- 2071 Magh · 5 marks
Amplifier vs oscillator in terms of stability factor
Short answer
Amplifiers require unconditional stability, K > 1 and |Δ| < 1, with positive port resistances; oscillators need K < 1 and negative resistance. Amplifier terminations are chosen in the stable region, while an oscillator operates in the unstable region and satisfies ΓTΓin = 1 for oscillation.
Discuss the difference between an amplifier circuit and an oscillator circuit in terms of stability factor.
An amplifier and an oscillator use the same active device, but they need opposite stability conditions: an amplifier must never oscillate, while an oscillator is deliberately made unstable at one frequency.
Stability is measured by the Rollett factor and Δ:
Δ = S11S22 − S12S21
K = (1 − |S11|² − |S22|² + |Δ|²)/(2|S12S21|)
| Point | Amplifier | Oscillator |
|---|---|---|
| Required K | K > 1 (or μ > 1) | K < 1 |
| Δ | Magnitude of Δ < 1 | Magnitude of Δ > 1 often helps |
| Port reflection | Γin, Γout magnitude < 1 | Γin magnitude > 1 (negative R) |
| Input/output R | Positive | Negative at port |
| Γs, ΓL choice | In stable region | In unstable region |
| Feedback | Avoided/neutralised | Positive feedback added |
| Condition | No output without an input signal | Output with no input: ΓT·Γin = 1 |
Amplifier: if K < 1, stability circles are drawn and Γs, ΓL are chosen in the stable region; resistive loading may be added. The aim is GT with no oscillation at any frequency.
Oscillator: a transistor with K < 1 is chosen, or made so by adding series/shunt feedback (e.g. inductor in the common terminal). The terminating network ΓT is chosen inside the unstable region so that Rin < 0 and the oscillation condition holds:
Rin + RL = 0 and Xin + XL = 0
(start-up: |Rin| > RL, about 3×)
So the stability factor is a "design for stability" target in an amplifier and a "design for instability" target in an oscillator.
✓ Answer checked by Claude Opus 5.5 (high effort)
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