Why not just use a standard Class E amplifier?

A standard Class E amplifier achieves incredible efficiency by ensuring the transistor only switches when voltage is zero (ZVS). However, to achieve this, the resonant tank allows the voltage to swing massively high during the 'off' state—typically 3.56 times the DC supply voltage. If you power the amplifier with 28V, the peak spike is 100V. This massive stress often destroys the transistor via avalanche breakdown, severely limiting the maximum power a pure Class E amplifier can output.

How does the 'F2' modification solve the voltage spike?

The 'F2' denotes Class F harmonic tuning applied specifically to the 2nd harmonic. The engineer adds a resonant trap to the output network that presents a dead short-circuit (0 ohms) to the 2nd harmonic frequency. By destroying the 2nd harmonic voltage, the overall voltage waveform physically flattens out, changing from a sharp spike into a blunt, half-sine shape. This reduces the peak voltage stress from 3.56x down to roughly 2.3x the supply voltage, saving the transistor from destruction.

Why is Class E/F2 popular for GaN transistors?

Gallium Nitride (GaN) transistors have very high breakdown voltages, making them excellent for switch-mode operation. However, they also suffer from severe parasitic drain-to-source capacitance (Cds). Class E/F2 is explicitly designed to absorb that parasitic capacitance directly into its fundamental resonant tank while simultaneously using the F2 harmonic trap to manage voltage stress. This unique synergy allows engineers to push massive power levels (100W+) at microwave frequencies (S-band and X-band) where pure Class E would fail.

PA Architecture

Class E/F2 Amplifier

An engineer builds a 50-Watt pure Class E amplifier for a 2.4 GHz drone transmitter. To get 50W of power, they must use a 28V power supply. Because a Class E circuit forces the voltage into a sharp, narrow pulse to achieve zero-voltage switching, the peak voltage swings to an extreme 100 Volts, instantly blowing the transistor. To fix this, the engineer redesigns the output network into a Class E/F2 hybrid. They leave the fundamental frequency network tuned for Class E operation, but they insert a specialized LC trap that presents a perfect short-circuit to the 2nd harmonic (4.8 GHz). By shorting out the 2nd harmonic, the shape of the voltage wave fundamentally changes, flattening out at the top. The peak voltage drops from 100V down to a safe 65V, while the zero-voltage switching (ZVS) condition remains perfectly intact. The amplifier survives, delivering 50 Watts at 85% efficiency.

Category: PA Architecture

Harmonic Strategy: Short-circuit the 2nd harmonic (0 Ω)

Primary Advantage: Reduces peak voltage stress by ~35% vs pure Class E

Switch-Mode PA Stress Comparison

Amplifier Class	Harmonic Tuning Strategy	Peak Voltage Stress (normalized to Vdd)	Peak Current Stress
Pure Class E	None (Reactance absorbed)	~ 3.56 × Vdd	~ 2.8 × I_dc
Class F	Short even harmonics / Open odd harmonics	~ 2.00 × Vdd	~ 1.5 × I_dc
Class E/F2 (Hybrid)	Short 2nd harmonic only	~ 2.30 × Vdd	~ 2.5 × I_dc

The 2nd Harmonic Short:
To achieve the E/F2 condition, the output matching network must present a specific impedance at the fundamental frequency (to satisfy the Class E ZVS condition), but it must also present a massive admittance (a short circuit) exactly at the 2nd harmonic (2ω₀).
Z_L(2ω₀) = 0 + j0 Ω
This is typically achieved by placing a series L-C resonant trap directly from the transistor's drain to ground. The trap acts as an open circuit at the fundamental frequency, but perfectly shorts out the 2nd harmonic to ground.

Power Capability Limit (P_out / V_maxI_max):
Because the E/F2 hybrid lowers the peak voltage without severely compromising the current waveform, its overall "Power Capability" (how much RF power you get for a given size of transistor) is significantly higher than a pure Class E amplifier, allowing engineers to use smaller, cheaper transistors to generate the same wattage.

Common Questions

Frequently Asked Questions

What is the difference between Class E/F2 and Class E/F3?

The number indicates which harmonic is being manipulated. E/F2 shorts out the 2nd harmonic, resulting in a flatter voltage wave. E/F3 applies an 'open circuit' to the 3rd harmonic. E/F3 actually flattens the *current* wave instead of the voltage wave, which is highly beneficial for bipolar junction transistors (BJTs) that are sensitive to peak current stress, whereas E/F2 is preferred for FETs and GaN devices that are sensitive to peak voltage breakdown.

Does the F2 tuning make the amplifier narrower in bandwidth?

Yes. Pure Class E can actually be designed with decent bandwidth because it doesn't rely on razor-sharp harmonic traps. The moment you introduce a high-Q resonant trap to short out the 2nd harmonic, the amplifier becomes highly frequency-dependent. If the frequency shifts by 5%, the trap no longer shorts the 2nd harmonic, the voltage spike returns, and the transistor blows up. E/F2 is strictly for narrowband applications.

Can it amplify amplitude-modulated (AM) signals?

No, not directly. Like all switch-mode amplifiers (D, E, F), the transistor is acting purely as a digital switch (on/off). It can only output full power. To transmit a complex signal with amplitude variations (like 5G or Wi-Fi), the E/F2 amplifier must be used in a complex transmitter architecture like Envelope Elimination and Restoration (EER) or an Outphasing (LINC) system.

Related Terms

← Class D Amplifier Class EF →

PA Architecture

Class E/F2 Impedance Calculator

Input your target frequency, DC voltage, and desired power output. Synthesize the optimal fundamental impedance required for ZVS, and calculate the exact L-C component values needed to construct the 2nd-harmonic short-circuit trap.

Calculate Hybrid Network