What is load-pull?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

Load-pull characterizes a transistor under large-signal drive by varying the load impedance (and in source-pull, the source impedance) presented to it and measuring output power, efficiency, gain and linearity at each point. The results are contours on a Smith chart that show the optimum impedances, which small-signal S-parameters cannot predict.^[1]^[2]

What load-pull systems exist?

Passive mechanical tuners: slugs move along a slab line; limited by tuner loss at extreme impedances.
Active load-pull: injects a signal to synthesize any Γ, including |Γ| near 1.
Harmonic load-pull: controls impedances at 2f₀ and 3f₀ to design class F and other harmonic-tuned PAs.

How are load-pull results used?

Designers trade contours: the maximum-P_out and maximum-PAE points differ, and linearity contours (ACLR, IM3) add a third dimension. The chosen impedance becomes the target for the output matching network, defined at the transistor’s package or die reference plane. Large-signal device models can be validated against load-pull data.

What are the practical pitfalls?

Reference-plane errors from fixtures; careful de-embedding is required.
Oscillation when tuners present unstable impedances out of band.
Thermal drift during measurement of high-power devices.

Frequently asked questions

Can simulation replace load-pull?

With validated nonlinear models, simulated load-pull is common; measurement remains the reference.

What is source-pull?

Varying source impedance to optimize gain, PAE or noise.

Why harmonic load-pull?

Harmonic terminations strongly affect efficiency in high-efficiency classes.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“RF Power Amplifiers for Wireless Communications, 2nd ed.”. S. C. Cripps, Artech House. 2006.
“Microwave Circuit Design Using Linear and Nonlinear Techniques, 2nd ed.”. G. D. Vendelin, A. M. Pavio, U. L. Rohde, Wiley. 2005.

Categories: Amplifiers

Articles on RF Design Wiki

Start here	Main page What is RF design Learning path Design workflow
Fundamentals	Decibels and dBm RF power Wavelength and frequency Thermal noise Noise figure Noise temperature SNR and Eb/N0 Skin effect Q factor Nonlinearity and harmonics Intermodulation and IP3 P1dB compression Dynamic range and SFDR
Transmission lines	Transmission line theory Characteristic impedance Γ, VSWR and return loss Input impedance of a line Quarter-wave transformer Microstrip Stripline Coplanar waveguide Coaxial cable Waveguides RF connectors
Matching and S-parameters	Impedance matching Maximum power transfer Smith chart L-network matching Pi and T networks Stub matching Bode–Fano limit S-parameters Touchstone files Z, Y, ABCD parameters De-embedding Mixed-mode S-parameters
Filters and passives	RF filter basics Filter approximations LC filter design Microstrip filters Cavity and dielectric filters SAW and BAW filters Diplexers and duplexers Wilkinson divider Couplers and hybrids Baluns Circulators and isolators RF attenuators Passive components
Amplifiers	Gain definitions Amplifier stability LNA design Power amplifier basics PA classes Efficiency and PAE Doherty amplifier Linearization and DPD Load-pull Transistor technologies Transistor biasing ACPR and EVM
Mixers and frequency generation	RF mixers Mixer specifications Image frequency I/Q modulators Oscillator basics Crystal oscillators VCO design Phase noise Jitter from phase noise PLL basics PLL loop filter Fractional-N synthesis Direct digital synthesis Multipliers and dividers
Antennas and propagation	Antenna fundamentals Gain and directivity Dipole antenna Monopole antenna Patch antenna Arrays and beamforming Small antenna limits Polarization Friis equation Free-space path loss Link budget Multipath and fading Atmospheric and rain loss Path loss models Fresnel zones
Receivers and systems	Receiver architectures Superheterodyne Direct conversion Low-IF receiver Software-defined radio Receiver sensitivity Blocking and selectivity Transmitter architectures Modulation ADCs for RF Radar basics
PCB layout and materials	RF PCB layout RF PCB materials Grounding and vias Controlled-impedance stackup Decoupling RF shielding Thermal design
EMC and EMI	EMC and EMI basics FCC Part 15 CISPR standards Immunity and ESD EMI filters and ferrites Precompliance testing RF exposure limits
Test and measurement	Vector network analyzer VNA calibration Spectrum analyzer Noise figure measurement Phase noise measurement RF power measurement Signal generators TDR Antenna measurement Oscilloscopes for RF
Simulation	Simulation overview Harmonic balance EM simulation EM solver methods SPICE for RF
Standards and bands	Frequency bands Radar band letters ISM bands 5G NR bands Wi-Fi and Bluetooth RF GNSS frequencies Amateur radio bands Spectrum regulation
Reference	Formula cheat sheet Textbooks and references Glossary FAQ All sources
About	About Editorial policy