Why can Class E achieve higher efficiency than Class B or AB?

In Class B and AB, the transistor operates as a current source with a sinusoidal voltage swing. There is always a period during the RF cycle where both voltage and current are non-zero simultaneously, and the product V times I is dissipated as heat. In Class E, the transistor operates as a switch: when the switch is on, current flows but voltage is near zero; when the switch is off, voltage is high but current is zero. The load network shapes the drain voltage so that it returns to zero with zero derivative just as the switch turns on, ensuring no overlap even during the transition. Theoretically, there is no V-times-I dissipation in the device.

What limits Class E efficiency in practice?

Three things. First, the transistor is not an ideal switch: it has finite on-resistance (R_on) that dissipates I-squared-R power during the on state. Typical R_on for a GaN device is 1 to 3 ohms, costing 2 to 5 percent efficiency. Second, the finite switching speed means there is a brief period of V-I overlap during transitions, especially at multi-GHz frequencies. Third, the output capacitance (C_oss) of the transistor must be absorbed into the Class E shunt capacitance; if C_oss exceeds the design value, the waveform no longer reaches zero at switch-on. At 2 GHz, practical Class E achieves 75 to 85% drain efficiency.

Can Class E handle amplitude-modulated signals?

Not directly. Because the transistor operates as a switch, the output amplitude is fixed. To handle signals with varying envelopes (like OFDM), Class E must be combined with an envelope modulation technique. In envelope tracking (ET), the drain supply voltage is modulated to follow the signal envelope while the Class E PA operates at constant saturated efficiency at each instantaneous power level. In outphasing (LINC), two Class E PAs are driven with constant-envelope phase-modulated signals and combined through a Chireix combiner to reconstruct the amplitude modulation.

Active Components

Class E Amplifier

In every linear amplifier class, power is wasted because voltage and current coexist in the transistor at the same time. Nathan Sokal eliminated this problem in 1975 by treating the transistor not as a controlled current source but as a switch. When the switch is closed, current flows through near-zero resistance. When open, voltage builds across an open circuit. The load network, a carefully tuned combination of shunt capacitance and series resonance, shapes the drain voltage so that it glides smoothly to zero at exactly the moment the switch closes. No voltage times current overlap means no dissipation. Theoretical efficiency: 100%.

Category: Active Components

Peak Drain Voltage: 3.56 × V_DD

Theoretical Efficiency: 100%

Switching Away the Overlap Problem

Class E Load Network Design Equations

Zero-voltage switching (ZVS) conditions at switch turn-on:
V_DS(t_on) = 0 and dV_DS/dt(t_on) = 0

Shunt capacitance (including device C_oss):
C_shunt = 0.1836 / (ω × R_load)

Series inductance:
L_series = 1.1525 × R_load / ω

Optimum load resistance:
R_load = 0.5768 × V_DD² / P_out

Example: 10 W at 2 GHz, V_DD = 28 V:
R_load = 0.5768 × 784 / 10 = 45.2 Ω
C_shunt = 0.1836 / (2π × 2×10⁹ × 45.2) = 0.32 pF
If the GaN device has C_oss = 0.4 pF, it exceeds the design requirement. The device is too large for this frequency and power; select a smaller die or lower V_DD.

Switched-Mode PA Class Comparison

Class	Peak V_DS	Waveform Shape	Theoretical η	Practical η at 2 GHz	Signal Compatibility
E	3.56 × V_DD	Half-sinusoidal V, rectangular I	100%	75 to 85%	Constant envelope only
F	2.0 × V_DD	Squared V, half-sine I	100%	70 to 80%	Constant envelope only
F⁻¹	3.14 × V_DD	Half-sine V, squared I	100%	70 to 80%	Constant envelope only
D	2.0 × V_DD	Square wave V	100%	<1 GHz only	Constant envelope only

Common Questions

Frequently Asked Questions

Why can Class E beat Class AB efficiency?

Class AB has simultaneous voltage and current in the transistor (V×I = heat). Class E uses the transistor as a switch: current flows through near-zero R_on when on, voltage is high but current is zero when off. The load network ensures zero voltage at turn-on. No overlap = no dissipation.

What limits practical efficiency?

Finite R_on (2 to 5% loss), finite switching speed (V-I overlap during transitions at GHz), and device C_oss exceeding the design shunt capacitance. At 2 GHz, practical Class E achieves 75 to 85% drain efficiency.

Can Class E handle OFDM signals?

Not directly (it is a constant-envelope topology). Use envelope tracking (modulate V_DD) or outphasing (two Class E PAs with Chireix combining) to reconstruct amplitude modulation while maintaining high efficiency.

Related Terms

← Class D Amplifier Class F Amplifier →

Efficiency Design

Switched-Mode PA Design Flowchart

Step-by-step decision tree for choosing between Class E, F, and inverse-F topologies based on your frequency, device technology, and signal requirements.

View the Flowchart