Question

An op amp has an offset voltage of 1mV and is ideal in all other respects. If this op amp is used in the circuit shown in figure the output voltage would be:

(A) 1 mV
(B) 1 V
(C) \[0.1{\text{V}}\]
(D) 0 V.

Answer 1

Hint: In such a circuit, the offset voltage (also known as the input offset voltage) is the input voltage to the op amp. The ratio of the output voltage to the input voltage is equal to the ratio of the resistance across the op amp and the input resistance of the op amp
Formula used: In this solution we will be using the following formulae;
\[{V_{out}} = \dfrac{{{R_F}}}{{{R_{in}}}}{V_{off}}\] where \[{V_{out}}\] is the output voltage of the op amp, \[{R_F}\] is the resistance connected across the op amp, \[{R_{in}}\] is the input resistance usually connected to the negative part terminal of the op amp, and \[{V_{off}}\] is the offset voltage.

Complete Step-by-Step solution:
To solve the above question, as told, we note that the op amp is ideal. This means that for such a circuit the offset voltage would be amplified. This offset voltage is a parameter of an amplifier which gives information about the required input voltage for the operation of the amplifier.
Generally, the ratio of the output voltage to the input voltage is equal to the ratio of the resistance across the op amp and the input resistance of the op amp
\[{V_{out}} = \dfrac{{{R_F}}}{{{R_{in}}}}{V_{off}}\] where \[{V_{out}}\] is the output voltage of the op amp, \[{R_F}\] is the resistance connected across the op amp, \[{R_{in}}\] is the input resistance usually connected to the negative part terminal of the op amp, and \[{V_{off}}\] is the offset voltage.
By inserting known values, we have
\[{V_{out}} = \dfrac{{1000k\Omega }}{{1k\Omega }}0.001{\text{V}} = 1{\text{V}}\]

Hence, the correct option is B.

Note: As seen from the diagram, there is no input signal in the system as both the negative and positive are grounded. Hence, why the offset voltage is used as the input. Also, this is ideal, an non-ideal op amp may give a zero volt at output even at a non zero offset voltage.