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•Biasing circuits are essential parts of the PAs.
–Simply; no biasing, no amplification
–Moreover; no proper biasing, no good performance
•Influence on the PA performance
–Stability (mostly low frequency)
–Efficiency (RF and DC losses)
–Linearity and memory effects
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Design Considerations of Gate Biasing Network
•Biasing is possible with resistor, inductor or λ/4 microstrip lines.
•Using a resistor improves stability and isolation.
•Gate current should be considered for the very high power GaN HEMT devices.
•A gate current flows around 1mA/mm at the saturation region.
•Biasing voltage swing at the gate causes nonlinearities.
•Lossless and wideband gate biasing network design is possible due to negligible current.
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Linearity Issues Induced by Biasing Networks
∆V = ids(t) x Rout + Zout1ids(∆1,t) + Zout2ids(∆2,t)
Resistive part Inductive part
•Thick and wide line
•Total inductance from drain pad to RF-bypass
capacitors should be as small as possible
ΣL = Lwire + Lpackage + Lbiasline
5-8% 1-2% 90%
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λ/4 Microstrip Line
Inductance of a RF by-passed
microstrip line at the low frequency *
Impedance at low frequency
Impedance at operating frequency
*S. C. Cripps, RF Power Amplifiers for Wireless Communications, 2nd ed. Norwood, MA: Artech House, 2006.
Impedance at fo
RF BW RF
Narrow ▲ ▼ ▲ ▼ ▲ ▼
Wide ▼ ▲ ▼ ▲ ▼ ▲
25 Ω3.1 nH
50 Ω6.5 nH
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•λ/4 wavelength microstrip
•Better RF bandwidth
•Poor low frequency performance (high inductance)
•λ/8 wavelength microstrip
•Good low frequency performance (low inductance)
•Embedded to matching network
•Issues with 2nd harmonic termination
•Integrated to power combiner
•Balun or coupler
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Drain Biasing Network Considerations
➢Very low DC resistance
➢Power handling capability
➢Low RF loss in operation band
➢Wide Video bandwidth
➢Low frequency stability
•Proper width of biasing microstrip line (wide enough)
•Thickness of the line
•Additional surface finishing
•Soldering a metal plate onto the line
Very low impedance at the low frequency region
•Very low ESR RF-bypass capacitors
•Specific design regarding the requirements
•Additional attention to the second harmonic
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Design Example 1
•Dual stage GaN MMIC
•5W output power, 27 dB linear gain
M. Acar, O. Ceylan, F. Kiebler, S. Pires and S. Maroldt, "Highly efficient GaN RF power amplifier MMIC
using low-voltage driver," 2017 12th European Microwave Integrated Circuits Conference (EuMIC),
Nuremberg, 2017, pp. 188-191.