Assertion (A) : The series pass transistor in a regulator is in class A mode. Reason (R) : Class A is a switching mode and yields high efficiency.
(A) is true, but (R) is false.
The assertion is correct; the reason is wrong twice over — class A is not a switching mode, and it has the lowest efficiency of all the classes. The answer is option 3.
Why the pass transistor is class A. In a linear regulator the series transistor conducts continuously, for the whole cycle, adjusting its resistance to absorb whatever voltage the input has in excess of the wanted output. Conducting for 360° of the cycle is the definition of class A operation.
| Class | Conduction angle | Maximum efficiency |
|---|---|---|
| A | 360° | 25 % (50 % transformer-coupled) |
| B | 180° | 78.5 % |
| AB | 180° – 360° | Between the two |
| C | < 180° | > 80 % |
| D | Switching | > 90 % |
The reason confuses class A with class D. A switching mode is one in which the device is either fully on or fully off, never in between — class D. There, dissipation is small in both states because voltage or current is near zero:
\(P=V\times I\approx0\)
Class A is the exact opposite. The device sits in the middle of its active region, carrying substantial current with substantial voltage across it, so \(VI\) is large at all times. That is why class A dissipates most when there is no signal at all.
The consequence for a linear regulator follows immediately:
\(P_{diss}=\left(V_{in}-V_{out}\right)I_{L}\qquad \eta=\dfrac{V_{out}}{V_{in}}\)
A 12 V input regulated to 5 V at 1 A wastes 7 W as heat to deliver 5 W — 42 % efficient, and needing a substantial heat sink.
Why linear regulators survive nonetheless. The class A pass transistor is a continuously variable resistor with no switching edges, so it generates no switching noise and needs no output inductor. Its noise and ripple are orders of magnitude below a switcher's, and its transient response is fast — which is why sensitive analogue and RF circuits are still fed from linear regulators, often placed after a switching pre-regulator that removes most of the excess voltage first.
Hence, (A) is true, but (R) is false.
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