Page 235 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

X
I I
where gm(t) is a time-varying transconductance with its shape depending on the

oscillator amplitude. This shape resembles the collector current of a bipolar
transistor with a large overdrive signal, as seen in Figure 14-6. For large signals
such as 100 mV to 200 mV peak to peak, the time-varying transconductance has a
shape approaching a pulse. And the narrowing nature of this pulse as larger and

larger amplitudes of oscillator signals are combined into a transistor mixer will limit
the conversion transconductance.

·5

FIGURE 14-6 Collector current of the transistor for various input levels
into the base-emitter junction. The Xaxis shows time, whereas the Yaxis

shows the amplitude.
As seen in the figure, because transconductance is directly related to the collector
current, the transconductance is time-varying and has the same pulse "shape" as
the OSCillator amplitude is increased into the transistor mixer. A topic of

mathematics known as modified 8esse/ functions explains the time-varying