Q.Distinguish between open loop and closed loop gain.

Open-loop gain is an amplifier's gain measured with no feedback path connected, while closed-loop gain is its gain measured with a feedback network returning part of the output to the input.

[!ANSWER]

Open-loop gain $A = V_o/V_i$ is the ratio of output voltage to the amplifier's own input voltage (feedback disconnected). Closed-loop gain $A_f = V_o/V_s$ is the ratio of output voltage to the source signal voltage with feedback connected; for negative feedback $A_f = A/(1+A\beta)$, which is smaller than $A$ but far more stable.

[!TLDR]

Open-loop gain $A = V_o/V_i$ is measured without feedback; closed-loop gain $A_f = V_o/V_s$ is measured with the feedback loop connected, and $A_f = A/(1+A\beta)$ links the two.

An amplifier's gain is quoted differently depending on whether the feedback path is present. With the feedback loop open, the whole signal reaches the amplifier and the gain is the open-loop gain $A = V_o/V_i$, where $V_i$ is the voltage appearing directly at the amplifier input. When the feedback network samples the output and returns a fraction $\beta V_o$ to the input, the effective input becomes the source voltage $V_s$, and the gain from source to output is the closed-loop gain $A_f = V_o/V_s$.

The two are related by $A_f = A/(1+A\beta)$ for a negative-feedback amplifier. This is a standard 2-mark distinction in the Karnataka 2nd PUC Electronics question bank.

[!ANSWER]

Open-loop gain $A = V_o/V_i$ is the gain without feedback; closed-loop gain $A_f = V_o/V_s$ is the gain with feedback, related by $A_f = A/(1+A\beta)$ — smaller than $A$ but much more stable.