Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1995-07-31
1997-06-03
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330260, 330290, H03F 345, H03F 134
Patent
active
056358741
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the field of electronics for audio signals. It concerns an analog circuit for the processing or amplifying of audio electric signals.
The audio circuits should be linear, that is to say, generate the least possible distortion. This expression covers the deformations which the circuit imposes on the signal and which are related to this signal, in opposition to the noise or parasitic signals which are uncorrelated to the signal.
The distortion produced by a circuit is traditionally measured by using signals of constant amplitude and it consists in measuring the ratio between the energy of the signals generated by the distortion and the energy of the test signal. This method of measurement fails to take two facts into account: generated by the distortion does not depend only on their energy; completely define the behavior of the circuits tested; thus, the frequent variations in level of the signals customarily amplified by the audio signals (signals of musical origin) may, particularly in transistor amplifiers, generate parasitic signals of very low frequency related to the amplitude of the signal which, while they remain inaudible and are generally eliminated by the circuits themselves, can modulate the distortions, making them thus more audible.
For example, in the case of a circuit generating a distortion formed of a second order distortion with an amplitude of 0.5% and third order distortion with an amplitude of 0.25% (and therefore very linear), the superposing of an interference signal of very low frequency (a few hertz) will lead to a modulation of the distortion of the desired signal at the frequency of the interference signal. This phenomenon generally remains negligible, but if the amplitude of the interference signal is substantial, it becomes significant; in the case of our example, an amplitude of the interference signal which is 10 times greater than the desired signal leads to a modulation of the second order distortion by about 6 dB, thus making it much more perceptible by the auditory system (a phenomenon known as emergence in psychoacoustics).
The phenomena generating such very low frequency interference signals may be: variations in temperature of their junctions related to variations in the power dissipated in these transistors as a function of the amplitude of the signal; calculated to filter the frequencies of the desired signals while the consumption may vary at the slower rate of the variations in amplitude of the signal, particularly if all the signals are not operating in Class A.
Transistor audio circuits customarily employ high feedback ratios in order to be as linear as possible, but these high feedback ratios lead to substantial open-loop gains even for the continuous signals. The output quiescent point is therefore subject to important drifts in open loop. The global feedback therefore has two roles: to reduce the non-linearities and to stabilize the output quiescent point. However, this double role leads, as will be shown subsequently, to greatly amplifying, very low frequency interference signals (which correspond to the drifts of the output quiescent point) as compared with the desired signal at the input of the amplifier, generating an intermodulation between the desired signal and the deviations of the continuous signal at a point critical for the distortion. The present invention makes it possible to produce transistor circuits (or transistor and tube circuits) using high feedback ratios while avoiding these distortion modulating phenomena. For this, the circuits developed in accordance with the invention separate the linearizing action of the feedback from the stabilizing of the output quiescent point; they filter the signals reinjected into the input by the feedback loops eliminating upstream of the feedback loop the interference signals generated in the amplifier and assuring the stabilization of the output quiescent point, and they stabilize the operation of the input stage. The influence of the offset voltages and of the very low frequ
REFERENCES:
patent: 3916333 (1975-10-01), Zuk
patent: 4301421 (1981-11-01), Yokoyama
patent: 4769617 (1988-09-01), Mizuide
"Ultra-Low-Noise Amplifiers and Granularity Distortion", D.R.G. Self, The Journal of Audio Engineering Society, vol. 35, No. 11, Nov. 1987, pp. 907-915.
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