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. Fig. 6.14 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.

Figure 6.14: HF circuit diagram of a Colpitts oscillator.

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 V_a 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.

Figure 6.15: Circuit diagram of the simulated Colpitts oscillator.

Table 6.4: Parameters of the lumped circuit elements of the Colpitts oscillator.

element	value
V0	3.5 V
Va	0 V
R1	9 k$\Omega$
R2	11 k$\Omega$
Re	100 $\Omega$
C1	56 pF
C2	5.6 pF
Ck1	100 nF
Ck2	100 nF
L	500 nH
Ld	1 mH