Test: Feedback Amplifiers - Electronics and Communication Engineering (ECE) MCQ

In a series shunt feedback network, feedback is connected in series with signal source but in shunt with the load. Error voltage from feedback network is in series with the input. Voltage fed back from output is proportional to output voltage, hence parallel or shunt connected. The current in and voltage out connection refers to a shunt-shunt connection.

R_L = 10kΩ
I_F = βI_L
I_L = I_F/β = 10/10 = 1mA
V_L = I_LR_L = 10V.

R_I = V_I/I_I = 4/2m = 2kΩ
R_IF = R_I(1+A.β) = 2k(1+500) = 1002kΩ.

Resistor R5 causes global feedback. It is connected to the input node, causing shunt mixing but not to output node, meaning current sampling. Hence it’s a current shunt feedback. Resistors R6 and R7 are neither connected to input nor the output, causing series mixing and current sampling, hence causing current series feedback.

Given that input is 14V, feedback is 6V and source is 20 V, we can see
V_I = V_S – V_F, which is voltage mixing. Also, β is in ohms that is voltage/current. Since output of feedback is voltage and input is current, the output has current sampling. Thus, configuration is a series-series feedback/current – series feedback.

When feedback network is in shunt with load, then output voltage appears as input to feedback. In above case, output current appears as the feedback input, hence B is a current sampling network. Also, feedback network is in shunt with the signal source, hence it’s called shunt mixing or current mixing.

The feedback network is connected directly to output node, so voltage sampling occurs. However, it’s not connected directly to the input node. Hence it’s series mixing at the input. Voltage sampling is a shunt network.

In series-series feedback, the output is current sampled, that is it is in series with the load. Also, input is a voltage mixer, which is in series with signal source. So feedback factor
Β = V_F/I_L in Ohms.

In feedback systems, the feedback signal is in proportion with the output signal.
X_F ∝ X_O
X_F = βX_O, where β is the feedback factor or reverse transmission factor.

Negative feedback decreases lower cutoff frequencies and increases the higher cutoff frequency.
f_HF = f_H(1+Aβ)
f_LF = f_L/(1+Aβ)
Total bandwidth is thus increased.