Distributed theory for microwave bipolar transistors

A transistor large enough in one dimension to span a length significant compared to a quarter wavelength within the device can be treated as two-coupled transmission lines. The general solution of such lines is applied here for the small-signal linear case treated as a classical boundary value problem of TEM waves on coupled lines, which are uniform but have no other restrictions regarding transmission line parameters, coupling, symmetry, similarity, or terminations. This solution is extended to include the parasitic inductance and capacitance usually present in connecting the device into a circuit. Electrical characteristics of a distributed device begin to deviate noticeably from those of the equivalent lumped device even at a relatively short electrical length. Computer simulation of a large distributed transistor indicates that, if properly employed, distributed effects in small-signal operation may lead to advantageous characteristics such as in device impedance levels, stability, and gain-band product. Other characteristics may prove to be disadvantageous or troublesome in large signal or class C operation. If unrecognized or ignored, distributed effects can result in highly unsatisfactory transistor operation. As microwave transistors increase in size and frequency capability, distributed effects will undoubtedly need to be taken increasingly into account.