AM-PM conversion represents a shift in the phase delay of a signal when a transistor changes from small-signal to large-signal operating conditions.|
As the input signal level applied to a transistor amplifier is increased until some degree of gain compression is produced, further increases in signal amplitude will result in a slight shift of the amplifier phase delay. This phenomenon is known as AM-PM conversion and can be thought of as a result of the change of the transistor operating parameters from the small-signal to large-signal conditions. Many Agilent Technologies amplifiers include a guaranteed specification that AM-PM conversion will not exceed a certain value, on the order of a few tenths of a degree per dB increase in power output at a nominal power output level. If the input signal is further increased, the amount of AM-PM conversion will continue to increase reaching a maximum value when one of the amplifier stages is driven into full saturation. The maximum value will normally never exceed a few degrees/dB near amplifier saturation, and may generally be ignored.
Any limiters in a system are usually the major contributors to overall AM-PM conversion. Perhaps the worst case example is when a transistor amplifier is used in a receiving system in close proximity to a nearby transmitting system operating on a different frequency and the leakage power is sufficient to drive the limiters into their operating region. The result will usually be a noticeable slope in the baseband frequency response which will take place only when the transmitter is operating.