Question

The open loop voltage gain in an opamp
(A) reduces if voltage across the inverting and non inverting terminal are same
(B) reduces if voltage across the inverting terminal is zero
(C) increases if voltage across the inverting and non inverting terminal are same
(D) remains the same, if voltage across any of these terminals are changed

Answer 1

Use the formula for open loop voltage gain given by ${A_V} = \dfrac{{{V_0}}}{{{V_ + } - {V_ - }}}$ to observe that if voltage across the inverting and non inverting terminal will become same, ${A_V} \to \infty$ ,i.e. very high.

Complete step by step solution:
In an Op amp, while finding the voltage gain, two situations can be considered, when no feedback loop is applied and when a feedback loop is in operation.
When there is no feedback loop applied in the Op amp circuit, the voltage gain is termed as open loop voltage gain. When a feedback loop is active in the Op amp circuit, the voltage gain measured is called closed loop voltage gain.
The general formula for open loop voltage gain is ${A_V} = \dfrac{{{V_0}}}{{{V_ + } - {V_ - }}}$.
Where ${V_0}$ is the output voltage
${V_ + }$ is the voltage across the inverting terminal
${V_ - }$ is the voltage across the non inverting terminal
If the value of voltage across the inverting terminal, i.e. ${V_ + }$, approaches the value of voltage across the non inverting terminal, i.e. ${V_ - }$, or we can say when ${V_ + } \simeq {V_ - }$, the denominator term in the expression of open loop voltage gain, i.e. ${A_V}$, approaches a very small value. Therefore, the value of open loop voltage gain ${A_V}$ increases.

Therefore, option (C) is correct.

Note: For the inverting and non inverting feedback voltage values to be exactly equal, i.e. for ${V_ + } = {V_ - }$, the denominator in the expression for ${A_V}$ becomes zero and the value of open loop voltage gain ${A_V}$ eventually becomes infinitely high.