Photovoltaic Bias for Depletion-Mode Devices in Low-Noise Amplifier Applications [Application Notes]

It can be a designer’s nightmare: choosing a high-performance microwave transistor to design a low-noise amplifier (LNA) or a transmitter output stage, only to find out that many of them are depletion-mode devices. These require an additional negative voltage since the gate bias must be of the reverse polarity to the drain one. When a single power supply is used for such a device, which is typically the only viable solution, generating additional negative voltage can be a challenging task because it quickly increases the complexity and cost. Furthermore, a switching power supply (the most common solution to this problem) demands sufficiently large capacitors for reliable operation and noise filtering. However, large capacitor values in bias networks might affect amplifier stability and so introduce an extra challenge that engineers quickly overlook.