Amplifiers Circuit Diagram

A hybrid configuration receiving new focus could be the Doherty amplifier, invented in 1934 by William H. Doherty for Bell Laboratories (whose sister organization, Western Electric, was then a vital manufacturer of radio transmitters). The Doherty amplifier consists of a class B primary or carrier stage in parallel with a class C auxiliary or peak stage. The input signal is split to drive the two amplifiers plus a combining network sums the two output signals. Phase shifting networks are employed within the inputs along with the outputs. In the course of intervals of low signal level, the class B amplifier effectively operates on the signal along with the class C amplifier is cutoff and consumes tiny power. For the duration of periods of high signal level, the class B amplifier delivers its maximum energy and also the class C amplifier delivers up to its maximum power. The efficiency of prior AM transmitter styles was proportional to modulation but, with regular modulation usually around 20%, transmitters were restricted to under 50% efficiency. In Doherty's design, even with zero modulation, a transmitter could achieve at the least 60% efficiency.

As a successor to Western Electric for broadcast transmitters, the Doherty idea was considerably refined by Continental Electronics Manufacturing Organization of Dallas, TX. Maybe, the ultimate refinement was the screen-grid modulation scheme invented by Joseph B. Sainton. The Sainton amplifier consists of a class C major or carrier stage in parallel with a class C auxiliary or peak stage. The stages are split and combined through 90-degree phase shifting networks as in the Doherty amplifier. The unmodulated radio frequency carrier is applied to the manage grids of each tubes. Carrier modulation is applied for the screen grids of both tubes. The bias point from the carrier and peak tubes is distinct, and is established such that the peak tube is cutoff when modulation is absent (and the amplifier is producing rated unmodulated carrier power) whereas each tubes contribute twice the rated carrier power in the course of 100% modulation (as 4 times the carrier energy is essential to achieve 100% modulation). As each tubes operate in class C, a significant improvement in efficiency is thereby achieved in the last stage. In addition, because the tetrode carrier and peak tubes call for very small drive energy, a significant improvement in efficiency inside the driver stage is achieved too (317C, et al.). The released edition with the Sainton amplifier employs a cathode-follower modulator, not a push-pull modulator. Preceding Continental Electronics designs, by James O. Weldon and others, retained the majority of the characteristics with the Doherty amplifier but added screen-grid modulation with the driver (317B, et al.).

The Doherty amplifier remains in use in very-high-power AM transmitters, but for lower-power AM transmitters, vacuum-tube amplifiers in common had been eclipsed within the 1980s by arrays of solid-state amplifiers, which may be switched on and off with much finer granularity in response towards the specifications from the input audio. However, interest inside the Doherty configuration has been revived by cellular-telephone and wireless-Internet applications exactly where the sum of numerous constant-envelope users creates an aggregate AM result. The principle challenge in the Doherty amplifier for digital transmission modes is in aligning the two stages and acquiring the class-C amplifier to turn on and off very quickly.