Transient Distortion in Transistorized Audio Power Amplifiers

This paper discusses a new kind of distortion mechanism found in transistorized audio power amplifiers. It is shown that this distortion arises from the multistage feedback loop found in most high-quality amplifiers, provided that the open-loop transient response of the power amplifier is slower than the transient response of the preamplifier. The results of the analysis are verified by measurements from a simulated power amplifier, and a number of constructional rules for eliminating this distortion are derived. Manuscript received December 3, 1969; revised January 23, 1970. introduction An ordinary transistorized audio amplifier consists of a preamplifier and a power amplifier. The typical preamplifier incorporates two to eight stages with local feedback. The power amplifier has, however, usually a feedback loop enclosing three to four stages. The power amplifier generally determines the frequency response and the distortion of the whole amplifier, For stationary signals, the harmonic distortion of the power amplifier decreases proportionally with increasing feedback, provided that the transfer function of the amplifier is monotonically continuous and that the gain is always greater than zero. (These assumptions are not valid, of course, in case of overload or crossover distortion.) With the same assumptions, the intermodulation distortion decreases similarly. The frequency response is also enhanced in proportion with the feedback. It would seem, then, that feedback is highly beneficial to the power amplifier. The purpose of this paper is, however, to show that the usable frequency response of the amplifier does not necessarily become better due to feedback, and that, under certain circumstances, the feedback can cause severe transient distortion resembling intermodulation distortion. These facts are well known among amplifier designers and have been discussed on a phenomenological basis (for instance [l]). They have not, however, received a. quantitative ,treatment except in some special cases [2], [3 I. Transient Signals in Amplifiers Sound in general, and especially music, consists largely of sudden variations. The steep rise portion of these transient signals can be approximated with a unit step function, provided that the transfer functions of the microphone and the amplifiers are considered separately. We may, therefore, divide the amplifier as in Fig. 1. A is the preamplifier including the microphone, C is the power amplifier, and B is the feedback loop around it. If resistive feedback is to be applied in the power amplifier, stability criteria necessitate its transfer function to have not more than two poles and a single zero in the usable frequency range. The transfer function without feedback can thus be approximated to be of the form F,(s) = d o SD? 1 (1) (s + wo)(s + 4 where A. is the midband gain without feedback, and w1 and w0 are the upper and lower cutoff angular frequencies, respectively. The transfer function of the signal source and the preamplifier can be arbitrary. Usually, however, it can be considered as having several poles and zeros, often multiple. In the following we will consider two special cases. Case a: The transfer function is flat in the midband and has a 12 dB per octave rolloff in both the high-frequency 234 lEEE TRANSACTIONS ON AUDIO AND ELECTROACOUSTICS VOL. AU-18, NO. 3 SEPTEMBER 1970 V1 A v 2 + v 3 0 C 1 ; O v4 Fig. 1. The analyzed circuit. A is the preamplifier which includes the transfer function of the signal source. B is the feedback path around the power amplifier C. Fig. 2. The preamplifier f equency response asymptotes used in the analysis. Asymptote o corresponds to a flot response and asymptote b corresponds to o cose where the high-frequency tone control has been turned to maximum. and the low-frequency ranges. This characteristic is shown in Fig. 2 with asymptote a. Case b. The transfer function in the low-frequency range is similar to Case a. A $6 dB/octave emphasis is applied in the high-frequency range starting at an angular frequency w4 and resulting in asymptote b in Fig. 2 . These two cases are, of course, arbitrary, but are con-. sidered as being representative: the first for the flat response case, and the second, for the worst case where the high-frequency tone control has been turned to maximum. The transfer functions of the preamplifier are then for Case a