This invention relates generally to power amplifiers, and more particularly the invention relates to switch-mode power amplifiers and transistors useful therein.
Switch mode power amplifiers have attracted a significant amount of interest for use in applications requiring highly efficient power amplification of high frequency signals. Examples of applications of such devices include power amplifiers for wireless communications systems, satellite communications systems, and advanced radar systems. In particular, high power, high frequency power amplifiers are needed for digital communication systems such as 3G and 4G PCS systems, WiFi, WiMax and digital video broadcast systems. For applications requiring high output power, the power amplifier accounts for a significant portion of the overall power consumed by the system. Thus, it is desirable to maximize the efficiency of the power amplifier circuit in a communication system.
Co-pending application Ser. No. 11/132,619, supra, discloses a single-stage switch mode amplifier circuit which includes an active device switch transistor configured to operate in either an ON state or an OFF state depending on the signal level of an input signal. The switch transistor has an output connected to a load network which filters the signal output from the switch transistor to provide a narrow-bandwidth output signal to a load impedance. Energy rejected by the load network is stored in a switch capacitor which continues to drive the output signal while the switch transistor is in the OFF state. Drain voltage to the switch transistor is provided through a drain inductor which prevents instantaneous changes in source current. In some embodiments, the amplifier operates in Class E mode.
In some embodiments, a switch mode amplifier circuit includes an input matching stage, an active stage and an output matching stage. The active stage includes an active device switch transistor in parallel with a switch capacitor. The switch transistor has an output connected to a load network and a load impedance. The output of the device, which comprises the voltage across the load impedance, is supplied to the output matching stage, which transforms the output impedance of the active stage to the desired output impedance of the circuit. In other embodiments, multiple active transistor stages and matching networks may be used to provide additional amplifier gain (e.g. 2-stage amplifiers, etc.)
The switch transistor can comprise a wide bandgap MESFET transistor capable of sustaining high drain voltages and/or high current levels while operating at frequencies in excess of 1.0 GHz. In some embodiments, the switch transistor comprises a gallium nitride (GaN) based high electron mobility transistor (HEMT). In some embodiments, the switch transistor comprises a GaN based HEMT having a total gate periphery of about 3.6 mm. In some embodiments, the switch transistor comprises a GaN MESFET. In other embodiments, the switch transistor comprises a different wide bandgap high frequency transistor, such as a SiC MESFET, SiC LDMOS, SiC bipolar transistor, or GaN MOSFET device.
Switch mode operation of field effect transistors in an amplifier requires robust operation at microwave frequencies under high compression. In practice, this is difficult to realize due to the very large forward currents that flow from gate to source under high input drive as needed for switch mode operation.