Op-Amp Parameters Questions and Answers

Answer: C

The key takes option (C): very high input resistance differential stages are used in an op-amp to minimise the input offset current. The input of the amplifier draws a small base current, and the input offset current is the difference between the two input bias currents. A very high input resistance, which comes from high-beta transistors or a FET input, keeps these currents very small and so reduces the offset current. Option (A), input offset voltage, depends on the mismatch of the transistors of the differential pair. Options (B) and (D) refer to the offsets at the output, which are set by the gain and by the circuit that follows.

Answer: C

The assertion is true: the offset voltage and currents of an op-amp are caused by the mismatch of the transistors in the differential input stage, and they drift with time and temperature. The key regards the reason as false, because offsets cause errors in dc signals and the output offset appears as a dc shift, and they are not described as introducing noise in a.c. signals, whose dc component can be blocked by coupling capacitors. So the reason does not state why offsets must be removed. The answer is that (A) is true and (R) is false.

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.

Answer: C

Statements 1, 3 and 4 are correct, the third option. For an ideal op-amp the power supply rejection ratio is taken as zero, in the sense that supply changes have no effect on the output, the thermal drift of the offset voltage is stated in \(\mu\)V/\(^\circ\)C, and the slew rate is stated in V/\(\mu\)s.

Statement 2 is wrong because the thermal drift is given in microvolts per degree, and a figure in amperes per degree is the drift of a current, which is not the drift defined in the question. So the options with 2 are wrong.

Answer: A

The correct match is a-iii, b-i, c-ii, d-iv, the first option. The alpha of a transistor is less than one, since the collector current is a little smaller than the emitter current, and the beta is greater than one. The common mode rejection ratio of an ideal op-amp is infinite, and the power supply rejection ratio is zero.

The other options exchange alpha and beta, or give the CMRR a value other than infinity, which does not agree with these definitions.