Small Signal Amplifiers Questions and Answers

Answer: C

Dividing the load by the intrinsic emitter resistance:

\[ A_{v} = -\frac{4700}{26} \approx -180 \]

giving about -180. The figure depends on the operating current through that 26 ohms, so it moves with temperature and with any drift in the bias - which is exactly the unpredictability that degeneration is added to remove.

Answer: D

The external resistance now dominates the denominator:

\[ A_{v} \approx -\frac{4700}{470+26} \approx -9.5 \]

so about -9.5, close to the simple ratio of ten. Nearly twenty times the gain has been traded for a figure set by two resistors, which will hold across devices and temperatures. Whether that trade is worth making depends entirely on how much gain the design has to spare.

Answer: A

Looking into the base gives beta times the emitter resistance, and the bias divider appears in parallel with it - often dominating, since a stiff divider is deliberately made low. The third option applies when the emitter resistor is unbypassed. Remembering that the bias network is part of the signal path is what stops input impedance being overestimated by a wide margin.

Answer: B

The reflected quantity is now the external resistor plus the intrinsic one, so the input impedance rises by roughly the same factor as the gain falls. It appears in series with the emitter rather than in parallel with the input, and it is certainly inside the reflected path. That gain and input impedance move in opposite directions under series feedback is a general result, and it is often the reason degeneration is added in the first place. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: C

The device's own output resistance is far higher than a practical collector resistor, so the load resistor dominates and is essentially the output impedance. The other options belong to the input side or to the emitter circuit. A high output impedance is why a common-emitter stage cannot drive a low-impedance load directly, and why a follower so often appears after it.

Answer: D

The gain is the emitter load divided by that load plus the intrinsic emitter resistance, so just below one and non-inverting. It cannot exceed unity, which rules out the second option immediately. There is no inversion, so the fourth is wrong in sign as well. A stage whose gain can be stated as a number rather than an expression is usually one dominated by feedback, and a follower has as much of that as a circuit can have. The shared properties explain why these schemes form a family, and the differences explain why more than one exists. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: A

The whole output voltage is fed back in series with the input, so the feedback factor is essentially one and the gain is pinned just below unity whatever the device does. The emitter resistance is small but not negligible, and the collector's role and the base current are beside the point. Complete feedback is what makes a gain of one the most reliable gain in electronics, and it is why followers appear wherever fidelity matters more than amplification.

Answer: B

The intrinsic resistance is thirteen ohms at two milliamps:

\[ A_{v} = \frac{1000}{1000+13} \approx 0.987 \]

giving about 0.987. A value above one is impossible for a follower, which rules out 1.013 without any calculation. Checking a computed gain against the physical bound the topology imposes is a quick and reliable sanity test.

Answer: C

With the base common, the collector-base capacitance no longer bridges input and output across an inverting gain, so there is no Miller multiplication. The capacitance itself is the same device property in every configuration. The low input impedance helps but is not the mechanism, and the current gain is unrelated. Bandwidth is nearly always about which capacitance sees which gain, rather than about the size of the capacitance. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: D

Driving the emitter directly presents the intrinsic emitter resistance, tens of ohms at typical currents, with no beta multiplication because the base is not the input. The second option is the common-emitter figure. A very low input impedance is a good match to a transmission line or to a current-source drive, which is why the configuration survives in high-frequency and cascode arrangements. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.