High-power amplifiers (HPAs) for RF/microwave applications were once automatically associated with electron-tube devices such as klystrons or traveling-wave tubes (TWTs). While those devices still provide some of the highest output-power levels available at microwave frequencies, solid-state HPAs are pushing output-power performance higher, largely due to the capabilities of gallium-nitride (GaN) wide-bandgap transistors that enable HPAs with high output-power levels in smaller packages.
The physical sizes of these amplifiers are generally limited by the thermal characteristics of the devices, their associated printed-circuit boards (PCBs), and their enclosures. Although GaN and other power transistors are also gaining in efficiency, even a device with 50% efficiency still wastes one-half of its supplied energy as heat and other forms of energy losses.
Amplifier output power comes in many forms. An amplifier designed for linear Class A or Class AB use (with transistors remaining turned on) with continuous-wave (CW) signals will generally deliver less output power than the peak power levels possible with an amplifier developed for pulsed signals. An example of the latter is a Class D amplifier that switches on and off to save power and boost efficiency. Solid-state RF/microwave amplifiers exploit several different transistor technologies, including silicon (Si) lateral diffused metal-oxide-semiconductor (LDMOS) devices for lower frequencies and GaN and GaAs field-effect transistors (FETs) at higher frequencies.
Some designs, such as the innovative model 2154-BB52E6ARR from Empower RF Systems, combine semiconductor technologies, using Si LDMOS at lower frequencies and GaN for upper-frequency operation. The Class AB linear amplifier system comes in three rack-mountable enclosures, with a Si LDMOS amplifier for 20 to 1000 MHz and a GaN amplifier for 1 to 3 GHz that feature as much as 500 W CW output power over each frequency range. The third rack-mount enclosure carries power-supply, control, and monitoring circuitry for the two amplifiers. The amplifier, powered by a single-phase power supply, is equipped with forced-air cooling and an assortment of self-test functionality for protection.
As an example of the solid-state power levels that can be reached with pulsed signals, the firm offers the GaN-based rack-mount model 2213 power amplifier for applications from 2900 to 3500 MHz. It also operates under Class AB conditions, but with pulsed signals, and is capable of 10 kW peak output power with 2- to 100-µs pulses at duty cycles from 0.5 to 6.0%.