Design and Optimization of a Highly Linear Power Amplifier with Novel Active Bias Circuit in InGaP/GaAs HBT MMIC Technology

This paper introduces a highly linear power amplifier (PA) that utilizes a 2-μm InGaP/GaAs hetero-junction bipolar transistor (HBT) process. As communication systems evolve in complexity, the demands for the linearity and static power consumption of HBT power amplifiers are becoming more stringent. Typically, there exists a trade-off relationship between linearity and static power consumption in the design process. Hence, this paper proposes a novel active bias circuit for HBT power amplifiers, along with its layout design. The proposed active bias circuit consists of a differential pair and a current mirror structure, designed to enhance PA linearity while maintaining low quiescent current. Additionally, a novel layout structure is implemented to achieve thermal coupling between the RF transistors and the active bias, further improving PA linearity. As a result of the implementation of the proposed active bias and layout structure, the PA demonstrates high linearity in both amplitude modulation to phase modulation (AM/PM) and amplitude modulation to amplitude modulation (AM/AM) performance, while simultaneously maintaining a low quiescent current. The three-stage PA prototype achieves notable performance metrics, including a peak gain of 32.6 dB at 5.6 GHz, gain flatness of ±0.5 dB from 5.15 GHz to 5.85 GHz, and P1dB of 32.1 dBm at a quiescent emitter current of 180 mA.


Introduction
HBT technology is widely adopted in civilian radio frequency chips due to its advantaged of low cost, high reliability and the use of single supply voltage compared to FET devices.However, achieving high linearity in HBT power amplifiers often requires a large base current to mitigate early gain compression, leading to unacceptably high overall quiescent currents, particularly for portable devices.To address this challenge, various active bias circuits have been proposed.
The active bias circuit using base-collector junction diode is firstly presented by T. Yoshimasu to compensate the base voltage drop of RF transistors [1] and the base-emitter junction diode is also proved able to enhance efficiency of PAs by H. Kawamura [2].Later, the active bias circuit is improved with serial diodes and transistors respectively to achieve better linearity of PAs by Y.S. Noh [3] [4].And active bias circuits designed with current mirror structure are employed for the delicate tradeoff between the quiescent current and linearity of PAs in [5] [6].However, the HBT power amplifiers still faces the challenges related to low power consumption requirements and linearity degradation caused by temperature variations during operation.
In this paper, a novel architecture of active bias circuit is presented and discussed.In the proposed active bias, a differential pair circuit along with a transistor-based current source are employed to enable complete adjustment of the quiescent current.Furthermore, a current mirror configuration is adopted to compensate for temperature variations.Additionally, a unique layout approach is proposed to achieve thermal coupling between both transistors in the current mirror, further improved the temperature adaptability of the amplifier.The proposed architecture is implemented using the 2-µm InGaP/GaAs HBT technology from WIN Company, chosen for its high maturity and excellent performance in MMIC applications.As a proof-of-concept demonstration, a highly linear PA prototype is designed and simulated incorporating the proposed active bias circuit.Simulation results show that the proposed PA operates from 5.15 GHz to 5.85 GHz with input return losses under -16 dB and gain up to 32.7 dB at 5.5 GHz.The simulated P1dB is 32.1 dBm at the quiescent emitter current of 180 mA.The findings highlight the significant enhancement in linearity achieved by the proposed active bias circuit.
This paper aims to contribute to the advancement of highly linear HBT power amplifiers.The proposed bias circuit improved the linearity performance of the power amplifier while minimizing quiescent current, which addresses the contradiction between static power consumption and linearity in power amplifier design.The proposed layout strategy for thermal coupling realizes the coordinated temperature change of both transistors in current mirror, offsetting the influence of temperature on the bias current and finally improve the linearity of PA.These improvements make the power amplifier more suitable for modern wireless communication systems.

