Radio Frequency Integrated Circuit Design - Webs

270 Radio Frequency Integrated Circuit Design
the desired output power. In traditional negative resistance oscillators, analysis
has shown that a small-signal open-loop voltage gain of 3 is optimal for output
power [2]. Fortunately, this is close to the optimum value for phase noise
performance.
8.10 Basic Differential Oscillator Topologies
The three main oscillators discussed so far can be made into differential circuits.
The basic idea is to take two single-ended oscillators and place them back to
back. The nodes in the single-ended circuits, which were previously connected
to ground, in the differential circuit are tied together forming an axis of symmetry
down the center of the circuit. The basic circuits with biasing are shown in
Figure 8.22.
8.11 A Modified Common-Collector Colpitts Oscillator with
Buffering
One problem with oscillators is that they must be buffered in order to drive a
low impedance. Any load that is a significant fraction of the R p of the oscillator
would lower the output swing and increase the phase noise of the oscillator. It
is common to buffer oscillators with a stage such as an emitter follower or
emitter-coupled pair. These stages add complexity and require current. One
design that gets around this problem is shown in Figure 8.23. Here, the commoncollector
oscillator is modified slightly by the addition of resistors placed in the
collector [3, 4]. The output is then taken from the collector. Since this is a
high-impedance node, the oscillator’s resonator is isolated from the load without
using any additional transistors or current. However, the addition of these
resistors will also reduce the headroom available to the oscillator.
8.12 Several Refinements to the −Gm Topology
Several refinements can be made to the −Gm oscillator to improve its performance.
In the version already presented in Figure 8.22, the transistors’ bases
and collectors are at the same dc voltage. Thus, the maximum voltage swing
that can be obtained is about 0.8V. That is to say, the voltage on one side of
the resonator drops about 0.4V while on the other side of the oscillator the
voltage rises by about 0.4V. This means that the collector would be about 0.8V
below the base and the transistor goes into saturation. In order to get larger
swings out of this topology, we must decouple the base from the collector. One