Current Series Feedback Amplifier Circuit:

Current Series Feedback Amplifier Circuit is also known as series-derived series fed feedback. In such a feedback circuit, a part of the output current is made to develop a voltage proportional to the output current and supplied back in series with the input, as shown in Fig. 19.10 (c). Since feedback network is in series with the amplifier on the output end as well as on the input end, both input and output impedances are increased with negative feedback.

The current feedback can be obtained by removing the bypass capacitor across the emitter resistor R_E, as shown in Fig. 19.32 in case of CE amplifier. The emitter resistance R_E provides the negative current feedback. The current I_e flowing through resistor R_E produces a voltage drop I_eR_E across emitter resistance R_E. This voltage is fed back to the input and opposes the input signal as it is in opposition. This negative voltage feedback is proportional to collector current because I_e ≈ I_c. Thus negative Current Series Feedback Amplifier is provided.

Circuit Analysis:

When an input signal is applied, the resulting collector I_c develops output voltage across the collector load and emitter current I_e develops ac voltage equal to I_eR_E or I_cR_E (I_e ≈ I_c) across the emitter.

Thus input voltage to amplifier, which is equal to V_be, is given as

Thus

Now voltage gain with feedback,

So effective voltage gain with negative current feedback is given by the expression A_f = R_C/R_E. So gain also depends upon the collector load R_C. If R_C depends upon the frequency i.e., if R_C is reactive then there will be distortion in the output.

Applications:

Such a negative feedback is widely employed in practical amplifiers such as in public address systems, record-players, tape recorders, stereo-amplifiers etc. Sometimes, only a part of the emitter resistance is bypassed so that the amplifier gain is not much reduced, and at the same time advantages of negative feedback are achieved.