The present invention relates to an audio-frequency power amplifying device.
Audio-frequency power amplifying circuits usually allow high electric powers to be delivered across a load. One of the principal characteristics of such amplifiers is the efficiency, i.e. the ratio between the power effectively delivered across the load and the total power provided by the supply source which feeds them. A second important characteristic is the maximum voltage which they can supply across the load for a given supply voltage. Furthermore, it is manifestly advantageous from an economical point of view to have an amplifier requiring only a single supply voltage source.
A number of known devices have enabled amplifiers to be constructed having good characteristics insofar as one or the other of the abovementioned points is concerned. Among these known devices, the class B amplifier has a relatively high efficiency, close on 0.8 in the case of amplification of a sine-wave signal whose amplitude is close to the admissible maximum. This amplifier, well-known in the technique, has an output terminal able to deliver a bidirectional current and whose potential may vary between zero and the maximum voltage of the supply source. The load is generally connected, on the one hand, to the output terminal of the amplifier and, on the other hand, to an intermediate potential point, this potential being most frequently equidistant from the zero potential and from the maximum potential V of the supply source. The voltage at the terminals of the load may then vary between -V/2 and +V/2, and generally its average value is zero. Under these conditions, the intermediate potential point may be obtained by connecting a high-value capacitor between the zero potential point or the +V potential point and the terminal of the load opposite the output terminal of the amplifier. For the parts of the signal higher than the average value, the supply source supplies to the load the required current from the output terminal of the amplifier, thus supplying a charge to the capacitor. For the parts of the signal less than the average value, the capacitor supplies to the load the required current, this current entering through the output terminal of the amplifier. This type of power amplifier has a good efficiency when the output signal has a value close to the maximum value. On the other hand, when such an amplifier is used to amplify signals of a lower value, its efficiency decreases rapidly, being for example in the neighborhood of 0.4 when the power is equal to a quarter of the maximum admissible output power.
Another known device, shown schematically in FIG. 1, consists in connecting together two class B amplifiers 1 and 2, supplied from the same supply source 3, the load 4 being connected between the output terminals 5 and 6 of the amplifiers. The input terminal 7 of amplifier 1 receives the signal to be amplified S, whereas the input terminal 8 of amplifier 2 receives the opposite of this signal -S. The efficiency of this arrangement is identical with that of a single class B amplifier, but enables further a voltage between +V and -V to be obtained across the load, V being the voltage of the single supply source 3.
Another known device is shown schematically in FIG. 2. This device, known under the name of class G amplifier, comprises two interconnected class B amplifiers supplied with different supply voltages. Transistors 20 and 21 form a first class B amplifier, supplied through diodes 28 and 29 from the middle-point supply source formed by voltage sources 30 and 31 connected in series. The load 34, connected between the output terminal 35 of this amplifier and the middle point 36 of the sources, is subjected to an output voltage which may vary between +V and -V, V being the voltage at the terminals of voltage sources 30 or 31. Assuming that transistors 20 and/or 21 are saturated, transistors 22 and 23, connected in series with these latter as shown in the figure, form a second class B amplifier, supplied from the middle-point source formed by the voltage sources in series 30, 31, 32 and 33. Generally sources 32 and 33 are used having the same voltage V'. When this second amplifier is operating, the voltage at the terminals of load 34, still connected between the output terminal 35 of the amplifier and the middle point 36 of the sources, is subjected to a voltage capable of varying from -(V+V') to +(V+V'). Diodes 24, 25, 26 and 27 connected to the bases of transistors 20, 21, 22 and 23 form a switching unit for controlling either the first amplifier or the second amplifier depending on the level of the input voltage applied to the input terminal 37. This arrangement permits, when the input signal at terminal 37 is low, solely the first class B amplifier formed by transistors 20 and 21 to be controlled. This amplifier, supplied from voltage sources 30 and 31, operates under better conditions than if it were supplied by the maximum voltage source formed by 30, 31, 32 and 33. Thus, its efficiency is higher than would be obtained by a single class B amplifier supplied with the maximum voltage. Similarly, when the input voltage at terminal 37 is high, with the output voltage at the terminal of load 34 exceeding the value V of sources 30 and 31, the second amplifier formed by transistors 22 and 23 is then controlled. It allows the signal portions corresponding to output voltages greater than V, and going up to V+V', to be amplified. The second amplifier also operates then under good conditions, since its output voltage is close to the maximum admissible voltage. The class G amplifier thus described has a higher efficiency than that of a class B amplifier, or of two bridge-mounted amplifiers such as described in FIG. 1. Nevertheless, it does not allow a peak voltage higher than half the maximum supply voltage formed by the sum of the voltages of sources 30, 31, 32 and 33 to be supplied across the load and, furthermore, it requires the presence of several power supply sources.