Small Signal Amplifiers Questions and Answers

Answer: A

Power gain is the product of voltage and current gain, which is why common emitter beats the other two configurations on that measure: common base has no current gain and common collector no voltage gain. The other combinations describe the intrinsic gain, an impedance ratio and the gain-bandwidth product. Which figure of merit to quote depends on what the stage is for, and power gain is the one that ranks the three configurations most clearly.

Answer: B

Common emitter offers both voltage and current gain, so their product is the largest of the three - typically tens of thousands. Common base gives voltage gain with a current gain just under one, and common collector the reverse, so each is limited to roughly the one gain it has. That the default choice for a general amplifier stage is also the one with the highest power gain is no accident, and it is the reason it became the default.

Answer: C

The first two are correct and the third is the reverse: a current source needs some voltage across it to remain a current source, so headroom is lost rather than gained. The gain and area benefits are real and are why the technique dominates integrated design. A statement that reverses the cost of a technique into a benefit is worth checking against what the component physically requires.

Answer: D

The first is true and the second is false: the gain rises because the load impedance is high, and the swing falls because the current source needs headroom of its own. The two move in opposite directions, which is exactly the trade the technique makes. Being able to name both directions of a change is what distinguishes understanding a technique from having heard of it.

Answer: A

A higher gain per device is not among them - each half of a pair gives about the same gain as a single stage, and the differential gain is comparable rather than larger. The other three are the real advantages, all following from the symmetry. A topology adopted for its rejection and stability rather than for raw gain is worth recognising as such, since expecting extra gain from it leads to disappointment.

Answer: D

The correct matching is a-iv, b-iii, c-ii, d-i.

The CE hybrid parameters are defined with either the output voltage held constant (short-circuit parameters) or the input current held constant (open-circuit parameters):

• \(h_{fe} = \Delta i_{c}/\Delta i_{b}\) at constant \(V_{ce}\): forward current gain (iv).
• \(h_{oe} = \Delta i_{c}/\Delta v_{ce}\) at constant \(I_{b}\): output admittance (iii).
• \(h_{ie} = \Delta v_{be}/\Delta i_{b}\) at constant \(V_{ce}\): input impedance (ii).
• \(h_{re} = \Delta v_{be}/\Delta v_{ce}\) at constant \(I_{B}\): reverse voltage ratio (i).

The other codes swap the output admittance with the input impedance or the reverse ratio, which is easy to check from the units: only h_oe is a current over a voltage.

Answer: D

The correct matching is a-iv, b-i, c-iii, d-ii.

Decibels use 20 log for voltage ratios and 10 log for power ratios:

• Voltage gain 10: \(20 \log 10 = 20\) dB (iv).
• Power gain 10: \(10 \log 10 = 10\) dB (i).
• Power gain 1000: \(10 \log 1000 = 30\) dB (iii).
• Voltage gain 100: \(20 \log 100 = 40\) dB (ii).

The codes pairing voltage gain 10 with 10 dB use the power formula for a voltage ratio, and the one pairing power gain 1000 with 40 dB mixes the two formulas in the other direction.

Answer: C

The keyed matching is a-iii, b-ii, c-iv, d-i.

• Common emitter: moderate input resistance, about 1 kΩ (iii).
• Common collector: low output resistance (ii).
• Common base: low input resistance, a few tens of ohms (iv).
• Emitter follower: high input resistance (i).

The common collector amplifier and the emitter follower are the same circuit, which has both a high input resistance and a low output resistance; the question assigns one of these properties to each name. With that, only this code works: the others give the common emitter a high or low input resistance, or the common base a high or moderate one.

Answer: A

The matching is a-iii, b-iv, c-i, d-ii: voltage gain high in CB, current gain low in CB, power gain very high in CE, output resistance very low in CC.

The three transistor configurations behave differently. In common base the current gain is just under 1 (low) while the voltage gain is high. In common emitter both current and voltage gain are high, so the power gain is the largest of the three. In common collector (the emitter follower) the voltage gain is slightly below 1 but the current gain is high, the input resistance is high and the output resistance is very low.

Code D is the nearest rival: it pairs voltage gain with 'very low in CC' and output resistance with 'high in CB'. The CB output resistance is indeed high, but the CC voltage gain is close to unity, not very low, so the pairing of (a) fails. Codes B and C put the current gain or the output resistance against the wrong configuration.

Answer: D

The suitable device is a phase sensitive amplifier.

A signal a million times smaller than the noise cannot be recovered by amplification or ordinary filtering. A phase sensitive (lock-in) amplifier modulates or references the signal at a known frequency, multiplies the input by a reference at that frequency and phase, and averages the product over a long time. The signal gives a steady output while noise at other frequencies and phases averages towards zero, giving an extremely narrow effective bandwidth.

A high-gain linear amplifier amplifies signal and noise alike. A band pass filter cannot be made narrow enough to pull out a microvolt from a volt of noise, and a notch filter removes one frequency, which is the opposite of what is needed.