Analysis and Design of the Proposed Active Bias Circuit
As shown in figure 1, the proposed active bias circuit comprises four components.Part A focuses on the transistor HBT0, which serves as a crucial element in the active bias circuitry.The first and second order transconductance of HBT0 are denoted as  21 and  22 , respectively.Referring to the calculation in [7], the base current  , of transistor HBTRF can be expressed using the following equation.The careful derivation process is presented in [7].Notably, the polynomial  ,1 2 +  ,2 2 , which stands for the sum of the first and second order of the input signal voltage, indicates that the base current   increases with the power of the input signal.The analysis in [7] reveals that the second-order distortion component of HBT0 includes a significant DC component, which realized the self-compensation of the base current of HBTRF through the active bias.This compensation mechanism is a critical feature of active bias circuits, enabling the simultaneous achievement of high linearity and low quiescent current.Figure 2 illustrates the P1dB of the two PAs that are both supplied with a base current of 106 uA and operating under the same match condition.The key distinction between the two PAs lies in the bias circuit employed: one utilizes a conventional resistor-based bias circuit, while the other incorporates the proposed active bias circuit.The comparison clearly demonstrates that the P1dB is noticeably delayed when the active bias circuit is utilized.
Equation ( 2) illustrates that the quiescent current (Iq) can be adjusted by the area ratio (m) of the two transistors, the reference voltage (Vref) and the resistance of R4.With this bias architecture, R4 and m can be elaborately designed to ultimately determine the quiescent current (Iq), significantly enhancing the adjustability of the power amplifier's quiescent current.The transconductance of transistors HBT3 and HBTRF is represented by β1 and β2, respectively, while VCE denotes the collector voltage of HBT3.In equation (3), the variable k is supposed to be constant for β1, β2 and VCE are determined by manufacturing factors.However, as a matter of fact, the transconductance of transistors is temperature-dependent.Consequently, the quiescent current (Iq) will vary with temperature during the operation of the power amplifier, which can lead to nonlinear behaviors such as AM/PM and AM/AM distortion.
To mitigate the influence of temperature variation, the proposed layout strategy places the transistor HBT3 of the active bias in the middle of the RF main transistor, as shown in the red box in Figure 5.The physical proximity enables HBT3 and HBTRF to achieve good heat conduction, achieving thermal coupling.This thermal coupling ensures that HBT3 and HBTRF experience similar temperature variations, thus maintaining the constant value of k. Figure 3 compares the performance of two different AM/PM configurations.The blue line represents the performance with thermal coupling layout, while the red line represents the performance without it.Additionally, in part A, the current source formed by HBT1 and R2 is utilized to share the current generated from the emitter of HBT0, as it is difficult for HBT0 to stably output such low emitter current due to its large current gain (β).The resistance of R2 precisely sets the base current of transistor HBTRF.To achieve proper and stable biasing of HBT0, a differential pair in part B is employed instead of resistors, mitigating the impact of temperature variation and manufacturing errors.Part D serves as a tail current source, stabilizing the output current of the differential pair.The resistance of R3 allows for adjustment of the output current of the differential pair.
In summary, the proposed active bias circuit offers the advantages of low quiescent current and high linearity for the power amplifier.The proposed structure significantly enhances both the AM/PM and AM/AM performance of the PA.

