Radio Frequency Integrated Circuit Design - Webs

274 Radio Frequency Integrated Circuit Design
A brief circuit description will now be provided. The circuit must be dc
biased at some low current. As the oscillation begins, the voltage rises on one
side of the resonator and one transistor starts to turn off while the other starts
to turn on harder and draw more current. As the transistor draws more current,
more current flows through R tail, and thus the voltage across this resistor starts
to rise. This acts to reduce the v BE of the transistor, which acts as feedback to
limit the current at the top and bottom of the swing. The collector waveforms
are shown conceptually in Figure 8.27. Since the current is varying dynamically
over a cycle, and since the resistor R tail does not require as much headroom as
a current source, this allows a larger oscillation amplitude for a given power
supply.
An alternative to the resistor R tail is to use a noise filter in the tail as
shown in Figure 8.26(b) [6]. While the use of the inductor does require more
chip area, its use can lead to a very low-noise bias, leading to low-phase-noise
designs. Another advantage to using this noise filter is that before startup, the
transistor Q 3 can be biased in saturation, because during startup the second
harmonic will cause a dc bias shift at the collector of Q 3, pulling it out of
saturation and into the active region. Also, since the second harmonic cannot
pass through the inductor L tail, there is no ‘‘ringing’’ at the collector of Q 3,
further reducing its headroom requirement.
8.13 The Effect of Parasitics on the Frequency of Oscillation
The first task in designing an oscillator is to set the frequency of oscillation
and hence set the value of the total inductance and capacitance in the circuit.
To increase output swing, it is usually desirable to make the inductance as large
as possible (this will also make the oscillator less sensitive to parasitic resistance).
However, it should be noted that large monolithic inductors suffer from limited
Figure 8.27 −G m oscillator with resistive tail collector currents.