Analysis and Design of BJT Differential Amplifier

A differential amplifier is a basic building block for analog integrated circuit design; the operational amplifier integrated circuit has wide applications in signal processing. In this paper, a bipolar junction transistor BJT differential amplifier circuit is analyzed and designed to determine the output resistance and the differential mode voltage gain. A cascode current mirror is used as a biasing circuit and the active load for the differential stage is a modified Widlar circuit. The calculations and results are performed using MATLAB software tool version 7.10.0 (R2010a) and the simulation is carried out with NI Multisim version 14.0.0, 2016. With standard resistors and transistors values the results showed that a decrease in the biasing current cause an increase in the output resistance of both the biasing circuit and active load stage, and a decrease in the differential voltage gain. The output resistance of the differential amplifier is also increased with increasing in the load resistance but the differential voltage gain is increased in this case.