A – Voltage gain of amplifier
β – Feedback ratio.
\[{{A}_{V}}=\frac{A}{\beta A}\left[ \beta A>>1 \right]\]
\[{{A}_{V}}=\frac{1}{\beta }\]
It can be observed that the gain depends on feedback factor in a voltage divider network. Temperature, transistor parameters and frequency changes do not affect the gain. Hence, the gain of the amplifier remains stable.
Reduction of Non-linear Distortion: Negative voltage feedback amplifier reduces non-linear distortions in large signal amplifiers occurred due to the variations in voltage gam.
\[{{D}_{{{V}_{F}}=}}\frac{D}{1+A\beta }\]
Where, D – Distortion in amplifier without feedback
DVF – Distortion in amplifier with feedback
Hence, from equation (1), It can be observed that the distortion in amplifier reduces by a factor 1 + Aβ.
Improvement in Frequency Response or Increases Bandwidth: The negative feedback in an amplifier increases the upper cut-off frequency by (1 + Aβ) and decreases lower cut-off frequency by (1 + Aβ), which results increase in the bandwidth by a factor of (1 + Aβ).
Increase in Circuit Stability: The stability of the circuit increases inspective of temperature changes or other. When the output of negative feedback amplifier varies due to temperature amplification is either increased or decreased. The circuits thus opposes such increased or decreased amplification and hence adjusts the circuit to achieve stability.
Increase in Input Impedance and Decrease in Output Impedance: Negative feedback in an amplifier increases input impedance by a factor of (1 + Aβ) and decreases output impedance by a factor of (1 + Aβ).
Advantages of Negative Feedback Amplifier
The advantages of negative feedback amplifiers are given as follows,
- In negative feedback amplifiers, the voltage gain of an amplifier remains stable.
- It reduces the non-linear distortion produced in large signal amplifiers.
- It improves the frequency response of the amplifier.
- It increases the stability of the circuit.
- Negative feedback increases the input impedance and decreases the output impedance of the amplifier.
- It decreases the noise voltage in the amplifier.
Disadvantages of Negative Feedback Amplifier
The main disadvantage of using negative or degenerative feedback in amplifier is reduction in gain. The required gain can be attained by increasing the number of amplifier stages.