Radio Frequency Integrated Circuit Design - Webs

276 Radio Frequency Integrated Circuit Design
Eventually, with increasing signal amplitude, the effective g m will decrease to
the point where it just compensates for the losses in the circuit and the amplitude
of the oscillator will stabilize.
The saturation and cutoff linearity constraint will also put a practical limit
on the maximum power that can be obtained from an oscillator. After reaching
this limit, increasing the bias current will have very little effect on the output
swing. Although increasing the current causes the small-signal g m to rise, this
just tends to ‘‘square up’’ the signal rather than to increase its amplitude.
Looking at the common-base or common-collector Colpitts oscillators as
shown in Figure 8.10, it can be seen how this effect works on the circuit
waveforms in Figures 8.28 and 8.29. In the case of the common-base circuit,
when vc is at the bottom of its swing, v ce tends to be very small, causing the
base collector junction to be forward biased. This also tends to make v be quite
large. These two conditions together cause the transistor to go into saturation.
When vc reaches the top of its swing, v be gets very small and this drives the
Figure 8.28 Waveforms for a common-collector oscillator that is heavily voltage-limited.
Figure 8.29 Waveforms for a common-base oscillator that is heavily voltage-limited.