Distributed theory for microwave bipolar transistors

When a transistor is large enough to span a length which is significant compared to an electrical quarter wavelength within the device, its operation can be treated as two coupled transmission lines. The approach used here is to solve the problem rigorously from first principles for the small signal linear case by treating it as a classical boundary value problem of TEM waves in one rectangular dimension on coupled lines which are uniform but have no other restrictions regarding transmission line parameters, coupling, symmetry, similarity, or terminations. The solution is given in terms of voltage and current everywhere in both lines and is extended to include the parasitic inductance and capacitance usually present in connecting the device into a circuit. Numerical examples show that electrical characteristics 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 recognized and employed, distributed effects may lead to some advantageous characteristics such as in device impedance levels and stability. Other characteristics may prove to be disadvantageous or troublesome, as in large signal or class C operation. If unrecognized or ignored, distributed effects can result in highly unsatisfactory transistor operation, The general conclusion is that as microwave transistors increase in size and frequency capability, distributed effects will need to be taken increasingly into account.