HF power gains and f max
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1 HF power gains and f HF power gains and f max max LECTURE 19 • Clarification of power gains: MAG, MSG, U
description
HF power gains and f max. LECTURE 19 Clarification of power gains: MAG, MSG, U. MAG: Maximum Available Gain. Y S conjugately matched to Y in Y L conjugately matched to Y out. MAG. ρ is real part, and γ is any, of y-, z-, s-, etc. MAG: lower-level equation. MAG: oscillation. MAG. - PowerPoint PPT Presentation
Transcript of HF power gains and f max
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HF power gains and fHF power gains and fmaxmax
LECTURE 19
• Clarification of power gains: MAG, MSG, U
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YS conjugately matched to Yin
YL conjugately matched to Yout
MAG: Maximum Available GainMAG: Maximum Available Gain
Sout
Lin
yy
yyy
v
iY
yy
yyy
v
iY
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211222
2
2
22
211211
1
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ρ is real part, and γ is any, of y-, z-, s-, etc
MAG
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MAG: lower-level equationMAG: lower-level equation
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MAG: oscillationMAG: 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 fmax.
fmax is determined by extrapolation from lower frequencies at -20dB/decade.
BUT what if the FET is not stable?
* with passive feedback
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StabilityStability
MAG can also be written as:
≡
where k is the stability factor
MAG
If k<1, the MAG is complex and unrealistic.
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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 stabilityInherent stability
Maximum Stable GainMaximum 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
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Example of MSGExample of MSG
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Unilateral gainUnilateral gain
In Section 14.4.1 of the text , Zin is computed, for conjugate-matching purposes, by assuming z32 ≈ 0.
When ρ32 = 0 for the transistor and any associated network, the system is unilateralized.
ggggd
mBC RCC
gU
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Obviously falls-off at -20dB/decade.
The extrapolated fmax is
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Mason’s unilateral gainMason’s unilateral gain
Quote from S.J. Mason, Trans. IRE, vol. CT-1, 20-25, 1954
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Mason’s unilateral gain: Mason’s unilateral gain: resultsresults
