Small Signal Amplifiers Questions and Answers

Answer: A

A single pole falls at six decibels per octave, equivalently twenty per decade. Three decibels is the drop at the break frequency itself rather than a rate, twelve per octave is two poles, and twenty per octave is not a standard figure - twenty per decade is the same slope expressed differently. Two ways of stating one slope, and one number that is a point rather than a slope, is exactly the confusion this item is built on. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

A single pole approaches ninety degrees of lag well beyond its break, passing through forty-five degrees at the break frequency itself. One hundred and eighty degrees would require two poles, which is why stability analysis counts them so carefully. Attaching each phase figure to its position relative to the break, rather than learning them as a set, is what makes them usable in a stability argument. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: C

The first two are correct and the third is not: the high-frequency limit comes from junction and stray capacitances shunting signal away, not from series coupling capacitors. A series capacitor blocks low frequencies and a shunt capacitance blocks high ones, which is the distinction to hold. Whether a capacitance is in series or in parallel with the signal path decides which end of the response it affects.

Answer: D

The pairing is A-P, B-Q, C-R. Two series capacitors set low breaks at different frequencies because they work into very different resistances, and one junction capacitance sets the high limit because it shunts the signal. Sorting the capacitances by whether they are in series or in shunt, and by the resistance each works into, accounts for the whole shape of an amplifier's response.

Answer: A

The first three are correctly paired and the fourth is not, so three match. Transconductance is the collector current over the thermal voltage and has nothing to do with beta, which is precisely why the voltage gain is the predictable specification and the input impedance is not. Tracing each specification to the parameters it actually depends on is the quickest way to know which will survive a change of device. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Pinch-off voltage belongs to a field effect transistor, describing where its channel closes; the hybrid-pi model of a bipolar device has no such element. The other three are its core elements, corresponding to the operating current, beta, and the Early effect respectively. Spotting a parameter that presupposes a structure the device does not have is a quick way to keep the two families' models apart. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: C

The factor is one plus beta, because the base supplies only a beta-th of the current at the same voltage. The transconductance and the thermal voltage appear in the gain and in the intrinsic emitter resistance respectively, and the collector load appears in the gain. One rule, applied in two directions, accounts for every input and output impedance in the three configurations, which makes it the most economical thing to learn in the topic.

Answer: D

A black box description makes no assumption about what is inside, so the same four parameters describe a transistor, a valve or a complete amplifier module. It has the same number of parameters as the hybrid-pi model, is equally restricted to small signals, and must be measured rather than derived. Generality bought at the cost of insight is a real trade, and it is why the two-port form survives in measurement and specification even where the physical model is preferred for design.

Answer: A

The forward current ratio is beta in the common-emitter case, and it is the parameter that carries the device's threefold spread. The input impedance corresponds to the reflected emitter resistance, the reverse voltage ratio to the Early effect seen backwards, and the output conductance to the slope of the output characteristics. Mapping each two-port parameter onto its physical counterpart is what makes the black box description intelligible.

Answer: B

The reverse ratio is a few ten-thousandths, so the error from ignoring it is far smaller than the spread in beta and the tolerance of the resistors. It is measurable, does not vanish with bypassing, and applies at all frequencies. Neglecting a term because it is small compared with the uncertainties already present is legitimate, and stating that comparison is what distinguishes a justified approximation from a careless one.