Amplifiers – With semiconductor amplifying device – Including d.c. feedback bias control for stabilization
Patent
1983-07-11
1985-07-23
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including d.c. feedback bias control for stabilization
330270, 330288, 330296, H03F 304
Patent
active
045311005
DESCRIPTION:
BRIEF SUMMARY
BRIEF DESCRIPTION OF THE PRIOR ART
The present invention relates to an amplifier suitable for low supply voltage operation, and in particular to an amplifier of which the performance is consistent over a wide range of supply voltage.
Amplifiers in general, by virtue of the biassing arrangements chosen for their output amplifying elements, will not operate satisfactorily (or at all) under low supply voltage conditions. Furthermore, where an amplifier has been designed to operate at a specific supply voltage, often quite a small voltage drop from that specific value will upset the biassing arrangement and cause unsatisfactory operation of the amplifier. It is an object of the present invention to provide an amplifier which is relatively insensitive to such supply voltage variations.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an amplifier comprising a signal amplifying stage and a control stage for optimizing the D.C. current to the signal amplifying stage, the signal amplifying stage including a series combination of a semiconductor device and an output element which combination is arranged to be connected across a power source, the control stage acting to control D.C. current to the semiconductor device so as to clamp voltage excursion peaks appearing between the semiconductor device and the output element to a value equal to, or otherwise related to, that at the opposite side of the output element.
In general, the semiconductor device is a bipolar transistor of which the signal path is series-connected to a suitable output element or load such as a loudspeaker, earphone, or transformer for driving same. The amplifier may be single-ended or may use two such circuits in push-pull configuration.
Preferably, the D.C. current to the semiconductor device is controlled by the control stage by means of continuous comparison of the minimum instantaneous voltage across the output load with a very low reference voltage arising from the difference in the base-emitter voltages of two similar transistors operating at unequal collector current values, for example in the so-called "current mirror" configuration. The circuit performance is thus related to a difference voltage between two voltages which are both already stabilized to a first order. As a result, the effects of variations in supply voltage and in temperature are small.
When using silicon bipolar transistors, the circuit can operate from a supply voltage of which the lowest workable value, which varies slightly according on the application to which the amplifier is put, is generally in the range 0.85.+-.0.1 volts D.C. The quiescent power computation (i.e. in the absence of input signal) is low; its magnitude naturally depends on the D.C. resistance of the output load but it can be of the order of 0.1 milliwatt for earphone output or of the order of 20 milliwatts for loudspeaker output. This aspect is clearly beneficial as regards battery operation, in particular in applications where there is little or no input signal for long periods of time, one such application being the so-called "baby alarm".
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the present invention may be more readily understood, embodiments thereof will now be described by way of example, with reference to the accompanying drawings in which:
FIG. 1 shows one form of basic amplifier circuit in accordance with the invention, but omitting signal paths for clarity;
FIGS. 2A and 2B show the amplifier circuit of FIG. 1 including first and second possible signal path modes of connection, respectively;
FIG. 3 shows a form of amplifier circuit arranged to operate in push-pull; and
FIG. 4 shows a developed form of amplifier circuit including further features.
DETAILED DESCRIPTION
By reference to FIG. 1, the D.C. operation of the amplifier in the absence of signal is described. The amplifier includes control stage 10 and signal output amplifying stage 11 connected to D.C. voltage source 9. The latter is a conventional external power source such as a battery, having p
REFERENCES:
patent: 2979666 (1961-04-01), Erath
patent: 3202924 (1965-08-01), Myers et al.
patent: 3434066 (1969-03-01), Huntley
patent: 3868582 (1975-02-01), Haferl
patent: 3984783 (1976-10-01), Bickley
patent: 4121168 (1978-10-01), Stitt
patent: 4387346 (1983-01-01), Fackler
Murari et al., "A High-Power Hi-Fi Monolithic Amplifier", IEEE Transactions on Broadcast and Television Receivers, vol. B TR 20, No. 4, Sep. 17, 1974, pp. 311-320.
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