Why does Voltage-Mode Class D fail at high frequencies?

In standard Voltage-Mode Class D (VMCD), the power supply acts as a constant voltage source. The transistors violently chop this voltage into a square wave. However, real transistors have parasitic drain-to-source capacitance (Cds). Every time the transistor switches on, it must violently discharge this capacitor to ground, wasting power as heat. At 1 MHz, this loss is negligible. At 2 GHz, the capacitor is charging and discharging 2 billion times a second, generating so much heat that the transistor instantly melts.

How does Current-Mode Class D solve the capacitance problem?

By changing the output filter. CMCD uses a parallel-resonant LC tank instead of a series-resonant tank. A parallel tank naturally includes a capacitor. When designing the tank, the engineer simply calculates the value of the transistor's parasitic Cds and absorbs it into the tank's equation. The parasitic capacitance stops acting like a heat-generating short-circuit and instead becomes a useful, necessary component of the resonant filter. This allows switch-mode efficiencies at microwave frequencies.

What is Zero-Voltage Switching (ZVS)?

ZVS is the holy grail of switch-mode amplifiers. If a transistor turns on while there is still voltage across its drain, that voltage multiplying against the surging current creates a massive spike of wasted heat (switching loss). In CMCD, the parallel-resonant tank forces the voltage into a perfect sine wave. If tuned correctly, the voltage wave naturally swings down to exactly zero volts right at the precise moment the transistor is scheduled to turn on. Because V=0 when the switch closes, switching loss is mathematically eliminated.

PA Architecture

Current-Mode Class D (CMCD)

An engineer attempts to build a highly efficient Class D switch-mode amplifier for a 1 GHz transmitter. They use a standard Voltage-Mode (VMCD) design. When they turn it on, the transistors violently discharge their parasitic drain capacitance to ground 1 billion times a second, instantly turning the chip to slag. To survive microwave frequencies, they switch to Current-Mode Class D (CMCD). They place a massive RF choke in series with the power supply, forcing it to act as a constant current source. The two push-pull transistors now chop current (not voltage) into a square wave. The output is fed into a parallel-resonant LC tank. The beauty of the parallel tank is that the transistor's parasitic capacitance is no longer a destructive flaw; it is mathematically absorbed into the tank's parallel capacitor. The tank shapes the voltage into a perfect sine wave, ensuring the transistor only ever switches when the voltage is naturally at zero, eliminating heat and allowing theoretical 100% efficiency at gigahertz frequencies.

Category: PA Architecture

Switching Condition: Zero-Voltage Switching (ZVS)

Primary Advantage: Absorbs parasitic capacitance for high-frequency operation

Voltage-Mode vs. Current-Mode Switch-Mode PAs

Feature	Voltage-Mode Class D (VMCD)	Current-Mode Class D (CMCD)
Power Supply Feed	Constant Voltage	Constant Current (via RF Choke)
Generated Waveform	Square Voltage / Sine Current	Square Current / Sine Voltage
Output Filter	Series Resonant Tank	Parallel Resonant Tank
Parasitic Cds Impact	Destructive (Discharge loss = fCV²)	Beneficial (Absorbed into parallel tank)

Peak Voltage Stress in CMCD:
Because the parallel tank is shaping a constant current into a sine wave voltage, the peak voltage swing across the transistor is significantly higher than the power supply.
V_peak = π · V_DD
If you are powering the CMCD with a 28V supply, the transistor must be able to survive a peak voltage swing of 88 Volts without suffering avalanche breakdown.

The Zero-Voltage Switching (ZVS) Requirement:
For CMCD to achieve its theoretical 100% efficiency, the parallel tank must be tuned slightly inductively. This phase shift guarantees that the sine-wave voltage reaches exactly 0 Volts slightly *before* the transistor is driven 'ON', ensuring no power is burned during the transition.

Common Questions

Frequently Asked Questions

Is CMCD the same as Class E?

They are very similar in that both use Zero-Voltage Switching (ZVS) and both absorb parasitic capacitance. The difference is the topology. Class E is a "single-ended" amplifier, meaning it uses only one transistor to chop the wave. CMCD is a "push-pull" amplifier, meaning it uses two transistors operating exactly 180 degrees out of phase. Because it uses two transistors, CMCD can generate more power and naturally suppresses even-order harmonics.

Why does the RF Choke create a constant current?

An inductor strongly resists any change in current. By placing a massive inductor (an RF Choke) between the DC voltage supply and the transistors, the inductor refuses to let the high-frequency RF switching alter the flow of current. It forces a steady, unyielding stream of DC current into the transistors, which they then route back and forth to create the square wave.

Why isn't CMCD used for 5G cellular signals?

Because it is a switch-mode amplifier. The transistors are acting purely as digital on/off switches. A switch can only output full power or zero power; it cannot output 50% power. Therefore, a CMCD amplifier cannot amplify an amplitude-modulated signal (like 5G OFDM) on its own. To use CMCD for 5G, designers must pair it with complex architectures like Outphasing (LINC), which requires massive digital signal processing.

Related Terms

← Crosstalk Cut-off Frequency →

PA Architecture

CMCD ZVS Tank Calculator

Input your target frequency, output power, and transistor parasitic Cds. Calculate the required RF choke inductance, the parallel tank component values, and the exact phase offset needed to guarantee Zero-Voltage Switching.

Calculate ZVS Tank