Radio Frequency Integrated Circuit Design - Webs

Voltage-Controlled Oscillators
oscillator into cutoff. A similar argument can be made for the common-collector
circuit, except that it enters cutoff at the bottom of its swing and saturation at
the top of its swing.
8.15 Bias Shifting During Startup
Once the oscillator starts to experience nonlinearity, harmonics start to appear.
The even-ordered harmonics, if present, can cause shifts in bias conditions since
they are not symmetric. They have no negative-going swing so they can change
the average voltage or current at a node. Thus, they tend to raise the voltage
at any node with signal swing on it, and after startup, bias conditions may shift
significantly from what would be predicted by a purely dc analysis. For instance,
the voltage at the emitter of the common-base or common-collector Colpitts
oscillators will tend to rise. Another very good example of this is the −Gm
oscillator with resistive tail as shown in Figure 8.26. The node connected to
R tail is a virtual ground; however, there is strong second-harmonic content on
this node that tends to raise the average voltage level and the current through
the oscillator after startup.
8.16 Oscillator Amplitude
If the oscillator satisfies the conditions for oscillation, then oscillations will
continue to grow until the transistor nonlinearities reduce the gain until the
losses and the negative resistance are of equal value.
For a quantitative analysis of oscillation amplitude, we first start with a
transistor being driven by a large sinusoidal voltage, as shown in Figure 8.30.
Figure 8.30 Transistor driven by a large sinusoidal voltage source.
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