11.1 Low-noise amplifiers (LNAs)
The first amplifier in any receiver. Its noise figure dominates the entire system noise figure, so optimizing this single stage is critical.
Noise figure (NF) is the ratio (in dB) of input SNR to output SNR. A perfect amplifier has NF = 0 dB; real amplifiers add their own thermal and shot noise. Modern microwave LNAs:
- Cooled HEMT LNAs for radio astronomy: NF below 0.3 dB (a few K equivalent noise temperature).
- Room-temperature pHEMT LNAs: NF 0.5–1 dB at GHz, maybe 1.5–3 dB at mm-wave.
- CMOS LNAs: NF 1–3 dB at GHz, integrated into modern phone front ends.
Key parameters in addition to NF:
- Gain: 10–25 dB typical.
- IIP3 (input third-order intercept): a measure of linearity. Higher means less distortion when multiple signals are present.
- Bandwidth: octave for some LNAs, single-frequency for others.
Real-world LNAs: every cellphone, satellite-TV LNB, GPS receiver, Wi-Fi card, radio telescope, military radar. The first transistor in your phone's RF path is a CMOS or SiGe LNA.
11.2 Power amplifiers
The last amplifier in a transmitter; delivers the watts (or kilowatts) to the antenna.
Classes at microwave:
- Class A: linear, low efficiency (max ~50%, real ~25%). Used where linearity matters more than efficiency.
- Class AB: middle ground.
- Class F: harmonic-tuned for high efficiency (70%+) at the cost of bandwidth.
- Class E: switching amp, 80%+ efficient, narrowband.
- Doherty: two amps in parallel, one for low power, both for peak power. Standard in modern cellular base stations for OFDM signals with high peak-to-average ratio.
Devices: GaAs HEMT (older, mid-power), GaN HEMT (modern high-power), LDMOS silicon (high-power below 4 GHz, e.g., cellular base stations until GaN took over).
11.3 Balanced amplifiers
Two identical amplifiers fed in quadrature (through a hybrid coupler) and combined in quadrature. The hybrid couplers cancel reflections from the amplifier inputs, so the overall input return loss is dominated by the hybrid (typically 20+ dB) regardless of how badly mismatched each individual amplifier is. Trades 3 dB of theoretical gain (split-and-combine) for excellent input/output match and graceful degradation if one amp fails.
Used widely in broadband military and instrument PAs.