Oscillators Questions and Answers

Answer: C

Oscillation grows only where the phase condition and the gain condition are satisfied at the same frequency, and the frequency-determining network arranges for that coincidence at one point. The amplifier is far broader, the non-linearity acts on amplitude, and the transient is broadband. It is the coincidence rather than either condition alone that selects, which is why a sharp phase characteristic gives a precisely defined frequency.

Answer: D

A Wien bridge whose amplitude is regulated by a lamp or a slow control loop never clips, so the output stays a clean sine with distortion in the hundredths of a per cent. A multivibrator and a unijunction produce square and sawtooth waveforms by construction, and a clipping Colpitts carries strong harmonics. The limiting mechanism decides the purity, so a topology plus its amplitude control has to be named together before any distortion figure means anything.

Answer: A

Both are true and the second is the reason: a steeper phase characteristic means any phase disturbance in the loop is corrected by a far smaller frequency excursion. That is the whole of the stability argument, and it explains why raising the quality factor is the only reliable route to a better frequency source. Every other improvement, from oven control to careful buffering, addresses a particular disturbance rather than the loop's sensitivity to disturbance in general.

Answer: B

The reason states the criterion correctly, but the assertion misapplies it: a loop gain of exactly one would sustain an existing amplitude and could never build one from noise, so a practical oscillator starts with a loop gain above one and is pulled back to unity by a non-linearity. The criterion describes the steady state rather than the starting condition. Distinguishing the condition for sustaining from the condition for growing is the central practical point in oscillator design.

Answer: C

Both hold and the second is the mechanism: the transistor's junction capacitances drift with temperature and bias, but a small series capacitor with a good coefficient decides the frequency and their contribution is negligible beside it. Making the frequency depend on the most stable available component, rather than on a sum including unstable ones, is a general technique and this is its clearest application in oscillator design.

Answer: D

The reason is true but the assertion is false: a higher supply charges the capacitor faster and also raises the voltage it must reach, so the two effects largely cancel and the period is nearly independent of supply. That cancellation is why the circuit works acceptably without a regulated supply. A mechanism that appears twice with opposite signs is worth tracing before concluding that a quantity matters.

Answer: A

All three are correct, and the third is the one most often left unstated even though it does the real work: it is the coincidence of the two conditions at one frequency that picks the oscillation out of the noise. A circuit meeting the gain condition at one frequency and the phase condition at another will not oscillate at either. Stating a criterion completely, including the conditions under which its parts must hold, is what makes it usable rather than merely quotable.

Answer: B

The first two are correct and the third understates by orders of magnitude: a crystal's quality factor runs from tens of thousands to over a million, while a few hundred is what a good LC tank achieves. That gap is the entire reason crystals are used, so getting the magnitude right matters more than the exact figure. A statement that is qualitatively right and quantitatively wrong by four orders of magnitude is still wrong in the way that counts.

Answer: C

The first is true and the second is false: the Wien network gives zero phase shift so a non-inverting stage completes the loop, while the phase shift network gives a hundred and eighty degrees and needs an inverting stage to supply the rest. Whether a network shifts by zero or by a half turn decides which kind of amplifier it must be paired with, and that pairing is the first thing to establish about any oscillator topology.

Answer: D

The pairing is A-P, B-Q, C-R. All three are the same amplifier and tank with three ways of extracting a fraction of the tank's signal, which is why they share a frequency formula and differ only in how the fraction is set. Recognising a family as one circuit with one element varied is far more economical than learning three circuits.