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6.5 Simulation of a Colpitts Oscillator

6.5 Simulation of a Colpitts Oscillator

The Colpitts oscillator is the combination of a transistor amplifier and an LC circuit which is used for feedback of the output signal. A figure shows the circuit diagram without the DC circuit for setting the bias point.
The oscillation frequency is determined not only by the inductance L and the capacities C1 and C2 but also by the base-collector and the base-emitter junction capacities of the distributed device.

The circuit diagram of the simulated Colpitts oscillator in common emitter configuration is shown in Fig. 6.15. The parameters of the lumped circuit elements are listed in Table 6.4.

The ideal voltage source Va is used to monitor the base current and to introduce an excitation at the begin of the transient simulation to start the oscillation. Otherwise the oscillation would be started by numerical noise and could not be reproduced reliably. The first 100 ns of the oscillation were simulated using a constant time step size of 250 ps. Within this time the steady state is reached.

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