An amplifier reproduces an enlarged copy of a signal applied to it; an oscillator manufactures a periodic signal by itself. Test each statement against that distinction.

A — "Oscillators do not have any input voltage." TRUE. An oscillator has only a d.c. supply and no external signal input. Oscillation builds up from the circuit's own thermal noise or a power-supply transient: the noise component at the design frequency is fed back in phase, is amplified round the loop, and grows until the amplifier's non-linearity limits the amplitude.

B — "Amplifiers do not have any input voltage." FALSE. With no input, an amplifier's output is just its quiescent d.c. level; it produces an output only in response to an applied signal.

C — "Oscillators do not have positive feedback." FALSE. Positive (regenerative) feedback is exactly what an oscillator needs. Sustained oscillation requires the Barkhausen criterion:

\(|A\beta| = 1 \qquad \text{and} \qquad \angle A\beta = 0^{\circ}\ (\text{or } 360^{\circ})\)

i.e. unity loop gain with the feedback signal in phase with the input. (For start-up one designs |Aβ| slightly greater than 1.)

D — "Amplifiers do not have positive feedback." TRUE in the sense intended: practical amplifiers use negative feedback, which trades gain for benefits — stabilised gain set by passive components, wider bandwidth, lower distortion, and controlled input/output impedances. Applying positive feedback to an amplifier would push it toward instability, which is precisely how one turns an amplifier into an oscillator.

E — "Both of them are stable circuits." FALSE as a difference. The question asks for differences, and this statement asserts a similarity; moreover an oscillator is deliberately operated at the edge of instability, so it is not "stable" in the control-theory sense at all.

The valid differences are A and D.

Underlying gain relation. With feedback, \(A_f=\dfrac{A}{1\mp A\beta}\): the minus sign (positive feedback) drives the gain toward infinity as Aβ → 1, which is the oscillation condition, while the plus sign (negative feedback) reduces and stabilises the gain, which is the amplifier condition.

Hence, the correct answer is A and D only.