TABLE OF CONTENTS Several refinements can be made to the - G m oscillator to improve its performance. As mentioned in the previous section, for bipolar negative G m oscillators, to increase signal swing beyond V BE, the collector and base of the switching transistors can be decoupled at dc with capacitors or transformers. In either case, proper dc bias must be supplied to the base, either with resistors in the capacitively decoupled case or by providing dc to a center tap in the inductively decoupled case. Obviously, care must be taken to prevent large increases in dc power dissipation or increases in noise due to these extra components.
A modification that can be made to the - G m oscillator is to replace the high-impedance current source connected to the sources of M 1 and M 2 in Figure 8.2(c) with a resistor. Since the resistor is not a high impedance source, the bias current will vary dynamically over the cycle of the oscillation. In fact, the current will be highest when the oscillator voltage is at its peak and lowest during the zero crossings of the waveform. Since the oscillator is most sensitive to phase noise during the zero crossings, this version of the oscillator can often give very good phase noise performance. This oscillator is shown in Figure 8.15(a).
