Principles of Negative Voltage Feedback in Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

Principles of Negative Voltage Feedback In Amplifiers 

A feedback amplifier has two parts viz an amplifier and a feedback circuit. The feedback circuit usually consists of resistors and returns a fraction of output energy back to the input. Fig. 1.3 shows the principles of negative voltage feedback in an amplifier. Typical values have been assumed to make the treatment more illustrative.
The output of the amplifier is 10 V. The fraction m_V of this output i.e. 100 m_V is fedback to the input where it is applied in series with the input signal of 101 m_V. As the feedback is negative, therefore, only 1 m_V appears at the input terminals of the amplifier. Referring to Fig. 1.3, we have, Gain of amplifier without feedback,

The following points are worth noting :

• When negative voltage feedback is applied, the gain of the amplifier is reduced.
• Thus, the gain of above amplifier without feedback is 10,000 whereas with negative feedback, it is only 100.
• When negative voltage feedback is employed, the voltage actually applied to the amplifier is extremely  small. In this case, the signal voltage is 101 m_V and the negative feedback is 100 m_V so that voltage applied at the input of the amplifier is only 1 m_V.
• In a negative voltage feedback circuit, the feedback fraction m_V is always between 0 and 1.
• The gain with feedback is sometimes called closed- loop gain while the gain without feedback is called open-loop gain. These terms come from the fact that amplifier and feedback circuits form a loop. When the loop  is ―opened by disconnecting the feedback circuit from the input, the amplifier's gain is Av, the open- loop gain. When the loop is closed b y connecting the feedback circuit, the gain decreases to Av_f, the ―closed- loop gain.

Gain of Negative Voltage Feedback Amplifier:

Consider the negative voltage feedback amplifier shown in Fig. 1.4. The gain of the amplifier without feedback is A_v. Negative feedback is then applied by feeding a fraction m_v of the output voltage e_o back to amplifier input. Therefore, the actual input to the amplifier is the signal voltage e_g minus feedback voltage m_v e₀ i.e.,

Actual input to amplifier = e_g − m_v e₀

The output e₀ must be equal to the input voltage (e_g − m_v e₀₎ multiplied by gain Av of

But e₀/e_g is the voltage gain of the amplifier with feedback. Voltage gain with negative feedback is

It may be seen that the gain of the amplifier without feedback is A_v. However, when negative voltage feedback is applied, the gain is reduced by a factor 1 + A_v m_v. It may be noted that negative voltage feedback does not affect the current gain of the circuit.

 Advantages of Negative Voltage Feedback:

The following are the advantages of negative voltage feedback in amplifiers :

(i) Gain stability. An important advantage of negative voltage feedback is that the resultant gain of the amplifier can be made independent of transistor parameters or the supply voltage variations.

for negative voltage feedback in an amplifier to be effective, the designer deliberately makes the product A_v m_v much greater than unity. Therefore, in the above relation, 1 can be neglected as compared to A_v m_v  and the expression becomes :

It may be seen that the gain now depends only upon feedback fraction m_v i.e., on the characteristics of feedback circuit. As feedback circuit is usually a voltage divider (a resistive network), therefore, it is unaffected by changes in temperature, variations in transistor parameters and frequency. Hence, the gain of the amplifier is extremely stable.

(ii) Reduces non- linear distortion. A large signal stage has non- linear distortion because its voltage gain changes at various points in the cycle. The negative voltage feedback reduces the nonlinear distortion in large signal amplifiers. It can be proved mathematically that :

It is clear that by applying negative voltage feedback to an amplifier, distortion is reduced by a factor 1 + A_v m_v.

(iii) Improves frequency response. As feedback is usually obtained through a resistive network, therefore, voltage gain of the amplifier is independent of signal frequency.
The result is that voltage gain of the amplifier will be substantially constant over a wide range of signal frequency. The negative voltage feedback, therefore, improves the frequency response of the amplifier.

(iv) Increases circuit stability. The output of an ordinary amplifier is easily changed due to variations in ambient temperature, frequency and signal amplitude. This changes the gain of the amplifier, resulting in distortion. However, by applying negative voltage feedback, voltage gain of the amplifier is stabilised or accurately fixed in value.

This can be easily  explained. Suppose the output of a negative voltage feedback amplifier has increased because of temperature change or due to some other reason. This means more negative feedback since feedback is being given from the output. This tends to oppose the increase in amplification and maintain it stable. The same is true should the output voltage decrease. Consequently, the circuit stability is considerably increased.

(v) Increases input impedance and decreases output impedance. The  negative voltage feedback  increases the input impedance and decreases the output impedance of amplifier. Such a change is profitable in practice as the amplifier can then serve the purpose of impedance matching.

(a) Input impedance. The increase in input impedance with negative voltage feedback can be explained by referring to Fig. 1.5. Suppose the input impedance of the amplifier is Z_in without feedback and Z ′_in with negative feedback. Let us further assume that input current is i₁. Referring to Fig. 1.5, we have,

But e_g/i₁ = Z ′_in , the input impedance of the amplifier with negative voltage feedback.

It is clear that by applying negative voltage feedback, the input impedance of the amplifier is increased by a factor 1 + A_ν m_v. As A_ν m_v is much greater than unity, therefore, input impedance is increased considerably. This is an advantage, since the amplifier will now present less of a load to its source circuit.

(b) Output impedance. Following similar line, we can show that output impedance with negative voltage feedback is given by :

It is clear that by applying negative feedback, the output impedance of the amplifier is decreased by a factor 1 + A_ν m_v . This is an added benefit of using negative voltage feedback. With lower value of output impedance, the amplifier is much better suited to drive low impedance loads.