Process · Photonics, MEMS & Sensors
Load-pull
A measurement technique that systematically varies the impedance presented to a transistor's output and records power, gain and efficiency, mapping the optimum matching conditions.
- Also known as
- Load-pull characterisation, Source-pull
- Type
- Process
- Domain
- Photonics, MEMS & Sensors, Compound semiconductors
Notes
Essential for power amplifier design because large-signal optimum impedance differs from the small-signal conjugate match.
Overview
Load-pull is a large-signal measurement in which the impedance presented to a transistor's output is changed step by step while its output power, gain and efficiency are recorded at each setting. The result shows which load gets the best performance from the device, which is the starting point for designing the matching network of a power amplifier. It is needed because at high drive levels a transistor no longer behaves linearly, so small-signal models and S-parameters cannot predict its optimum.
How it works
The transistor, either packaged in a test fixture or probed on the wafer, is biased and driven with a signal source. An impedance tuner on its output presents a chosen reflection coefficient; a second tuner on the input does the same for the source side, which is called source-pull. Power sensors or vector receivers measure what goes in and comes out, and the system steps through many impedances. Results are plotted on a Smith chart as contours of equal output power, gain or efficiency, so the designer can see the best point and how sensitive performance is to moving away from it.
Passive load-pull uses mechanical tuners, typically built from slab transmission lines or waveguides, which are repeatable but cannot reflect all the power back: losses in cables, fixtures and probes reduce the highest reflection reachable at the device. Active load-pull injects a controlled signal back into the output instead, which can reach the edge of the Smith chart and is fast, at the cost of extra amplifiers. Hybrid systems combine a passive tuner for most of the reflection with active injection for the rest.
More advanced set-ups also control impedances at harmonic frequencies, measure with modulated signals to capture linearity, or use pulsed signals. The quality of the result depends on careful calibration and de-embedding of everything between tuner and device.
Examples
- Characterising a power transistor for the final stage of a mobile base station or handset amplifier to choose its output matching network.
- On-wafer active load-pull at millimetre-wave frequencies, where probe and tuner losses make passive tuning impractical.
- Harmonic load-pull to study how the terminations at the second and third harmonics affect a power amplifier's efficiency.
- Source-pull and load-pull together to find the input and output impedances that maximise transducer gain and output power.
Why it matters
Related terms
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