Why make the peaking amplifier larger than the carrier?

In a standard symmetric Doherty, the carrier and peaking amplifiers are identical in size. This arrangement provides a secondary efficiency peak at 6 dB back-off from maximum power. However, modern LTE and 5G waveforms have a Peak-to-Average Power Ratio (PAPR) of 8 to 10 dB. If a symmetric Doherty handles an 8 dB PAPR signal, its average operating point falls below its efficiency peak, wasting massive amounts of power as heat. By making the peaking amplifier twice as large as the carrier amplifier (a 1:2 asymmetric ratio), the active load modulation mechanics change. The carrier amplifier hits voltage saturation—and thus peak efficiency—much earlier. This shifts the secondary efficiency peak down to 9.5 dB back-off, perfectly aligning the amplifier's most efficient operating point with the average power level of a modern cellular signal.

How does the impedance modulation work in an asymmetric design?

In a standard 1:1 Doherty, the carrier amplifier operates into an impedance of 2 times Ropt at low power. In a 1:2 asymmetric Doherty, because the peaking amplifier can deliver twice the current of the carrier at peak power, the impedance scaling is more extreme. At low power, the carrier amplifier operates into an impedance of 3 times Ropt. This highly mismatched impedance causes the carrier to hit maximum voltage swing (saturation) at only one-third of the total system power. As power demands increase, the massive peaking amplifier turns on and injects a huge amount of current into the combining node. Through the quarter-wave inverter, this drives the apparent impedance seen by the carrier down from 3 times Ropt to exactly Ropt at peak power.

What are the design trade-offs of going asymmetric?

The primary trade-off is combining complexity and bandwidth. You cannot combine unequal power levels efficiently using a standard 50-ohm Wilkinson combiner; doing so would result in power dissipated as heat in the isolation resistor. You must use an amplitude-compensated combiner where the characteristic impedances of the quarter-wave arms are scaled based on the power ratio. Additionally, driving a massive Class C peaking amplifier requires significantly more input power. Designers usually must implement an unequal input power splitter, routing more RF drive to the peaking amplifier to ensure it turns on hard and fast enough to modulate the carrier properly. This unequal splitting and specialized output combining severely restricts the operational frequency bandwidth compared to a symmetric design.

Active Components

Asymmetric Doherty

A standard symmetric Doherty amplifier provides peak efficiency at 6 dB below maximum power. For older 3G networks, this was perfect. But modern 5G signals have Peak-to-Average Power Ratios (PAPR) of 9 dB. If a symmetric Doherty is forced to operate at 9 dB back-off, its efficiency plummets, generating massive amounts of heat. The asymmetric Doherty solves this by abandoning equality. By making the peaking amplifier physically twice as large as the carrier amplifier, the active load modulation changes dramatically. The carrier amplifier hits voltage saturation—and peak efficiency—much earlier in the power curve. This shifts the "sweet spot" of the amplifier from 6 dB down to 9.5 dB back-off. Though it requires complex unequal input splitters and amplitude-compensated output combiners, the asymmetric Doherty perfectly aligns the amplifier's maximum efficiency with the actual average power level of a modern 5G waveform.

Category: Active Components

Scaling Ratio: Typically 1:2 or 1:3 (Carrier:Peaking)

Efficiency Peak: 8 to 12 dB back-off

Doherty Ratio vs. Efficiency Peak

Carrier:Peaking Ratio (K)	Power Distribution (Peak)	OBO Efficiency Peak	Target Application
1:1 (Symmetric)	50% / 50%	6.0 dB	WCDMA, GSM, LTE (Low PAPR)
1:1.5 (Asymmetric)	40% / 60%	8.0 dB	LTE-Advanced
1:2 (Asymmetric)	33% / 67%	9.5 dB	5G NR, Wi-Fi 6
1:3 (Asymmetric)	25% / 75%	12.0 dB	DVB-T, Wideband OFDM

Back-off extension formula:
Output Back-Off (OBO) = 20 · log₁₀(1 + K) dB
Where K is the current ratio of the Peaking amplifier to the Carrier amplifier. For a 1:2 asymmetric design, K = 2. OBO = 20 · log(3) = 9.54 dB.

Carrier Impedance Modulation (1:2 Ratio):
Low Power (>9.5 dB OBO): Z_carrier = (1 + K) · R_opt = 3 · R_opt
Peak Power (0 dB OBO): Z_carrier = R_opt
Because the carrier starts at 3× impedance, it saturates at one-third of the current, achieving peak efficiency very early.

Common Questions

Frequently Asked Questions

Why make the peaking amp larger?

To shift the efficiency peak deeper into back-off. A symmetric Doherty peaks at 6 dB back-off. A 5G signal operates at 9 dB back-off. By making the peaking amplifier twice as large as the carrier, the carrier is forced to saturate earlier (at 9.5 dB back-off), keeping the amplifier cool during average signal transmission.

How do you combine unequal powers?

You cannot use a standard Wilkinson combiner, as the voltage difference between the unequal arms will dissipate power into the isolation resistor. You must use an amplitude-compensated combiner. The quarter-wave line on the higher-power peaking side is designed with a lower characteristic impedance than the carrier side, balancing the voltages at the combining node.

What are the drawbacks?

Bandwidth reduction and drive complexity. An oversized Class C peaking amplifier is difficult to turn on. It requires an unequal input splitter (delivering more RF drive to the peaking device). The highly asymmetric matching networks and compensated combiners limit the fractional bandwidth of the amplifier compared to a standard symmetric design.

Related Terms

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