TABLE OF CONTENTS As has been seen in the previous chapters modern GaAs FET devices are capable of producing noise figures of less than 2 dB at 18 GHz with almost 10 dB associated gain whilst at the lower frequencies noise figures of less than 0.5 dB are possible. Power FETs are now capable of producing 15 watts up to X-band in Class A operation with promise of considerably more power in the pulsed mode of operation.
The GaAs FET offers in its simplest form a device having somewhat more convenient impedances for matching than the bipolar device and it is also capable of exhibiting much superior performance above approximately 4 GHz than the bipolar transistor. This chapter deals with the theory and design of amplifiers using GaAs FETs.
The measurements which are usually made on a three terminal device are those where one of the terminals is effectively earthed. In the case of a microwave GaAs FET it is usual to measure the device in common source configuration, i.e. where the source electrodes are bonded to r.f. ground. This arrangement usually provides the design engineer with the most stable arrangement and low feedback capacitance between drain and gate. Alternative configurations such as common gate and common drain are dealt with later.
The gain parameters of a microwave transistor can be completely specified by a set of 2 port parameters, the so-called scattering or S-parameters . Other descriptions of the transistor such as H or Y...
