LINEAR, HIGH POWER IMPATT AMPLIFIERS USING CONSTANT VOLTAGE BIA

A new type of IMPATT amplifier using constant voltage bias has been developed which provides a practical solution for handling heretofore troublesome diode instabilities. For an amplifier using constant current bias the gain drops to unusually low levels at high input powers, with consequent high distortion levels and low efficiency. For an amplifier using constant voltage bias and a stable microwave circuit, the bias current rises at high input powers thus making it possible to maintain a negative conductance approximately half that obtained with small signal input. This limited drop in the negative conductance at high input power yields an amplifier with reasonably low distortion and essentially constant gain over the entire input power range.;Two constant bias voltage amplifiers have been built and tested. One amplifier used a Varian grown-junction hi-lo single drift diode, and the other used a Raytheon Schottky-barrier lo-hi-lo single drift diode. The system performance of both amplifiers was very attractive for linear amplifier applications.;The basic theory of constant voltage operation has been published by Lee et al. However, to study the physics involved in the constant voltage operated IMPATT amplifiers studied here a computer model was developed that avoids some of the analytical approximations made earlier and extends the modelling of the drift zone. This computer model predicts the chip admittance as a function of the r.f. voltage amplitude. Input parameters were found from basic measurements such as the doping profile, ionization rates, and thermal resistance. Only the saturation current was assumed rather than measured. These calculations were found to be in excellent agreement with measurements of the chip admittance taken with a large signal automatic network analyzer.