EFFECT OF LOAD IMPEDANCE in power amplifier
An important factor affecting distortion is the load impedance. The optimum value at the fundamental frequency is always specified in Philips’ transistor data sheets for the frequency range of interest. The load reactance at the second harmonic is also important. Often, this is solely the output capacitance of the transistor. The collector or drain current contains a substantial second harmonic component which due to the presence of the load reactance causes a second harmonic output voltage component. A relatively small component can be tolerated. However, if it is above say 10% of the voltage at the fundamental frequency, the amplifier will saturate at a lower power than intended, so the allowable distortion is reached at lower power.
This can be solved by adding an external capacitor between collector (or drain) and earth. Though this will reduce gain and efficiency somewhat, it will reduce intermodulation significantly. A good practical rule is that the reactance of the parallel combination of internal and external capacitance at the second harmonic should be about 2.2 times the load resistance at the fundamental frequency.