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Design and Optimization of Biasing Networks for Wideband High Power Amplifiers


Abstract and Figures

The demand for very high data rate 5G systems requires the use of wider spectrum bandwidths and phased array systems to increase the network capacity. Although the individual power level of each RF amplifier decreases due to the array factor, in the sub-6GHz bands it is still high enough to place significant challenges on its development, to fulfill the increasing performance requirements. The RF bandwidth requirement of 200 to 600 MHz, associated with the high back-off efficiency and linearity needed by the use of wideband modulated signals, leads to even more complex architectures. One of the consequences is the design and optimization of the bias networks that becomes a challenging issue as it needs to consider even more parameters, such as low-frequency stability, video bandwidth, power handling capability, linearity, and reliability. All of these parameters have a direct influence one the performance of the PAs. In this study, we present (i) a detailed analysis of the PA biasing networks regarding low-frequency stability, (ii) an in-depth examination of the several types of RF-bypass capacitors, (iii) modeling of the RF-bypass capacitors at low frequency range, (iv) a comparison of different biasing topologies by considering video bandwidth, and (v) finally a design methodology for biasing network of the wideband RF PAs. Moreover, a low-frequency resonance problem-solving example and video bandwidth extension study with a low voltage driven dual-stage GaN MMIC PA are given. The links and trade-offs between the requirements and technical restrictions are pointed out to guide to the designers.
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Design and Optimization of Biasing Networks
for Wideband High Power Amplifiers
Osman Ceylan, Taha Dogan, Sergio Pires
Ampleon Netherlands BV
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Gate Biasing
Low frequency response & stability
Design suggestions
Drain Biasing
Design considerations
Biasing Topologies
Some design examples
Resonance Investigation
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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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High data rate communication systems require
wideband PAs.
Design of a wideband high power amplifier for the
high data rate communication systems is a challenge.
Gain flatness
Memory effects
DPD performance
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Structure of a PA
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Effect of the gate resistor
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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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Drain Biasing
DC Condition
Current handling capability
DC resistance
Low Frequency Condition
Low frequency stability
Biasing induced memory effects
High-Frequency Condition
RF power loss
Zbias= 0 Ω@DC
|Zbias|= 0 Ω@low freq
|Zbias|= ꝏ Ω @fo
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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.
Width Characteristic
Impedance at fo
Inductance at
Low frequency
Narrow ▼ ▲
Wide ▲ ▼
25 Ω3.1 nH
50 Ω6.5 nH
100 Ω
12.6 nH
@low frequency
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Biasing Topologies
λ/4 wavelength microstrip
Better RF bandwidth
Poor low frequency performance (high inductance)
λ/8 wavelength microstrip
Narrower bandwidth
Good low frequency performance (low inductance)
Embedded to matching network
Sensitive structure
Issues with 2nd harmonic termination
Integrated to power combiner
Balun or coupler
Complex design
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Drain Biasing Network Considerations
Very low DC resistance
Power handling capability
Low RF loss in operation band
Better performance
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
Low inductance
Specific design regarding the requirements
Additional attention to the second harmonic
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Resonance Investigation with
S-parameter measurement
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Design Example 1
Before tuning
Dual stage GaN MMIC
5W output power, 27 dB linear gain
2.1 GHz
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.
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Design Example 2
1.9-2.6 GHz, Doherty Amplifier
42 dBm output power @3pdB
>900 MHz video bandwidth
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Design Example 3
GaN Doherty Amplifier
1.8-2.1 GHz
250W peak power @p3dB
>300 MHz video bandwidth
Output stage of the Doherty Amplifier
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