Amplifiers – Modulator-demodulator-type amplifier
Patent
1982-04-26
1984-10-09
Mullins, James B.
Amplifiers
Modulator-demodulator-type amplifier
330251, H03F 3387
Patent
active
044764367
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates generally to a power amplifier and is directed more particularly to a power amplifier which is capable of amplifying a PWM (pulse width modulation) signal efficiently.
BACKGROUND ART
As a power amplifier for amplifying a PWM signal, known in the prior art is a so-called class D amplifier. Since such class D amplifier has a transistor at its output stage which can operate in a switching mode, the output transistor produces less heat and hence, it is generally used as a power amplifier which is small in construction but produces a large output. In general, since if a switching frequency is higher, the efficiency becomes higher and higher, for the switching transistor, is used a field effect transistor which can operate in high speed.
But, the field effect transistor for use in power amplification has a defect such that it becomes more expensive than a bipolar transistor.
On the other hand, since the bipolar transistor can not set its switching frequency as high because of its carrier concentration effect, it has also such drawback that is can not carry out the efficient power amplification.
DISCLOSURE OF INVENTION
Therefore, it is a main object of the present invention to provide a power amplifier which can obviate the aforesaid defects.
It is another object of the present invention to provide a power amplifier utilizing a bipolar transistor at its power amplifying stage to thereby perform an efficient amplification.
According to an example of the present invention, there is provided a power amplifier having a pulse width modulation signal generating circuit proportional to an amplitude of an input signal, an amplitude modulating circuit for amplitude-modulating the pulse width modulation signal in response to the level of the input signal and a power amplifying stage being driven by the pulse width modulation signal, the amplitude of which is modulated to perform power amplification efficiently.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a circuit diagram schematically showing an example of a conventional power amplifier of a PWM type;
FIGS. 2, 3A-3C and 4 are characteristic graphs used to explain the operation of FIG. 1;
FIG. 5 is a circuit diagram for showing an embodiment of the present invention;
FIGS. 6A-6E are signal waveform diagrams used to explain the operations of FIGS. 1 and 5; and
FIGS. 7 and 8 are characteristic graphs used to explain the operation of FIG. 5.
BEST MODE FOR CARRYING OUT THE INVENTION
Prior to describing the present invention, as one of the conventional power amplifiers of such kinds, let us first describe an example which is provided at its final stage with a transistor of an emitter grounded saturation type as shown in FIG. 1. That is, in FIG. 1, reference numeral 1 generally denotes an input signal source for generating an input signal S.sub.2, a waveform of which is nearly a sine wave as shown in FIG. 6B, 2 denotes a reference signal source for generating a clock signal S.sub.1, a waveform of which is shown in FIG. 6A, 3 denotes an integrator for integrating the input signal S.sub.2 on the basis of the clock signal S.sub.1 so as to produce a saw-tooth wave signal S.sub.3, a waveform of which is illustrated in FIG. 6C, and 4 denotes a comparator for comparing the saw-tooth wave signal S.sub.3 with a reference level (zero level) so as to produce a PWM signal S.sub.4 shown in FIG. 6D.
Further, 5 to 8 designate transistors forming a voltage amplifying section in which diodes 9 and 10 for use in absorbing charges are respectively connected in parallel to emitter-collector paths of the transistors 7 and 8 at the final stage. If each of these transistors is made of a non-saturation type transistor, these diodes are each connected to base-collector paths of the transistors. Then, the output signals of the transistors 7 and 8 are delivered through a filter 15, which is comprised of coils 11 and 12 and capacitors 13 and 14, to a speaker 16 to be sounded. 17 and 18 denote DC voltage supply sources.
By the way, in the case of the po
REFERENCES:
patent: 4293817 (1981-10-01), De Michele
patent: 4390849 (1983-06-01), Miskin
Koizumi Akio
Yamagishi Hiroshi
Mullins James B.
Sony Corporation
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