Linear Amplifier
Understanding Linear Amplifiers
Linearity is the most demanded PA specification for modern communication systems. High-order QAM modulations (64-QAM, 256-QAM, 1024-QAM) carry information in both amplitude and phase, requiring the amplifier to faithfully reproduce the input signal amplitude at higher power.
Linearity Metrics
- P1dB: Power level where gain compresses by 1 dB. Operating 6-10 dB below P1dB for linear operation.
- IP3: Hypothetical intercept of fundamental and 3rd-order IM. Higher is better. OIP3 = P1dB + 10 dB (approximate).
- ACLR: Adjacent channel leakage ratio. The regulatory metric for transmitter linearity.
- EVM: Error vector magnitude. Measures modulation quality including amplitude distortion.
Frequently Asked Questions
What is a linear amplifier?
A linear amplifier faithfully reproduces the input at higher power. Linearity measured by P1dB, IP3, ACLR, and EVM. Essential for QAM signals where amplitude carries information. Class A (most linear) or Class AB with DPD.
How much backoff is needed for linear operation?
Operating point below P1dB: 6-8 dB for single carrier. 8-10 dB for multi-carrier OFDM (without DPD). 3-6 dB with DPD. More backoff = more linear but less efficient.
Does DPD make any PA linear?
DPD can improve linearity by 15-25 dB but requires the PA to have some inherent linearity (Class AB). Class C and switching PAs have too much distortion for DPD alone. The PA must have sufficient instantaneous bandwidth for the DPD correction signal.