In current-series (series–series) feedback topology, output current (usually a voltage proportional to the output current) is sampled and mixed in series with the externally applied input signal.
Figure below shows the block schematic arrangement of a generalized feedback amplifier with current-series feedback.
Schematic arrangement of current-series (series–series) feedback amplifier
Figure below shows the equivalent circuit for the schematic arrangement of current-series feedback amplifier.
Equivalent circuit for schematic arrangement of current-series feedback amplifier
The gain parameter for a current-series feedback amplifier is trans-conductance. Equation below gives the expression for trans-conductance with feedback (GMf)
Where, GM is trans-conductance without feedback β is the feedback factor
Expression for input resistance is given by
Therefore,
Where, GM is the trans-conductance taking load resistance RL into account GM is expressed as
Where, Gm is the short-circuit trans-conductance, that is, without taking the load resistance RL into account. It is also expressed as
Common-emitter amplifier with un-bypassed emitter resistor as shown in figure below is an example of current-series feedback
Common-emitter amplifier with current-series feedback
As is evident from the circuit diagram, a voltage proportional to the output current is fed back in series with the source of input signal. This is true if the base current were considered as negligible. The feedback factor β in this case equals RE.
If we consider the amplifier circuit in Q6 as voltage-series feedback, then the feedback factor would equal RE/RC. This violates one of the fundamental assumptions of negative-feedback amplifiers according to which the feedback factor should be independent of load and source resistances. Hence, the amplifier in Q62 is having current-series feedback.
Common source amplifier with un-bypassed source resistor is an example of current-series feedback.
Op-amp based non-inverting amplifier (refer to figure below) where the output taken is the current Io across the feedback resistor R2 is an example of current-series feedback. As shown in the figure, it is the voltage developed across R1 (= Io × R1) that is fed back in series with the input.
Op-amp based amplifier circuit with current-series feedback
Current-series feedback leads to very high input impedance and very high output impedance. These are the desirable traits of a good trans-conductance amplifier.