Op-Amp Parameters Questions and Answers

Answer: B

The answer is both (A) and (R) are true but (R) is not the correct explanation of (A).

An op-amp does have a high input impedance, typically megohms for a bipolar input stage and far higher with FET inputs; it comes from the differential input stage working at very small bias currents. Op-amps also have a low input offset current, the difference between the two input bias currents, because the input transistors are matched.

Both are true, but offset current is a measure of mismatch between the inputs, not of how much current the input draws for a given voltage, so it does not explain the high impedance.

Answer: A

The key's answer is that both statements are true and (R) is the correct explanation of (A). I could not confirm this reading in a standard source, so it follows the key.

Input offset current is the difference between the two input bias currents of an op-amp. The bias currents are small when the input stage has a very high input impedance (nanoamperes for bipolar inputs, picoamperes for FET inputs), and their difference is smaller still, which is the link the key draws between (R) and (A). A low offset current is wanted because the difference current flowing through the source resistances makes an unwanted offset voltage at the output.

(A) is therefore true, and (R) is true too. Option B would say (R) is true but unrelated; the key judges it related. Options C and D are ruled out because both statements are true.

Answer: C

The key gives High CMRR.

The LM725 is an instrumentation-grade operational amplifier, intended for amplifying small differential signals from transducers in the presence of large common-mode voltages. Its data sheet stresses low offset, low drift and low noise, with a common-mode rejection ratio of about 120 dB, and textbook lists of special-purpose op-amps place it among the instrumentation amplifiers chosen for high CMRR.

It does have a large open-loop gain, but so does almost every op-amp, so 'very high gain' does not distinguish it. It is a slow, internally limited precision device, so neither very high bandwidth nor high slew rate is its advantage; those belong to high-speed op-amps.