Linearization Techniques for Bipolar Transistor Transconductance Stage

Conventional BJT differential pairs have a relatively small input voltage dynamic range. This paper reviews and compares several linearization techniques, which are used to expand the input transconductance (gm) stage’s dynamic range. Two techniques are emphasized: the first one is the multi-tanh principle, and the other one is based on classical emitter degeneration. The main concept of multi-tanh is that individual nonlinear gm functions can be divided along the input voltage axis to obtain a function with higher linearity. This modification expands the linearity and results in a wider operating dynamic range. Emitter degeneration is an elementary technique used to increase stability and lessen thermal runaway. Adding an emitter resistor creates negative feedback, this keeps the transistor’s operation stable. Simulation results are provided for both, and the layout is presented for the emitter degeneration circuit. Both circuits are designed using state-of-the art SiGe BiCMOS technology provided by IHP.