Investigation of the effect of impedance matching on stability and bandwidth in RF amplifier design for 2.4 and 5 GHz frequencies

While analog circuits operating at low frequencies can be expressed with measurable quantities such as admittance, impedance, voltage and current, it is not possible to measure circuits operating at RF/microwave frequencies directly with these variables. It is more convenient to use variables such as voltage reflection and transmission coefficients, which are easier to measure and related to power flow. In this study, a class A amplifier for small signals has been designed and examined in simulation based on the center frequency of 2.4 and 5 GHz for RF/microwave. For this purpose, the BFP540 transistor, which is easily accessible and has the necessary data integrated in the AWR Design Environment software, has been used. L-type matching circuits have been used in order to have a high bandwidth in the circuit. Due to the high operating frequency of the circuit, it has been preferred to design the input and output matching circuits with distributed elements instead of lumped elements. The stubs have been designed as symmetrical and asymmetrical, open and short-circuit for matching circuits. Then, they have been examined again in terms of bandwidth among themselves. Gain reduction methods such as using negative feedback and shunt resistance at the output have been also tested to provide unconditional stability and increase bandwidth. Smith chart has been used while calculating transmission lines and stubs for matching circuits. The effects of the solutions for the first and second intersection points and the designed stub types and their combinations among themselves, and their effects on the bandwidth in the spectrum have been re-examined. The circuit has been then designed with lower collector current and included in the comparison. While designing, scattering (S) parameters received from the manufacturer have been used, a calculation software has been implemented and reflection coefficients and impedance/admittance variables have been calculated. Using these values, the stub positions and lengths to be added to the transmission lines have been found with the help of the Smith chart, and these values have been used in the simulation studies conducted with the relevant software. The simulation results of the design obtained with the relevant software have been examined in terms of gain, stability, noise figure, bandwidth and compatibility, and the differences between the results have been discussed. In addition, the calculation of these variables has been explained on an example in order to be useful for students and researchers who are new to the subject.