Current Shunt Feedback

In current-shunt (shunt–series) feedback, output current is sampled and mixed in shunt with the externally applied input signal.

Figure below shows the block schematic arrangement of a generalized feedback amplifier with current-shunt feedback.

Schematic arrangement of current-shunt (shunt–series) feedback amplifier

Figure below shows the equivalent circuit for current-shunt feedback amplifier.

Equivalent circuit for schematic arrangement of current-shunt feedback amplifier

Gain parameter for a current-shunt feedback amplifier is the current gain. Current gain with feedback A_If is given by

Where,
A_I is current gain without feedback β is the feedback factor

The expression for input resistance is derived as follows.

Substituting

in the above equation, we get

Now,

Therefore,

Also,

where A_I is the current gain without feedback taking load resistance into account. This gives

Where,
A_I is given by

Where, A_i is the current gain without taking the load resistance R_L into account. In fact, it is the short-circuited current gain. It can also be expressed as

Expression for output resistance is derived by considering I_s = 0, letting load resistance R_L = ∞ and applying a voltage V across the output. Ratio of applied voltage V to the resulting current I gives the output resistance.

For,

Where, Rof is the output resistance with feedback with R_L = ∞ Considering the effect of load resistance R_L, the output resistance with feedback R_of' is given by parallel combination of R_of and R_{L. R} of' can also be expressed by

Where,

Also, for R_L=∞ ,R'=R _o and A₁ =0 . Therefore,

Cascade arrangement of common emitter amplifier stages

The circuit in the figure is a cascade arrangement of two common-emitter amplifier stages with feedback from emitter of the second stage to the base of the first stage. It is an example of current-shunt feedback. The voltage across emitter resistor of the second stage is out of phase with the base voltage of the first stage, which confirms this to be a case of negative feedback. Also, the feedback current, that is the current flowing through resistor R, is proportional to the output current and the proportionality factor β is equal to R_e/(R_e + R).

Figure below shows op-amp-based inverting current amplifier having current-shunt feedback. The feedback factor in this case is given by [R₁/(R₁ + R₂)].

Op-amp-based current amplifier with current-shunt feedback

Current-shunt feedback leads to very high output impedance and very low input impedance. These are the desirable traits of a good current amplifier.