Audio-Frequency Class A1 Single Triode Power Amplifier

A convenient method of analysis¹⁹ for a single triode power amplifier is shown in Fig. 24. The direct plate voltage E_b is first selected at some convenient value. The correct grid voltage to use is approximately

The selected plate voltage and the calculated grid voltage establish the operating point P. The line X-Y is the load line and is drawn

Figure 23. With an ideal transformer, the load impedance reflected into the plate circuit equals ZL' multiplied by the turns ratio squared.

through the selected point of operation P at the angle, made with the vertical, θ = tan^-1R_L. It will be noted that in this figure the abscissas are in volts, the ordinates, in milliamperes, and R_L,in ohms. For Fig. 24, the grid bias E_c = -50 volts, the plate voltage E_b = +250 volts, and the load resistance is R_L = 3900 ohms, the grid is driven to zero value, and the plate-voltage and grid-current values are as shown. For a single tube such as in Fig. 23, the power output is

and the percentage second harmonic distortion is

Second harmonic distortion =

The plate-circuit efficiency, or ratio of signal power output to direct power input, is low for this amplifier. For maximum power output, the load resistance should equal the plate resistance (page 152). Because of second harmonic distortion, caused by the curvature of the dynamic characteristic curve of a triode, the relation just mentioned cannot be used. Experience has shown that for maximum "undistorted" power output the load resistance should be about twice the plate resistance. As used here, "undistorted" means that the nonlinear distortion is less than about 5 per cent.

The dynamic curve mentioned is the curve for the tube and its load resistance. This curve can be obtained experimentally or can be calculated.¹⁶ It corresponds to the load line of Fig. 24.

Figure 24. The applied grid-signal voltage swing causes the plate current and plate voltage to vary as shown. The total power delivered by the plate suppty source to the tube and resistance RL in the plate circuit is represented by the area oabc. The power delivered to the tube alone is given by the area oaPd. The alternating signal power output of the tube is shown by the area of the triangle d'Pc'. If a transformer is used as in Fig. 23, then the amount of direct-current power required is reduced because the tube is loaded with a reflected resistance instead of a resistor RL.