2020 A/L Physics Past Paper · Question 15 (English Medium)

Answer: (5) When an operational amplifier (op-amp) is used as a voltage comparator, it is used in the closed-loop configuration.

What the question asks

All five statements must be classified as "correct" or "not correct"; the question asks for the one that is not correct. Items: (1) an intrinsic semiconductor, (2) a full-wave rectifier, (3) a bipolar junction transistor, (4) a JFET, (5) an op-amp comparator.

Relevant theory

When the temperature of an intrinsic semiconductor is increased the intrinsic carrier concentration of it (ni) also increases, as more bonds are broken. Therefore the resistivity of it decreases and hence the temperature coefficient of resistance of the semiconductor is negative (-).

In both of the above rectifier circuits the output current is flowing along only one direction (DC) but the relevant output voltage is a series of pulses varying from zero to Vm.

The base region in the middle is a very thin layer of semiconductor and it has been doped with a very low doping level.

The collector region is doped with a moderate level and its area has to be made large.

ID has come to its maximum saturation on this occasion (when VGS = 0) and the particular ID is called the maximum saturation current which is denoted by IDSS (Figure 2.43 (b)).

The pinch - off voltage (VP) and the maximum saturation current at pinch - off state (IDSS) are constant for a given FET.

This happens practically as the linear region of the open loop op-amp is very much narrow.

Therefore, the op-amp compares the input voltage V1 and V2 and gives two output levels as, if V1 > V2 then V0 = + VS if V1 < V2 then V0 = – VS

What each statement is testing: (1) intrinsic carrier concentration rises with temperature; (2) a rectified output is a pulsing direct current; (3) by elimination — the base is lightly doped, the collector moderately, so the emitter (which must emit the most carriers) is the most heavily doped; (4) $I_{DSS}$ ($V_{GS} = 0$) is the largest drain current; (5) a comparator has no feedback — the open loop's very large gain pushes even a tiny difference input to $\pm V_S$.

Reading the options

Each statement is classified against the behaviour of its device.

(1) Intrinsic conductivity rises with temperature

Correct

Bonds break, $n_i$ rises, resistance falls

(2) A full-wave rectifier alone gives no steady d.c.

Output pulses from $0$ to $V_m$; smoothing is needed

(3) The emitter is doped more heavily than the collector

Emitter heaviest; collector moderate; base lightest

(4) $I_D$ is largest at $V_{GS} = 0$

$I_{DSS}$ — the largest $I_D$ under any condition

(5) A comparator is used closed-loop

Not correct

A comparator has no feedback; open loop with two output levels $\pm V_S$

Comparing the full classification with the options: (1)-(4) are correct; only (5) fits the "not correct" category — option (5).

Why the other options fail

A correct statement: intrinsic carriers increase with temperature.

A correct statement: a rectified output is a pulsing series.

A correct statement: the emitter is the most heavily doped of the three.

A correct statement: $I_{DSS}$ is the largest current.

Conclusion

A comparator works in the open loop; "used closed-loop" in (5) is not correct — the answer is (5).

Assuming closed-loop feedback is needed for every op-amp circuit.

Amplifiers need feedback; a comparator does not — its output is only $+V_S$ or $-V_S$.