**Current Shunt Feedback Amplifier Circuit:**

Current Shunt Feedback Amplifier Circuit is also known as **series-derived shunt fed feedback** or **current shunt inverse feedback**. In this circuit, the feedback network picks up a part of the output current and produces a feedback voltage in parallel with the input signal voltage, as shown in Fig. 19.10(d). Since the feedback shunts the input so input impedance is reduced with feedback whereas output impedance is increased because of feedback network being in series with the output.

A two-stage cascaded amplifier using such a feedback is given in Fig. 19.41. Here feedback is from the emitter of second stage to the base of first stage through resistor R_{f}. In this input voltage (V_{in2}) to second stage is much larger than input voltage (V_{in1}) to first stage, because of amplifying action of transistor Q_{1}, and is in phase opposition to it (i.e., phase difference of 180°). Voltage V_{e2 }is only slightly smaller than V_{in2}, because of emitter follower action. Also voltages V_{e2} and V_{in2 }are in phase. Thus V_{e2} is larger in magnitude than V_{in1} and has a phase difference of 180° with V_{in1}. When input signal increases causing I_{s} to increase, feedback current I_{f} also increases and so the input current V_{in1} would be smaller than it would be without feedback as it is difference of I_{s} and I_{f}. Thus negative feedback is provided.

With R_{f} much larger than R_{E}, the circuit shown in Fig. 19.41 approximates a current-shunt feedback pair and feedback current I_{f }is given as

because V_{in1} is quite small as compared to V_{e2}

Neglecting base current of transistor Q_{2} in comparison to its collector current and assuming R_{f} >> R_{E}

and so

Since feedback current is proportional to the output current, this circuit is an example of a Current Shunt Feedback Amplifier.

Transfer current gain with feedback,

Thus transfer current gain A_{if} would be stable provided that resistors R_{E} and R_{f }are stable.

Assuming input impedance with feedback R_{inf} to be zero, V_{s} = I_{s}R_{s}

and the voltage gain with feedback,

Thus voltage gain with feedback is stable i.e., independent of the transistor parameters, the temperature, or supply voltage variations because resistors R_{f}, R_{E}, R_{C2} and R_{S} are stable elements.

**Amplifier Without Feedback:**

Referring to the fourth topology in Table 19.4, the input circuit of the amplifier without feedback is obtained by opening the output loop at the emitter of Q_{2}. This places R_{f} in series with R_{E} from base to emitter of Q_{1}. The output circuit is obtained by shorting the input node (base of Q_{1}). This places R_{f} in parallel with R_{E}. The resultant equivalent circuit is shown in Fig. 19.42. Since the feedback signal is a current, the source is represented by a Norton’s equivalent circuit with

The feedback signal is the current I_{f }in the resistor R_{f}, which is in the output circuit.

From Fig. 19.42,

which is in agreement with Eq. (19.62).