RF Electronics Chapter 6: Oscillators Page 177
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The most important design parameter is the biasing of the transistor. The IV curve is very
useful in determining the correct biasing conditions. It should be realised that some of the
critical transistor parameters like can vary over a 5:1 ratio between transistors. The
transistor parameters used in the models of the computer simulation software, include
typical values. The design must be such that the oscillator will work under all permissible
variations of transistor parameters. The I-V curve can easily be obtained in MWO as
shown in figure 6.5. For linear biasing, a 10V supply and quiescent collector current of
between 40 and 5 mA is required. 15 mA is a good compromise to minimise the power
dissipation in the transistor and still have sufficient output power.
Figure 6.5. Circuit for determining the I-V characteristic of a transistor.
Step 2: Design an Amplifier
Figure 6.6. Amplifier for the Colpitts oscillator.
A typical amplifier configuration and one that is used for this design is shown figure 6.6.
The first step is to select the bias voltages. For a 10 Volt supply, and having an emitter
follower configuration for the amplifier, the emitter voltage should be close to half of the
supply voltage, minus one volt. An emitter voltage of 4.5 Volt and a resistor R1 = 300 ,
for a quiescent current of 15 mA is thus suitable. This results in a large swing without the
transistor saturating. The current through the biasing chain should be about one tenth of
the quiescent current, i.e. 1.5 mA, for good quiescent stability with temperature. For an
RF Electronics: Design and Simulation
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