
RF circuits and techniques 231
Figure 8.12(a) shows the collector and emitter waveforms of the circuit
shown in Figure 8.11. The collector waveform (channel 1, 5 V/division,
allowing for a xlO probe) was dc coupled, and the 0 V level has been
marked in. The emitter waveform - channel 2, 0.5 V/div - is ac coupled,
so the vertical position of the trace is immaterial. A Schottky diode was
used in the first instance, in the interests of fast switching; a 1N4148 was
then substituted with no observable difference at all in the waveforms. The
transistor, however, is another matter entirely. The BC109 is billed as an AF
transistor, although with it ...