5.1 Introduction – The transistor at Radio Frequency
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5.1.5 Gain
The gain that we are normally interested in for RF transistors is the power gain of the device,
rather than just the voltage or current gain. It is power gain that is important because of the
numerous impedance levels which are encountered in RF circuitry. When an impedance level
changes in a circuit, voltage and current gains alone no longer mean anything. Even a passive device
can produce a voltage or current gain but it cannot produce both simultaneously. That is what
transistors are for, to produce power gain.
The power gain of a transistor typically resembles a curve similar to that shown in Figure
5.1-7. This curve makes sense when we consider the equivalent transistor circuit of Figure 5.1-4.
Notice that what we have, in effect, is an RC low-pass filter with a gain which must fall off (neglecting
lead inductance) at the rate of 6 dB per octave. The maximum frequency at which the transistor
provides a power gain is called
. The gain curve passes through at 0 dB (i.e. Gain=1), and at
the rate of 6 dB per octave.
Figure 5.1-7 Gain Vs frequency for a transistor
Frequency
Power
Gain
(dB)
6dB/octave
max
f
Conquer Radio Frequency
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