Amplifier and Oscillator Circuits Flashcards

An RF amplifier has an input signal of 2 mV and produces an output of 400 mV. What is the voltage gain?

Answer: 200

Voltage gain = Vout / Vin = 400 mV / 2 mV = 200.

A voltage gain of 100 expressed in decibels (dB) equals:

Answer: 40 dB

Voltage gain in dB = 20 × log₁₀(100) = 20 × 2 = 40 dB.

For maximum power transfer between an amplifier's output and the following stage, the source impedance should be:

Answer: Equal to the load (input) impedance of the following stage

The maximum power transfer theorem states that maximum power is delivered when the source impedance equals the load impedance — this is the principle behind impedance matching networks.

The formula used to calculate the resonant frequency of a parallel LC tank circuit is:

Answer: f = 1 / (2π√(LC))

The resonant frequency is f = 1/(2π√LC), where L is inductance in henries and C is capacitance in farads; increasing either L or C lowers the resonant frequency.

A high Q factor in a tuned tank circuit indicates:

Answer: Low energy loss per cycle with narrow bandwidth and high selectivity

The quality factor Q represents the ratio of energy stored to energy lost per cycle; a high Q means low loss, a sharp resonance peak, and narrow bandwidth.

If a feedback network returns 10% of an oscillator's output voltage back to the input, the minimum voltage gain required to sustain oscillation is:

Answer: 10

The Barkhausen condition requires Aβ = 1; with β = 0.1 (10% feedback), the minimum required gain is A = 1/β = 1/0.1 = 10.

In a common-emitter transistor amplifier, the phase relationship between the input and output signals is:

Answer: 180° out of phase — signal is inverted

The common-emitter configuration inverts the signal — a positive-going base input produces a negative-going collector output — resulting in exactly 180° phase inversion.