1 HF power gains and f max LECTURE 19 Clarification of power gains: MAG, MSG, U

2 Y S conjugately matched to Y in Y L conjugately matched to Y out MAG: Maximum Available Gain ρ is real part, and γ is any, of y-, z-, s-, etc MAG

3 MAG: lower-level equation

4 MAG: oscillation MAG When MAG > 1, device is active; with appropriate feedback, an output could be maintained with no input – an oscillator. When MAG < 1, device is passive. The frequency at which MAG =1 is the maximum frequency at which oscillation can occur*, and is called f max. f max is determined by extrapolation from lower frequencies at -20dB/decade. BUT what if the FET is not stable? * with passive feedback

5Stability MAG can also be written as: ≡ where k is the stability factor MAG If k<1, the MAG is complex and unrealistic.

6 MAG > 1 (and real, i.e., k ≥ 1) is needed for an amplifier. But, also, no oscillation is wanted. If both of the above can be obtained, with no external feedback, the device is inherently stable. Inherent stability Maximum Stable Gain If stability can only be obtained by incorporating external feedback and non-conjugately matching terminations, then the Maximum Stable Gain is realized when k=1. MAG MSG

7 Example of MSG

8 Unilateral gain In Section of the text, Z in is computed, for conjugate-matching purposes, by assuming z 32 ≈ 0. When ρ 32 = 0 for the transistor and any associated network, the system is unilateralized. Obviously falls-off at -20dB/decade. The extrapolated f max is

9 Mason’s unilateral gain Quote from S.J. Mason, Trans. IRE, vol. CT-1, 20-25, 1954

10 Mason’s unilateral gain: results