Amplifiers – With semiconductor amplifying device – Including differential amplifier
Patent
1995-02-10
1996-04-30
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including differential amplifier
330258, 330260, H03F 345
Patent
active
055128580
DESCRIPTION:
BRIEF SUMMARY
The present invention pertains to a type of amplifier stage consisting at least of a first transistor whose base forms an input terminal for an amplified input signal and of a means to maintain an appreciably constant current in the collector-emitter path of the transistor.
Audio circuits make wide use of differential amplifiers, in particular as input stages for amplifier circuits. The most utilized circuit is the two-transistor differential amplifier; however, its linearity, the result of the transfer function of the two transistors, becomes limited as soon as the amplified signals reach a certain amplitude. To correct this problem, numerous circuits have already been proposed to yield more linear differential circuits, more so in the general context than in that of audio signals. This is specifically the case with the circuit of the prior art as mentioned in the above paragraph.
Another problem that affects these circuits, audio circuits in particular, is thermal distortion. The transfer function for these circuits is linked to the characteristics of the transistors utilized. Transistor characteristics are a function of the temperature at the junctions. In fact, these temperatures are a function not only of the ambient temperature conditions, but also of the instantaneous power dissipated in the junctions themselves. This power depends on the amplitude of the signal injected; in the case of audio signals, the amplitude often varies on a scale of approximately ten milliseconds, which is the same order of magnitude as that of the thermal time constant for the transistor junctions.
In the usual networks the circuit transfer functions are hence modulated by the amplitude variations of the signals treated. In the case of audio signals, this problem is quite pernicious, not only because the time constants of these variations have the same order of magnitude as the time constants of the level variations of the signals treated, but also because the ear can detect the variations, which escape standard measures used for signals having stable amplitudes.
The fundamental idea behind the invention is to stabilize the transfer function of an amplifier stage by stabilizing the power dissipated in the transistor(s), which is (are) most critical to the transfer function.
The invention thus pertains to an amplifier stage consisting of at least one module presenting a first transistor, of one semiconductor type, whose base constitutes the input terminal for an input signal to be amplified and of a means to maintain an appreciably constant current in the collector-emitter path of the first transistor (this transistor being utilized either as an emitter follower or as a common-emitter, particularly in a differential amplifier); and comprising a feedback circuit that provides a main current path between a first active terminal of the stage and the emitter of the first transistor; the amplifier stage is characterized in that said means consists of a first current source connected to the collector of the first transistor and in that the feedback circuit is arranged in such a way that the value of said main current is a function of the difference between the voltage of a reference-voltage source and the potential at the collector of the first transistor; the feedback circuit gain (transconductance) is selected such that the potential at the collector of the first transistor is appreciably constant and the voltage of the reference-voltage source is independent of the supply voltage and is selected in order that the first transistor function with an appreciably constant collector-base voltage, preferably a small collector-base voltage, that is, less than 1 volt.
As the current flow in the collector-emitter path of the first transistor is maintained by the current source and as the first transistor's collector potential varies little, since, with respect to the prior art, variations of this potential are reduced by means of the feedback circuit, the transistor operates with an appreciably constant power dissipation and therefore a
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