Active Biasing Solution for pHEMT Power Amplifiers

Pseudomorphic high electron mobility transistors (pHEMT) are depletion devices that have drain-source channels with resistances close to 0 Ω. This property allows the devices to operate at high gain at high switch- ing frequencies. However, the high conductivity of the drain channel might result in a burnout on the devices if a proper gate and drain bias sequencing is not applied. In this article, we will explore the operation of depletion mode pHEMT radio frequency (RF) amplifiers and how they can be practically biased. Depletion mode field-effect transistors (FETs) require a negative gate voltage and turn on/off must be carefully sequenced. Fixed gate voltage and fixed drain current circuits will be presented and compared. We will also take a close look at how the noise and spurs of these biasing circuits affect RF performance.