Power Amplification

Author: J.B. Hoag

Voltage amplifiers are designed to give the greatest possible alternating output voltage for a given input voltage. Power amplifiers are designed to deliver power (watts) to the load, voltage amplification being incidental.

If the amplifier is to be used to increase power rather than voltage, as is the case with the last tube in a radio receiver, a tube of different structure is used, and the design of the output circuit is different than in voltage amplifiers. The tubes are selected, not with high μ but with large mutual conductance g_m. The tube and the load impedance are chosen equal to each other for maximum power output. For minimum distortion, the load impedance is often made equal to approximately twice the internal impedance of the tube. Three types of audio-output coupling circuits are shown in Fig. 13 M.

Fig. 13 M. Types of output circuits: (a) transformer coupled; (b) LC coupled (L and C must both have large values); (c) frequency division, the low and high notes operating separate speakers

The power amplification is defined as the ratio of the output power to the a.c. power in the grid circuit. For tubes in which no power is consumed in the grid circuit the term power sensitivity is used. This is defined as the ratio of the output power to the a.c. grid voltage. There is a third term, plate efficiency, which is defined as the ratio of the output power to the d.c. power input to the plate. The latter is given by the product of the plate current and the plate voltage. In general, amplifiers free from distortion have low plate efficiencies and vice versa.