Design and Optimization of Biasing Networks for Wideband High Power Amplifiers

Abstract

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.

Full text

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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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Content
•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.
–Linearity
•Gain flatness
•Memory effects
•DPD performance

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Structure of a PA

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Effect of the gate resistor
Ztotal=R

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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
f2-f12f2-2f1➢Resistance
•Thick and wide line
➢Inductance
•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
RF BW RF
isolation
Current
Handling
Inductance at
Low frequency
Heat
dissipation
Narrow ▲ ▼ ▲ ▼ ▲ ▼
Wide ▼ ▲ ▼ ▲ ▼ ▲
Zo
Inductance
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
impedance

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Resonance Investigation with
S-parameter measurement

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Design Example 1
IM3H
IM3L
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
λ/8
λ/8
•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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Questions