Home/ Resources/ RF Knowledge Base/ RF Power Amplifier Classes
Semiconductor and Device Technology III-V Semiconductors Informational

How does the breakdown voltage of a transistor technology affect maximum output power?

Power amplifier classes are defined by the transistor's conduction angle and the output network topology. Linear classes: Class A: conducts 100% of the RF cycle (conduction angle = 360°). Maximum theoretical efficiency: 50%. Best linearity. Used for: driver stages, small-signal amplifiers, lab equipment. Class AB: conducts 180°-360° of the cycle. Efficiency: 50-78.5% depending on bias. Good linearity with moderate efficiency. Used for: most communication PAs (cellular, Wi-Fi). Class B: conducts exactly 180° (half cycle). Maximum efficiency: 78.5%. Crossover distortion at zero crossing. Used for: push-pull linear amplifiers. Class C: conducts < 180°. Efficiency: up to 90%. Highly nonlinear (suitable only for constant-envelope signals like FM). Switching classes: Class D: transistor switches between saturation and cutoff, drives a tuned output network. Efficiency: theoretically 100%. Used at lower frequencies (< 100 MHz). Class E: uses transistor as a switch with a specific output network that shapes voltage and current waveforms to achieve zero-voltage switching. Efficiency: theoretically 100%, practically 80-95%. Class F: uses harmonic tuning to shape the drain waveform into a square wave (voltage) and half-sine (current). Efficiency: theoretically 100%, practically 75-90%.
Category: Semiconductor and Device Technology
Updated: April 2026
Product Tie-In: Transistors, MMICs, Evaluation Boards

PA Classes

Doherty amplifier: the dominant PA architecture for modern cellular base stations. It uses a main (carrier) amplifier (Class AB) and a peaking amplifier (Class C) combined through a quarter-wave transformer. At low signal levels: only the main amplifier is active (high efficiency because it's near compression). At high signal levels: the peaking amplifier turns on and contributes additional power. The quarter-wave combiner modulates the load impedance seen by the main amplifier, maintaining high efficiency across a wide output power range. Typical Doherty efficiency: 40-55% at average power (compared to 15-25% for a single Class AB PA at the same back-off). Doherty PA is essential for OFDM signals with high PAPR.

Envelope tracking (ET) modulates the PA supply voltage to follow the signal envelope. At low signal amplitude: the supply voltage is reduced, keeping the PA close to compression (high efficiency). At high amplitude: the supply voltage increases to provide headroom. ET provides 25-40% efficiency improvement compared to fixed-supply Class AB. ET requires a wideband, high-efficiency envelope modulator that can track the signal envelope at 10-100 MHz bandwidth. Used in smartphones (where battery life is critical) and some base stations.

Load modulation and outphasing: Chireix outphasing PA uses two saturated (high-efficiency) PAs driven with phase-modulated signals. The phase difference between the two PAs controls the output power level. At the combining node: the vector sum of the two signals reproduces the desired amplitude-modulated signal. Efficiency is high because each PA operates near saturation. Challenge: the load impedance presented to each PA varies with the outphasing angle, causing efficiency degradation at deep back-off. Modern implementations use adaptive matching or digital compensation to maintain efficiency.

Common Questions

Frequently Asked Questions

Which PA class for my application?

For linear modulation (QAM, OFDM, 4G/5G): Class AB (simple) or Doherty (high efficiency at back-off). For constant envelope (FM, GMSK): Class C or Class E (highest efficiency). For high-power broadcast: Doherty (AM) or Class E (FM). For wideband military/SDR: Class AB with DPD (best linearity-bandwidth combination). For smartphone: Class AB with envelope tracking (best battery life).

How do I improve efficiency?

Doherty architecture: +10-15% efficiency improvement at back-off. Envelope tracking: +10-15% improvement. DPD: allows operating at reduced back-off, improving efficiency by 5-15%. Harmonic tuning (Class F, inverse Class F): +5-10% compared to Class AB at the same output power. GaN technology: higher breakdown voltage allows higher drain voltage, improving efficiency by 5-10% compared to LDMOS or GaAs at the same power level.

What about GaN vs. LDMOS vs. GaAs?

GaN HEMT: highest power density (5-10 W/mm), highest operating voltage (28-50V), broadband (high impedance simplifies matching), best for: 5G base stations, radar, military EW. LDMOS: mature, cost-effective, high power (up to 1 kW per device), moderate voltage (28-50V), best for: cellular base stations below 4 GHz, broadcast. GaAs HBT: highest efficiency at low voltage (3.3-5V), best for: smartphone PAs. SiGe: integrated with CMOS digital, best for: mmWave PAs (28/39/77 GHz).

Keep Reading

Related Questions

What is the difference between GaAs, GaN, InP, and SiGe for RF and millimeter wave applications?
→
When should I select GaN over GaAs for a power amplifier design?
→
What is the advantage of InP over GaAs for millimeter wave applications above 100 GHz?
→
What is the fT and fmax of a transistor and how do they determine the maximum operating frequency?
→
How do I select between HEMT, HBT, and MESFET topologies for a given application?
→
What is the difference between enhancement mode and depletion mode GaN transistors?
→
Glossary

Related Terms

P1dB → Noise Figure → IP3 → S-Parameters → Impedance → Dynamic Range →
Need expert RF components?

Request a Quote

RF Essentials supplies precision components for noise-critical, high-linearity, and impedance-matched systems.

Get in Touch