Explain, with a block diagram, how an amplifier and a feedback circuit together give sustained oscillations, and name an oscillator that uses an LC tank circuit.

1. 1 · Define it. An oscillator produces continuous AC oscillations of a set frequency without any input signal.
2. 2 · Explain the tank circuit. In an LC circuit, energy swaps between C and L, but losses make the oscillations die away.
3. 3 · Explain feedback. An amplifier (gain A) with feedback (factor β) returns part of the output to the input to make up the losses.
4. 4 · Draw the block diagram. [Block diagram: amplifier box A with input Vi and output Vo; feedback box β from the output back to the input; Aβ = 1 written]
5. 5 · State the condition. Aβ = 1, with 180° from the CE amplifier and 180° from the feedback: positive feedback.
6. 6 · Close in one line. So, a Colpitts oscillator, with L, C1 and C2, gives sustained oscillations at f = 1/(2π√(LC)).

Scoring tip: Write both parts of the condition: Aβ = 1 and a total phase shift of 360°. Many answers miss the phase.