Oscillators Questions and Answers

Answer: A

The frequency of any oscillator is unstable, in the sense that it drifts.

The passage opens: an oscillator having initially been set at a particular frequency will generally drift and wander about in frequency. It then explains that the deviations arise because circuit components, transistor parameters, supply voltage and stray capacitances do not stay constant. So an oscillator's frequency is never perfectly fixed.

'Constant' contradicts the passage. 'Not dependent on component selection' is wrong because the frequency depends on a few critical features, as the passage's later paragraph says. 'Decided by semiconductor device only' is wrong because the passage lists many causes (components, supply, stray elements) beyond the transistor.

Answer: B

The false statement is 'The tuned circuit oscillator is highly unstable'.

The passage ends by saying that a tuned circuit oscillator will have excellent frequency stability, because in a parallel resonant circuit with a high Q the change in phase at resonance is abrupt. So calling it highly unstable is the opposite of what the passage states.

The other three statements agree with the passage. An oscillator does not give a perfectly constant frequency, transistor parameters are described as inherently unstable, and stray circuit elements and couplings are listed among the things that disturb the frequency.

Answer: C

An effective solution is to choose a circuit configuration which does not have a large number of critical features.

The passage first calls keeping every feature constant an obvious but useless answer, because there are too many features and some, like stray elements, cannot even be located. It then says that in every oscillator only a few features set the frequency sensitively, so a circuit whose frequency depends on few critical features is easier to keep stable.

Switching off the circuit removes the oscillation altogether. Finding and bypassing every stray component is the impractical route the passage rejects, since their locations and sizes are hard to know. Making the impedance high is not mentioned and does not remove the dependence on critical features.

Answer: C

The frequency of a phase shift oscillator depends primarily on R and C only.

The passage gives this as its example: the frequency of a phase shift oscillator primarily depends on R and C and other parameters do not affect it too much. The resistors and capacitors of the phase-shifting network set the frequency at which the network supplies the required phase shift.

'Depends heavily on parasitic components' is the opposite of the passage. 'Decided by amplifier gain' is wrong because gain sets whether the oscillation is sustained, not the frequency, and the passage names R and C. 'Independent of temperature, current etc.' is the claim the passage makes for the tuned circuit oscillator, not the phase shift oscillator.

Answer: B

The tuned oscillator has excellent frequency stability because the change in phase is abrupt due to high Q.

In a parallel resonant circuit the impedance changes from inductive to capacitive as the frequency passes through resonance, and if Q is very high that phase change is abrupt. The oscillation can only occur at the frequency where the loop phase is right, so any small drift is pulled back and the frequency stays tightly fixed. The passage concludes that a tuned oscillator therefore has excellent stability.

The frequency of a tuned oscillator is set by L and C, not by R and C, so the first option describes the phase shift oscillator. The passage does not say the supply voltage is highly stable, and large stray capacitances would harm stability, not help it.

Answer: A

The Barkhausen criterion demands a loop gain of one in magnitude and a total phase shift around the loop of zero, or equivalently a whole number of full turns. Both must hold at the same frequency, and it is that coincidence which picks out the frequency of oscillation. Positive feedback and a resonant element are how the conditions are usually met rather than the conditions themselves, and bandwidth is a property of the amplifier. Two conditions at one frequency is the whole of oscillator theory, and every topology in this subject is an arrangement for satisfying them.

Answer: B

A loop gain of exactly one would sustain whatever amplitude already existed but could never build one, so a practical oscillator is designed with a loop gain somewhat above one and grows from circuit noise until something limits it. The amplifier's gain is constant until the amplitude limiting acts, the phase condition is amplitude-independent, and the supply's rise is incidental. Designing a loop to be marginally unstable, and then arranging what stops it growing, is the practical reading of the criterion.

Answer: C

Every circuit contains thermal and shot noise across a wide band, and the supply's own step excites the resonant circuit as well - the loop selects whatever component lies at its frequency and amplifies it repeatedly until limiting sets in. No injected pulse is needed, nothing is stored between runs, and offset is a direct quantity rather than a signal. That an oscillator needs no starting signal, only the absence of anything preventing growth, is what makes an unwanted oscillation so hard to suppress once a loop meets the conditions.

Answer: D

Growth continues until a non-linearity - clipping at the supply rails, a lamp's rising resistance, a diode network, or an automatic gain control loop - pulls the average loop gain back to exactly one. The phase condition is unaffected by amplitude, and neither supply current nor tank losses provide the regulation. Which non-linearity is used decides how clean the output is: hard clipping gives a distorted waveform, while a gentle amplitude control preserves the sine.

Answer: A

A small filament lamp heats as the amplitude grows, its resistance rises, and the negative feedback increases until the loop gain settles at exactly one - a gentle amplitude control that limits without ever clipping, so the output stays a clean sine. Its thermal inertia is what makes the control slow enough not to distort the waveform, though that is a consequence rather than the mechanism. Harmonics are what the arrangement avoids, and the frequency is set by the RC network. Regulating amplitude smoothly rather than clipping it is the difference between a laboratory oscillator and a squarewave generator.