RF Electronics Chapter 8: Amplifiers: Stability, Noise and Gain Page 292
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
to dominate the NF at those frequencies. At high frequencies the noise rises because of
changes in the transistor parameters. The NF in these the regions rises at 40 dB per
decade. In addition the noise voltage changes with collector current and voltage across
the transistor. The NF for the transistor is shown in figure 8.18. Coleman [12] shows how
the NF relates to different transistor parameters.
To start the design, a basic amplifier as shown in figure 8.19 is used. That circuit uses the
dual emitter transistor symbol for the BFP540. The transistor is designed to have the
emitter device pins connected to ground. The design uses the 5 mA collector current and
a V
ce
= 2 V corresponding to the values for which the NF is specified in the device data
sheet [11]. Note that the resulting NF is worse at 1 GHz than shown in figure 8.18.
The NF and gain can now be calculated using computer simulation and they are shown
in figure 8.20. The rise in NF at low frequency is due to the input coupling capacitor
causing an impedance mismatch at low frequency. The rise in NF at high frequency is
due to changes in transistor parameters and corresponds to a drop in gain. The supply
voltage, emitter resistor and bias resistor values are tuned, to obtain the best NF. The
values shown in figure 8.19 correspond to the lowest NF consistent with a high power
gain.
Figure 8.19. Basic low noise amplifier.
Figure 8.20. NF and gain of the amplifier.
RF Electronics: Design and Simulation
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