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Design method and Simulation of broadband amplifier
2020-09-11 17:07:06

Distributed amplifier can provide a wide frequency range and high gain. For a time, transmission lines were often used as input and output matching circuits. With the development and maturity of GaAs microwave monolithic integrated circuit, in order to improve efficiency, output power and reduce noise figure, many kinds of amplifier circuit types have been proposed, but distributed amplifier is still the mainstream design of broadband circuit (such as optical communication circuit). Understanding the design of GaAs MMIC distributed amplifier will be helpful to the application of many wideband circuits.

Johns Hopkins University has been offering MMIC design courses since 198? And is allowing students to film on TriQuint's production line. A distributed amplifier designed by Craig Moore (who served as a teaching assistant for the course from 198? To 2003) serves as a classic design example of the course. The design has even experienced low-temperature environmental experiments, and shows lower noise figure at low temperature of liquid nitrogen. The amplifier adopts 0.5 μ m GaAs MESFET process of TriQuint company, and its gain is slightly lower than the new circuit based on 0.5 μ m GaAs pseudo high electron mobility transistor PHEMT. In the new course in 2006, the new version of 0.5 μ m GaAs PHEMT distribution amplifier and some other circuits are used as examples.


The distributed amplifier uses a broadband transmission line to inject input signals into a group of active devices (as shown in Figure 1), while another parallel transmission line is used to collect the output signals of each active device and stack them. Each stage provides a considerable gain, but the gain is distributed over a wide frequency range. Compared with cascade design, the total gain is the sum of all levels of gain, not the product of all levels of gain. However, when lumped parameter elements are used to approximate the distributed transmission line (Fig. 2), the ground parallel capacitance of lumped parameter transmission line is replaced by the parasitic capacitance of transistor. The equivalent transmission line of lumped parameter element is used as a low-pass filter, and its cut-off frequency is inversely proportional to the parasitic capacitance of the transistor. Therefore, the size of the transistor directly determines the upper limit of the operating frequency of the circuit. Various parameters to be considered in the design include: the number of amplifiers, the size of active devices, the process type of devices (if there are multiple types) and the DC bias of each stage. More series means more gain bandwidth product, but also more power consumption. Once the size of the transistor is determined, the simulation software can be used to optimize the parameters such as gain, reflection coefficient, output power and noise figure.


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