Why is load modulation necessary for high efficiency?

A standard Class AB amplifier only achieves peak efficiency when its output voltage swings all the way to the supply rails (voltage saturation). This only happens at peak output power. If the signal drops by 6 dB (half voltage), the efficiency plummets because power is wasted as heat across the transistor. To stay efficient at 6 dB back-off, the amplifier needs to see an impedance that is twice as high, allowing it to hit maximum voltage swing with half the current. A fixed matching network cannot do this. Active load modulation solves the problem dynamically: at low power, the amplifier sees a high impedance and stays highly efficient. As power demands increase, a secondary amplifier turns on, modulating the load to a lower impedance so the primary amplifier can deliver more current without clipping.

Which PA architectures use active load modulation?

The Doherty amplifier is the most famous example. The carrier amplifier is connected to the load through a quarter-wave transformer (which acts as an impedance inverter). The peaking amplifier injects current directly at the load. As the peaking amplifier injects current, the impedance at the load node increases. The quarter-wave line inverts this, causing the impedance seen by the carrier amplifier to decrease dynamically as power increases. Outphasing (Chireix) amplifiers also use active load modulation. They drive two identical saturated amplifiers with constant-envelope signals that vary in phase. The phase difference between the two currents actively modulates the real and reactive impedance seen by each amplifier, controlling the combined output power while keeping both devices in high-efficiency saturation.

Active Components

Active Load Modulation

Q: How does injecting current change impedance?

Ohm's law states that Voltage = Current times Impedance (V = I x Z). Therefore, the apparent impedance seen by a current source is Z = V / I. If a primary amplifier pushes current I1 into a load resistor R, the voltage at that node is V = I1 x R, so the primary amplifier sees an impedance of R. However, if a secondary amplifier simultaneously injects current I2 into that exact same node, the voltage at the node rises to V = (I1 + I2) x R. Now, from the perspective of the primary amplifier, the apparent impedance it sees is Z_apparent = V / I1 = ((I1 + I2) x R) / I1 = R x (1 + I2/I1). By simply turning on the secondary amplifier and injecting current, the impedance seen by the primary amplifier increases without moving any physical components. This is active load modulation.

A standard power amplifier is matched to a fixed 50-ohm load. At peak power, it swings its maximum voltage and achieves 60% efficiency. But a modern 5G signal spends most of its time at 8 dB below peak power. At that backed-off level, the voltage swing is tiny, and efficiency drops to 15%. The solution is not to change the physical matching network, but to change the apparent impedance dynamically. By injecting current from a second "peaking" amplifier into the same load node, the voltage at that node rises. Ohm's law dictates that if the voltage rises while your current stays the same, you are seeing a higher impedance. This is active load modulation. It forces the primary amplifier to see a high impedance at low power (maximizing voltage swing and efficiency) and smoothly lowers that impedance as power demands increase. It is the engine driving the Doherty amplifier and every modern base station in the world.

Category: Active Components

Mechanism: Z = V / I (where V is altered by I₂)

Key Application: Doherty and Outphasing PAs

Load Modulation Architectures

Architecture	Control Variable	Impedance Change	Typical Back-off Peak
Doherty	Amplitude (Peaking PA turn-on)	Z decreases as power increases	6 dB (2-way), 9.5 dB (3-way)
Outphasing (Chireix)	Phase difference between PAs	Real & Reactive Z modulation	Signal dependent
Load Modulated Balanced PA (LMBPA)	Amplitude & Phase	Complex Z modulation	Wideband high-efficiency
Standard Class AB	None (Fixed load)	None	0 dB (Peak power only)

Active Impedance Equation:
Z₁ = V_load / I₁ = (I₁ + I₂)·R_L / I₁ = R_L · (1 + I₂/I₁)
As peaking current I₂ increases, the impedance seen at the combining node increases.

Doherty Impedance Inversion:
Z_carrier = Z₀² / Z_{combining_node}
A quarter-wave line connects the carrier to the combining node. As I₂ makes the combining node impedance rise, the carrier sees an inverted, decreasing impedance, allowing it to output more power.

Common Questions

Frequently Asked Questions

How does injecting current change impedance?

Impedance is just the ratio of voltage to current (Z = V/I). If you push current I₁ into a resistor, you see V. If a second amp pushes I₂ into that same resistor, the voltage rises to V'. Because your current I₁ hasn't changed but the voltage you are pushing against is now higher, the apparent impedance you "feel" has increased. You have modulated the load purely through active current injection.

Why is it necessary?

To keep an amplifier efficient, it must operate near voltage saturation. If the signal drops by 6 dB, voltage drops by half, and power is wasted as heat. To hit voltage saturation at half the current, the amplifier must see twice the impedance. Active load modulation provides this dynamically: high impedance at low power (high efficiency), transitioning to low impedance at peak power to deliver maximum current.

Where is this used?

The Doherty amplifier is the most widespread application, used in nearly every cellular macro base station. Outphasing (Chireix) amplifiers also use it, modulating the load by changing the phase angle between two constant-envelope amplifiers rather than the amplitude. Both rely on active current combining to manipulate apparent impedance.

Related Terms

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