RF Amplifiers

There are four classifications for RF amplifiers. These devices can be classed as either A, AB, B, or C depending on the phase-angle over which plate or collector current flows. The phase angle is the number of degrees of current flow during each 360 degree cycle.

Class A RF amplifiers will operate over a small portion of a tube’s plate-current or a transistor’s collector-current range and have continuous plate- or collector-current flow throughout each RF cycle. It should be noted that their effectiveness in converting DC (direct current) source power to RF output power is not very good. Whatever residual DC source power that is not converted to radio frequency output power is dissipated as heat.

It should also be noted that Class A RF amplifiers have greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. For this reason, they are often incorporated in smaller signal applications where linearity is more crucial than power efficiency. They are also occasionally used in larger signal applications but this is usually done only when the requirement for extraordinarily high linearity outweighs the higher cost and heat disadvantages associated with poor power efficiency.

Class B RF amplifiers will have their control grids (for tubes) or their transistor bases biased near plate- or collector-current cutoff. This results in causing the plate or collector current to flow only during 180 degrees of each RF cycle. What this means is that the DC source power to RF output power efficiency will be much higher than with Class A RF amplifiers. There will be severe output cycle waveform distortion but this can be reduced by using high-Q resonant output tank circuits to reconstruct what amounts to full RF cycles.

Class AB amplifiers are essentially compromises between Class A and Class B RF amplifiers. They are biased so plate- or collector-current flows less than 360 degrees, but more than 180 degrees, of each RF cycle.

What this means is that any bias-point between those limits can be used, and this allows for a continuous selection range. The range can now extend from low-distortion, low-efficiency on one end to higher-distortion, higher-efficiency on the other end.

Class C RF amplifiers are those devices that are biased well beyond cutoff, so that plate- or collector-current flows less than 180 degrees of each RF cycle. In practical terms this allows for higher power efficiency than Class B operation, but with the penalty of higher input-to-output nonlinearity. Because of this, the use of relatively high-Q resonant output tank circuits to restore complete RF sine-wave cycles becomes essential. It should be noted that high amplifying-nonlinearity makes these types of RF amplifiers unsuitable for amplifying AM, DSB, or SSB signals.