Distortion in Class A Amplifiers

Author: J.B. Hoag

Fig. 12 B. Characteristic curves of a triode. Plate current Ip vs. grid voltage Eg (From E. & N. P.)

As already pointed out, the output wave-form of Class A amplifiers is the same as the input wave-form. In order that this be true, they must be operated strictly as linear devices, over a straight-line characteristic curve. It should be obvious from the preceding section that distortion will occur in Class A amplifiers if the input signal voltage is so great that the upper and/or lower knee of the characteristic curve comes into use. Referring to Fig. 12 B, it can be seen that a longer straight curve is to be had when the plate voltages on a given tube are greater. For this reason, the successive stages in a Class A amplifier sometimes use higher and higher plate voltages and correspondingly greater and greater grid voltages. It was shown in Sec. 12.3 that the dynamic curves of a tube are straighter over a greater range of grid voltages (and also have smaller slopes) than the static curves, especially when the plate circuit resistance is large. This means that voltage amplification is sacrificed in order to prevent distortion. Thus larger plate circuit resistances are used to reduce distortion in Class A amplifiers.

When the C-bias on a triode is so adjusted that the operating point occurs near the bottom or the top of the curve, the output waves will be distorted, because part or all of the half-cycles of plate currents are squared off. It will be recalled that an irregular wave-form is the equivalent of a mixture of a direct current and a fundamental sine curve plus its various higher harmonics. When a particular device distorts a sine wave, the output wave will contain the fundamental frequency plus the higher harmonics. The more irregular the wave, or the sharper the bends in it, as at a square top, the stronger will be the harmonics in comparison with the fundamental.

When properly operated, Class A amplifiers are characterized by low power output for a given size of tube, by small distortion compared with Class B or C amplifiers, by high power amplification and by low plate efficiency (20 to 35 percent).