RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 296
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
In many cases, the amplifier may be conditionally stable outside the operational
frequency range. If the low noise amplifier is used at the input of a receiver, filters may
be required to prevent unwanted input signals from passing through the filter. Using
conventional filters is likely to make the amplifier less stable. However using
reflectionless filters, as described in chapter 7, or diplexers as used in example 2.2, to the
input and/or output may make the amplifier more stable. The impedance looking into
port 1 for both the circuits in figure 8.25 is exactly 50 Ω at all frequencies. The BGB707
MMIC at 3 V and 6 mA of figure 8.7, 8.9 and 8.10, for frequencies between 57 MHz and
457 MHz, is only stable if the input VSWR is <1.2. If this amplifier is to be used above
500 MHz, then a reflectionless high pass filter or diplexer with a 300 MHz cut off
frequency will ensure signals above 500 MHz are passed amplified without loss and will
have the same NF.
Figure 8.26. Stability circles with a reflectionless filter at the output.
Figure 8.27. Gain and NF of amplifier with reflectionless filters.
RF Electronics: Design and Simulation
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