Design of the Highly Linear PA
As shown in figure 4, a highly linear PA utilizing the proposed active bias circuit is demonstrated.To comply with the recommended maximum current density of 25 kA/um 2 , ten HBTs with the size of 2um×40um×4fingers are adopted for the output stage.Additionally, a ballast resistor is placed at the base of each stage to ensure the thermal stability.The inductors L3, L4 and L5 are implemented using gold bonding wires to achieve high quality factor (Q). Apparently, the inductance values of L3, L4, and L5 play a crucial role in determining the output matching of the power amplifier.Therefore, during the debugging phase, the shape of the bonding wires can be adjusted to fine-tune the output matching and optimize the overall performance of the power amplifier.Additionally, the capacitor C1 and inductor L1 form a trap for the second harmonic, aiming to improve linearity of the PA.The inductors L2 and L3 serve a dual purpose of contributing to matching and providing electrostatic discharge (ESD) protection for the input and output pads.Finally, a feedback circuit comprising a capacitor, resistor and inductor is employed in the first and second stage to optimize the gain flatness of the amplifier.
Figure 5 demonstrates the layout of the presented PA, which has a total area of 890×1700 um 2 .The layout design and simulation of the proposed PA are performed using the ADS platform developed by Keysight Technologies.In the layout, the proposed bias circuit is designed to be compact, located in the top left corner of the chip.However, the transistors for thermal coupling, as discussed in part 2, are not included in the bias circuit but rather placed strategically next to the power transistors.The output and input pads, the 2 nd harmonic trap and bonding wires are also marked in figure 5.

Simulation Results
Figure 6 presents the simulation results of the S-parameters for the proposed PA.The input return loss |S11| is lower than -16dB within the frequency band from 5.15 GHz to 5.85 GHz.The output return loss |S22| is lower than -8dB.And the peak gain |S21| for the small input signal is equal to 32.6dB.The gain flatness is within ±0.5 dB across the operating band.The large signal gain and power added efficiency (PAE) as a function of the output power are depicted in figure 7. The power amplifier exhibits a P1dB of 32.1 dBm.The peak efficiency is approximately 40%.In the DC simulation, it is observed that the total quiescent emitter current of the three stages amounts to 180 mA.

Summary
In this paper, a comprehensive exploration of a 5.15-5.85GHz power amplifier employing InGaP/GaAs HBT technology has been presented.The focal point of this research revolves around the introduction of an innovative active bias circuit, strategically designed to enhance the AM/PM and AM/AM performance of the power amplifier while concurrently maintaining a low quiescent current.
The active bias circuit, a pivotal element in our investigation, is intricately crafted through the integration of a differential pair and a current mirror.This synergistic combination contributes to the circuit's effectiveness in achieving a delicate balance between linearity and quiescent current, addressing the inherent challenges faced by HBT power amplifiers.Notably, the proposed active bias circuit surpasses its predecessors by providing enhanced adjustability of quiescent current and compensating for temperature variations, thus improving the overall performance of the power amplifier.
Furthermore, a unique layout strategy has been introduced to foster thermal coupling between critical components, specifically the transistors in the current mirror.This innovative layout design ensures consistent temperature conditions for the coupled transistors, thereby maintaining the desired bias conditions and enhancing the amplifier's performance.
In conclusion, our research marks a substantial contribution to the advancement of highly linear HBT power amplifiers.The introduced active bias circuit, along with the thermal coupling layout, collectively enhances the amplifier's suitability for modern wireless communication systems by improving linearity, minimizing quiescent current, and accommodating temperature variations.These findings underscore the potential of the proposed architecture in addressing contemporary challenges in RF power amplifier design.

Figure 1 .
Figure 1.The proposed active bias circuit composed of differential amplifier (B) and current mirror (C).

Figure 2 .
Figure 2. The P1dB of the PA with and without active bias at the same base quiescent current.In figure 1, the combination of part A, part B, and part C forms a buffered current mirror structure that determines the quiescent base current of HBTRF.The differential pair in Part B inherently offsets undesirable common mode signals, reduce extra phase distortion and suppresses voltage change at the collector of RF main transistor.The transistors of HBT3 and HBTRF in part C form a current mirror.According to the theory of a current mirror, the quiescent current of HBTRF could be expressed as followed.

Figure 3 .
Figure 3.The AM/PM performance of the PA with and without thermal coupling.

Figure 4 .
Figure 4. Schematic diagram of the proposed PA.

Figure 5 .
Figure 5. Layout of the proposed PA.

- 4 -Figure 7 .
Figure 7.The simulated power gain and PAE of the proposed PA.