How does a Class F amplifier achieve high efficiency?

Efficiency drops when a transistor has high voltage across it at the exact same time it is passing high current. In a standard Class B amplifier, the voltage and current are both half-sine waves, which overlap significantly. Class F solves this not by switching the transistor like a digital Class D, but by 'shaping' the waves. It forces the voltage to become a square wave and the current to become a half-sine wave. Because the square voltage wave stays perfectly flat at 0V while the current flows, the overlap is zero, resulting in a theoretical efficiency of 100%.

How do you force the voltage to become a square wave?

Using Fourier analysis. A perfect square wave is mathematically created by adding up a fundamental sine wave and all of its odd harmonics (3rd, 5th, 7th). The Class F output network is explicitly designed to reflect these odd harmonics back into the transistor exactly in phase. It presents an 'open circuit' (infinite impedance) to the odd harmonics so they generate voltage, and a 'short circuit' (zero impedance) to all even harmonics (2nd, 4th) so they are destroyed. The combination organically synthesizes the square voltage wave right at the drain terminal.

Why is Inverse Class F (Class F^-1) sometimes preferred?

Inverse Class F flips the harmonic conditions: it short-circuits the odd harmonics and open-circuits the even harmonics. This creates a square wave for the current and a half-sine wave for the voltage. This topology is often preferred in modern GaN designs because high-power transistors have massive parasitic drain-source capacitance (Cds). It is physically much easier to absorb this capacitance into a short-circuit at the high-frequency 3rd harmonic than an open-circuit, making Inverse Class F easier to tune at microwave frequencies.

PA Architecture

Class F Amplifier

An engineer designing a 2 GHz amplifier needs maximum efficiency but cannot use a switch-mode Class D amplifier because the parasitic capacitance destroys the switching speed. Instead, they use Class F. They keep the transistor operating as a linear current source but redesign the output matching network into a complex harmonic comb filter. By placing a quarter-wave transmission line stub exactly at the output, they create an impedance transformer that short-circuits the 2nd harmonic (4 GHz) and open-circuits the 3rd harmonic (6 GHz). According to Fourier theory, reflecting the odd harmonics back into the transistor causes the drain voltage to perfectly shape itself into a square wave, while the even harmonics are destroyed, leaving a half-sine current wave. Because the voltage drops to zero exactly when the current peaks, power dissipation plunges, pushing theoretical efficiency past 90% without requiring the transistor to act as a digital switch.

Category: PA Architecture

Mechanism: Harmonic Impedance Tuning

Goal: Square Voltage Wave / Half-Sine Current

Harmonic Termination Conditions

Harmonic	Standard Class F	Inverse Class F (F^-1)
Fundamental (f0)	Resistive Match (R_opt)	Resistive Match (R_opt)
2nd Harmonic (2f0)	Short Circuit (0Ω)	Open Circuit (∞Ω)
3rd Harmonic (3f0)	Open Circuit (∞Ω)	Short Circuit (0Ω)
Resulting Voltage	Square Wave	Half-Sine Wave
Resulting Current	Half-Sine Wave	Square Wave

Quarter-Wave Tuning Stub:
A transmission line exactly λ/4 long at the fundamental frequency (f0) acts as an impedance inverter.
At f0: Length is λ/4 (Inverts a short to an open).
At 2f0: Length is λ/2 (Repeats the termination; short stays a short).
At 3f0: Length is 3λ/4 (Inverts a short to an open).
This physical phenomenon makes the λ/4 stub the fundamental building block of Class F amplifiers, perfectly sorting the even harmonics from the odd harmonics.

Efficiency Limits:
Tuning only the 2nd and 3rd harmonics yields a theoretical efficiency of 90.7%. Tuning up to the 5th harmonic yields 94.8%. Tuning infinite harmonics yields 100%.

Common Questions

Frequently Asked Questions

Why does a square voltage wave improve efficiency?

Heat dissipation in a transistor is calculated as P = V × I. If the voltage is a sine wave, it slowly rises and falls. During the time it is rising/falling, current is also flowing. Multiplying a non-zero voltage by a non-zero current equals wasted heat. A square wave snaps instantly from 0V to Max Voltage. While it is at 0V, current flows freely with zero power loss (0 × I = 0). While it is at Max Voltage, the transistor cuts off the current entirely (V × 0 = 0).

Why is Class F strictly narrowband?

The entire architecture relies on placing precise short and open circuits at exact harmonic frequencies. A quarter-wave stub is only physically λ/4 at one specific fundamental frequency. If you change the input frequency by even 5%, the stub is no longer the correct length, the harmonics fall out of phase, the square wave collapses, and the amplifier burns up as heat.

How does parasitic capacitance ruin Class F?

Standard Class F requires an "Open Circuit" (infinite impedance) at the 3rd harmonic. However, high-power GaN and LDMOS transistors have massive parasitic drain-to-source capacitance (Cds). At high frequencies like the 3rd harmonic, this capacitor acts as a near short-circuit to ground. You cannot physically create an open circuit if the transistor itself is shorting the signal to ground. This is why designers often use Inverse Class F, which demands a short circuit at the 3rd harmonic, naturally absorbing the parasitic capacitance.

Related Terms

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Harmonic Tuning

Class F Waveform Synthesizer

Adjust the amplitude and phase of the fundamental, 2nd, and 3rd harmonic load impedances. Watch the Fourier synthesis organically shape the drain voltage from a sine wave into a perfect square wave and calculate the resulting efficiency jump.

Synthesize Harmonic Waveforms