Doherty amplifier

The Doherty amplifier is a modified class C radio frequency amplifier invented by William H. Doherty in 1936.

The amplifier is usually configured as a grounded-cathode, carrier-peak amplifier. Using two vacuum tubes, or two power transistors in modern implementations, one as a carrier tube and the other as a peak tube, the amplifier operates by cutting off the negative swing of the RF wave and adding it to the positive swing, greatly improving efficiency and wasting less energy than earlier plate-modulated RF amplifiers.

The amplifier's response is more linear than earlier designs, allowing it to respond to variations in RF power from the modulator more efficiently. Because the voltage and current of both tubes are present on the load, the amplifier constantly sees a varying load, which caused problems in early Doherty designs in balancing the tubes as early engineers didn't fully understand the amplifier's response characteristics. To balance the load more evenly between the carrier and peak tubes, an inductance is added between the two anodes to stabilize the load, allowing easier tuning of the amplifier.

References

• Doherty, M (1936). "A new high efficiency power amplifier for modulated waves". Annual convention of the Institute of Radio Engineers. http://sujan.hallikainen.org/BroadcastHistory/uploads/Doherty.pdf. Retrieved 2009-08-21. 
• "The Doherty Amplifier: New After 70 Years". Microwave product digest. August 2007. http://www.mpdigest.com/issue/Articles/2007/aug/freescale/Default2.asp. Retrieved 2009-08-21.