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 (If) is proportional to the output current ( Iout) of the amplifier. Fig. 1.7 shows the principles of negative current feedback. This circuit is called current- shunt feedback circuit. A feedback resistor Rf is connected between input and output of the amplifier. This amplifier has a current gain of Ai without feedback. It means that a current I1 at the input terminals of the amplifier will appear as Ai I1 in the output circuit i.e., Iout = Ai I1.
Now a fraction mi of this output current is fedback to the input through Rf.
The fact that arrowhead shows the feed current being fed forward is because it is negative
Feedback current, If = mi Iout
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, Iin = I1 + If = I1 + mi Iout
But Iout = Ai I1, where Ai is the current gain of the amplifier without feedback. Iin = I1+ mi Ai I1 (ä Iout = Ai I1) 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 Ai. However, when negative current feedback is applied, the current gain is reduced by a factor (1 + mi Ai).
(ii) The feedback fraction (or current attenuation) mi 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.
Zin = Input impedance of the amplifier without feedback
Z ′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 + mi Ai). 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 + mi Ai).
Z ′out = Zout (1 + mi Ai)
Zout = output impedance of the amplifier without feedback
Z ′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 + mi Ai).
BW ′ = BW (1 + mi Ai)
BW = Bandwidth of the amplifier without feedback
BW ′ = Bandwidth of the amplifier with negative current feedback
The Feedback Voltage Amplifier (Series- Shunt)