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**Feedback Circuit**

**The function of the feedback circuit is to return a fraction of the output voltage to the input of the amplifier. Fig. 1.6 shows the feedback circuit of negative voltage feedback amplifier. It is essentially a potential divider consisting of resistances R1 and R2. The output voltage of the amplifier is fed to this potential divider which gives the feedback voltage to the input. Referring to Fig. 1.6, it is clear that :**

**Principles of Negative Current Feedback**

In this method, a fraction of output current is fedback to the input of the amplifier. In ot her words, the feedback current (I_{f}) is proportional to the output current ( I_{out}) of the amplifier. Fig. 1.7 shows the principles of negative current feedback. This circuit is called current- shunt feedback circuit. A feedback resistor R_{f} is connected between input and output of the amplifier. This amplifier has a current gain of A_{i} without feedback. It means that a current I_{1} at the input terminals of the amplifier will appear as A_{i} I_{1 }in the output circuit i.e., I_{out} = A_{i} I_{1}.

Now a fraction m_{i }of this output current is fedback to the input through R_{f}.

The fact that arrowhead shows the feed current being fed forward is because it is negative

Feedback current, I_{f} = m_{i} I_{out}

Note that negative current feedback reduces the input current to the amplifier and hence its current gain.

**current Gain with Negative Current Feedback **

Referring to Fig. 1.6, we have, I_{in} = I_{1} + I_{f} = I_{1} + m_{i} I_{out}

But I_{out} = A_{i }I_{1}, where A_{i} is the current gain of the amplifier without feedback. I_{in} = I_{1}+ m_{i} A_{i} I_{1} (Ã¤ I_{out} = A_{i} I_{1}) Current gain with negative current feedback is

This equation looks very much like that for the voltage gain of negative voltage feedback amplifier. The only difference is that we are dealing with current gain rather than the voltage gain.

The following points may be noted carefully :

**i) **The current gain of the amplifier without feedback is A_{i}. However, when negative current feedback is applied, the current gain is reduced by a factor (1 + m_{i} A_{i}).

**(ii)** The feedback fraction (or current attenuation) m_{i} has a value between 0 and 1.

**(iii)** The negative current feedback does not affect the voltage gain of the amplifier.

**Effects of Negative Current Feedback **

The negative current feedback has the following effects on the performance of amplifiers :

**(i) Decreases the input impedance.** The negative current feedback decreases the input impedance of most amplifiers.

Let

Z_{in} = Input impedance of the amplifier without feedback

Z â€²_{in} = Input impedance of the amplifier with negative current feedback

** Referring to Fig. 1.8, we have,**

Thus the input impedance of the amplifier is decreased by the factor (1 + m_{i} A_{i}). Note the primary difference between negative current feedback and negative voltage feedback. Negative current feedback decreases the input impedance of the amplifier while negative voltage feedback increases the input impedance of the amplifier.

**Increases the output impedance**. It can be proved that with negative current feedback, the output impedance of the amplifier is increased by a factor (1 + m_{i} A_{i}).

Z â€²_{out} = Z_{out }(1 + m_{i} A_{i})

where

Z_{out} = output impedance of the amplifier without feedback

Z â€²_{out} = output impedance of the amplifier with negative current feedback

The reader may recall that with negative voltage feedback, the output impedance of the amplifier is decreased.

**Increases bandwidth**. It can be shown that with negative current feedback, the bandwidth of the amplifier is increased by the factor (1 + m_{i} A_{i}).

BW â€² = BW (1 + m_{i} A_{i})

where

BW = Bandwidth of the amplifier without feedback

BW â€² = Bandwidth of the amplifier with negative current feedback

**The Feedback Voltage Amplifier (Series- Shunt)**

** **

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