Figure 2-27.Frequency doubler waveforms.
Coupling the resonant frequency from the oscillator by different coupling methods also affects the
oscillator frequency and amplitude. A BUFFER AMPLIFIER decreases the loading effect on the
oscillator by reducing the interaction (matching impedance) between the load and the oscillator.
Figure 2-28 is the schematic diagram of a buffer amplifier. This circuit is a common-collector
amplifier. A common-collector amplifier has a high input impedance and a low output impedance. Since
the output of an oscillator is connected to the high impedance of the common-collector amplifier, the
buffer has little effect on the operation of the oscillator. The output of the common-collector buffer is then
connected to an external load; therefore, the changes in the output load cannot reflect back to the
oscillator circuit. Thus, the buffer amplifier reduces interaction between the load and the oscillator. Figure
2-29 illustrates a shunt-fed Hartley oscillator with a buffer amplifier. This is "one-way" coupling since the
oscillator signal is coupled forward, but load changes are not coupled back to the oscillator.
Figure 2-28.Buffer amplifier.