HF power gains and fmax

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HF power gains and fmax

• LECTURE 19
• Clarification of power gains MAG, MSG, U

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MAG Maximum Available Gain
YS conjugately matched to Yin YL conjugately
matched to Yout
MAG
? is real part, and ? is any, of y-, z-, s-, etc
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MAG lower-level equation
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MAG oscillation
MAG
When MAG gt 1, device is active with appropriate
feedback, an output could be maintained with no
input an oscillator. When MAG lt 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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Stability
MAG can also be written as

MAG
where k is the stability factor
If klt1, the MAG is complex and unrealistic.
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Inherent stability
MAG gt 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.
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 k1.
MSG
MAG
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Example of MSG
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Unilateral gain
When ?32 0 for the transistor and any
associated network, the system is unilateralized.
In Section 14.4.1 of the text , Zin is computed,
for conjugate-matching purposes, by assuming z32
0.
Obviously falls-off at -20dB/decade. The
extrapolated fmax is
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Masons unilateral gain
Quote from S.J. Mason, Trans. IRE, vol. CT-1,
20-25, 1954
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Masons unilateral gain results