Distributed amplifiers are known for their wide frequency range and high gain, making them ideal for broadband applications. Historically, the design of these amplifiers relied heavily on transmission lines for input and output matching. In 1948, Bill Packard, one of the founders of Hewlett-Packard, introduced a vacuum tube amplifier based on distributed design in his paper, marking an early milestone in this field. With the advancement of gallium arsenide (GaAs) microwave monolithic integrated circuits (MMICs), researchers have continuously aimed to improve efficiency, output power, and noise performance. Despite these developments, distributed amplifiers remain a key solution for broadband systems, such as optical communication, due to their inherent advantages.
At Johns Hopkins University, since the early 1980s, an MMIC design course has been offered, allowing students to access TriQuint's production line. A classic example from the course was a distributed amplifier designed by Craig Moore, who served as a teaching assistant from 1985 to 2003. This design underwent low-temperature testing, showing improved noise performance at cryogenic temperatures. The original design used TriQuint’s 0.5μm GaAs MESFET process, but later versions transitioned to 0.5μm GaAs PHEMT technology, offering better performance. In 2006, a new version of the distributed amplifier using PHEMT was introduced in the course, along with other circuit examples.
Figure 1 shows the schematic of a distributed amplifier using microstrip transmission lines. In this design, an input signal is injected into multiple active devices through a broadband transmission line, while another parallel line collects and combines the outputs. Each stage contributes similar gain, spreading it across a wide frequency range. Unlike cascaded designs where total gain is the product of individual gains, the overall gain here is the sum of each stage’s gain. When using lumped components to approximate transmission lines, the shunt capacitance is replaced by transistor parasitic capacitance, forming a low-pass filter whose cutoff frequency depends on the transistor size. Thus, the transistor’s dimensions directly affect the maximum operating frequency.
Figure 2 illustrates a lumped-component-based distributed amplifier, with CGS and CDS representing gate and drain capacitances. Due to the flexibility in application requirements, broadband gain remains a critical factor. In Craig Moore’s design, an enhanced PHEMT device was chosen to simplify the power supply configuration. To match the performance of TriQuint’s 0.5μm GaAs MESFET process, a 3-stage topology using PHEMT was implemented, operating at 3.3V for battery-powered applications. The design allows for adjustable voltage and current, maintaining stable performance even under varying conditions.
Using Agilent’s ADS software, linear simulations were conducted to optimize inductance values and PHEMT sizes for optimal gain and matching. Figure 3 presents simulation results for gain, noise figure, stability, and matching. A 6×30μm enhanced PHEMT was used, with additional components added to the drain to balance output and input capacitances, ensuring symmetrical phase delay. The design compared symmetric and asymmetric matching schemes, choosing the simpler approach due to minimal phase difference. For more realistic simulation, TriQuint’s models for inductors, resistors, and capacitors were used.
The final design operated at 30mA and 3.3V, limiting power consumption to 100mW while balancing output power and third-order intermodulation distortion. Figure 4 shows the layout, including two test structures for different PHEMT sizes, providing valuable data for performance evaluation.
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