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Seed account
Power amplifier linearity and efficiency pull in opposite directions. Key terms and how they connect:
P1dB: output (or input) power at which gain has compressed by 1 dB from its small-signal value. A simple single-tone measure of the onset of compression.
IP3 (third-order intercept): extrapolated point where the fundamental and third-order intermodulation products of a two-tone test would be equal. Third-order products rise 3 dB per 1 dB of input increase, while fundamentals rise 1 dB per dB. IP3 is useful well below compression; near compression the slopes no longer hold, so measure IMD at the operating level.
ACLR / ACPR: ratio of power in the adjacent channel to the main channel for a modulated signal. This is what standards specify for real transmitters, because modulated signals have peak-to-average power ratio (PAPR) that stresses the PA in ways a single tone does not.
EVM: in-channel distortion measure, also affected by compression and memory effects.
Why back-off costs efficiency
Signals like OFDM have high PAPR. To keep peaks out of hard compression, the average power must be backed off from saturation. For classic class-A or class-AB amplifiers, drain efficiency falls as output power decreases, so operating at back-off means much lower average efficiency than at peak.
Ways around the trade-off
- Doherty amplifiers maintain higher efficiency over a range of back-off using a main and a peaking amplifier.
- Envelope tracking modulates supply voltage to follow the signal envelope.
- Crest factor reduction (CFR) lowers PAPR digitally before the PA, within EVM limits.
- Digital predistortion (DPD) linearizes the PA, allowing operation closer to compression.
Memory effects (bias network, thermal, trapping) make PA behaviour depend on past signal history; they are why simple static predistortion often falls short on wideband signals.
P1dB: output (or input) power at which gain has compressed by 1 dB from its small-signal value. A simple single-tone measure of the onset of compression.
IP3 (third-order intercept): extrapolated point where the fundamental and third-order intermodulation products of a two-tone test would be equal. Third-order products rise 3 dB per 1 dB of input increase, while fundamentals rise 1 dB per dB. IP3 is useful well below compression; near compression the slopes no longer hold, so measure IMD at the operating level.
ACLR / ACPR: ratio of power in the adjacent channel to the main channel for a modulated signal. This is what standards specify for real transmitters, because modulated signals have peak-to-average power ratio (PAPR) that stresses the PA in ways a single tone does not.
EVM: in-channel distortion measure, also affected by compression and memory effects.
Why back-off costs efficiency
Signals like OFDM have high PAPR. To keep peaks out of hard compression, the average power must be backed off from saturation. For classic class-A or class-AB amplifiers, drain efficiency falls as output power decreases, so operating at back-off means much lower average efficiency than at peak.
Ways around the trade-off
- Doherty amplifiers maintain higher efficiency over a range of back-off using a main and a peaking amplifier.
- Envelope tracking modulates supply voltage to follow the signal envelope.
- Crest factor reduction (CFR) lowers PAPR digitally before the PA, within EVM limits.
- Digital predistortion (DPD) linearizes the PA, allowing operation closer to compression.
Memory effects (bias network, thermal, trapping) make PA behaviour depend on past signal history; they are why simple static predistortion often falls short on wideband signals.