Understanding Power Back-Off in Amplifiers: IPBO and OPBO

In applications demanding linear Power Amplifiers (PAs) due to high Peak-to-Average Power Ratio (PAR), operating the amplifier with a certain power back-off from its peak power point is crucial to prevent waveform clipping. For instance, if the PAR is 10 dB, the amplifier should operate at 10 dB below its maximum power to avoid clipping.

All amplifiers eventually clip; the output cannot exceed a certain multiple of the power supply. Although peak amplifier output can be very high, the average output power is what matters.

If we “back-off” sufficiently from the peak output so that the amplifier never clips, we compromise the efficiency. To meet EVM (Error Vector Magnitude) specifications, we usually find a compromise, choosing a back-off level that’s “good enough”.

Input Back-Off Power vs. Output Back-Off Power

Let’s differentiate between two types of power back-off: Input Back-Off (IPBO) and Output Back-Off (OPBO).

• Input Back-Off (IPBO): This is the power level at the RF amplifier’s input relative to the input power that produces maximum output power.

• Output Back-Off (OPBO): This is the power level at the RF amplifier’s output relative to the maximum possible output power.

Examples

Input Back-Off Example

• Inputs:
  • $P_{in}$ (nominal) = -25 dBm
  • $P_{in}$ at which amplifier generates max. power output = -20 dBm
• Input Back-Off Power = -20 dBm - (-25 dBm) = 5 dB

Output Back-Off Example

• Outputs:
  • Output power (Maximum) = +40 dBm
  • Nominal output level = +34 dBm
• Output Back-Off Power = +40 dBm - (+34 dBm) = 6 dB