Class BC power amplifier
BACKGROUND OF THE INVENTION 
     This invention relates to a class BC power amplifier which is intended for use in amplifying devices where the efficiency is of primary importance and the use of an output transformer is not obligatory. 
     A known class BC power amplifier comprises a symmetrical push-pull preamplifier and a pair of intermediate preamplifiers, the outputs of which are connected, respectively, to both power branches of the amplifier, the one power branch being connected to the positive poles of a pair of supply sources, while the other power branch is connected to the negative poles of the supply sources, both power branches and their corresponding intermediate preamplifying stages being totally symmetrical in which corresponding transistors are of opposite conductivities, wherein the outputs of the syymmetrical push-pull preamplifier are respectivly connected to the inputs of the intermediate preamplifiers, the outputs of which are respectively connected to corresponding transistors operaring in class B. 
     A drawback of this class BC power amplifier lies in that with the common collector connection circuit of th end transistors there cannot be used a control circuit with saturated transistor which can protect the power transistors from breakdown in transient processes caused by the inductive character of the load and it is necessary to build-in a special circuit for this purpose. Another drawback lies in that the end transistors are connected by a common collector circuit and the attacking transistor, which operates in class A conditions, must provide an amplitude greater than the whole output amplitude, which is impossible and leads to an incomplete saturation of the end power transistors, i.e. to a reduction of the maximum possible power and to great distortions of the output signal in the attacking stage itself. A further drawback lies in that the transistor, which operates in class A conditions, must be of a high-voltage type. 
     There are known circuits of class BC power amplifiers in which the transistor, operating in class C, is controlled by means of a Zener diode by the transistor operating in class B conditions. Thus, the aforementioned drawbacks are avoided, but this leads to a considerable reduction of efficiency, this effect being particularly strong in the range of commutation of the supply sources. The latter is a great drawback for such a control circuit for the transistors, operating in class C conditions, since the amplifier operates the longest time namely in the range of commutation. Thus, the most important characteristics of the class BC amplifier is affected, i.e. its high average statistical efficiency. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a general object of this invention to provide a class BC power amplifier which permits control by a circuit with saturated transistor, this circuit effecting a protection from transient processes caused by the inductive character of the load, and the circuit for control of the end power block can drive it to its maximum possible power and allows the use of low-voltage transistors, while the circuit for control of the end block and the circuit for control of the transistors, operating in class C conditions should be such that it does not lower efficiency typical for BC class amplifiers. 
     This object is achieved by a class BC power amplifier which comprises a symmetrical push-pull preamplifier and a pair of intermediate preamplifiers whose outputs are connected, respectively, to both power branches of the amplifier, the one power branch being connected to the positive poles of a pair of supply sources, while the other power branch is connected to the negative poles of the supply sources, both power branches and their corresponding intermediate preamplifying stages being totally symmetrical, corresponding transistors being of opposite conductivities, each output of the symmetrical push-pull preamplifier is connected to the input of the respective intermediate preamplifier, the output of which is connected to a transistor operating in class B and is characterized in that one output of each intermediate preampifier is respectively connected directly to the base of the transistor operating in class B, which is of the p-n-p type and is connected through a resistor to the emitter of a comparison n-p-n transistor. The collector of the transistor operating in class B is connected to the active end of the load, to which through a protective diode, in the same direction with the base current in the circuit, there is connected the base of the comparison n-p-n transistor, whose collector is connected directly to the base of a p-n-p transistor operating in class C. The second output of each intermediate preamplifier is respectively connected to the emitter of the transistor operating in class B, and at the same point there are connected a commutating diode and the collector of the transistor operating in class C. The anode of the commutation diode is connected to the positive pole of the low-voltage supply source, while the emitter of the transistor operating in class C is connected to the positive pole of the high-voltage source. The emitter of the comparison transistor is grounded through an emitter resistor. 
     The advantages of the class BC power amplifier according to this invention are: it permits control by a circuit with transistor which besides its control functions effects also a protection of the end stage from transient processes caused by the inductive character of the loud. The circuit for control of the end power block drives it to its maximum possible power and allows the use of low-voltage transistors. At the the efficiency is not lowered, but it remains sufficiently high--corresponding to the class BC. 
    
    
     DESCRIPTION OF THE DRAWING 
     For a better understanding of the ivention reference should be made to the accompanying drawing in which there is illustrated a preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing shows the circuit diagram of a class BC power amplifier. This power amplifier comprises a symmetrical push-pull preamplifier 1 whose outputs are connected, respectively, to both power branches 3 and 3&#39; of the amplifier. The one power branch 3 is connected to the positive poles of a pair of supply sources and the other branch 3&#39; is connected to the negative poles of the pair of supply sources. 
     Since both power branches 3 and 3&#39; are totally symmetrical, corresponding transistors being of opposite conductivities, only the power branch 3 will be described in detail in the following description. One output of the symmetrical push-pull preamplifier 1 is connected to the input of an intermediate preamplifier 2. One output of the intermediate preampifier 2 is connected both directly to the base of a transistor 4 operating in class B, which is of the p-n-p type, and, through a resistor 5, to the emitter of a comparison n-p-n transistor 6. The collector of transistor 4 is connected to the active end of the load 7, to which, through a protective diode 8, the pass direction with respect to the base current in the circuit, there is connected the base of the comparison n-p-n transistor 6, whose collector is connected directly to the base of a p-n-p transistor 9, operating in class C. The second output of the intermediate preamplifier 2 is connected to the emitter of the transistor 4 and, at the same point, is connected to a commutating diode 10 and the collector of transistor 9. The anode of the commutating diode 10 is connected to the positive pole of a low-voltage supply source +E 1 , while the emitter of transistor 9 is connected to the positive pole of a high-voltage supply source +E 2 . The emitter of the comparison transistor 6 is grounded through an emitter resistor. 
     The operation of the class BC power amplifier according to this invention is as follows: 
     At rest, there is established through the transistors 2 and 2&#39; of the intermediate preamplifier a maximum current close to the saturation current thereof, and a minimum rest current through the transistors 4 and 4&#39; operating in class B, while the transistors 9 and 9&#39;, operating in class C, are cut-off. When a signal is received, depending on its momentary polarity, one of the branches 3, 3&#39; remains cut-off, while the other is opened because of the resulting cut-off of the corresponding transistor 2 or 2&#39; of the intermediate preamplifier. When the momentary value of the input signal reaches a level about 50% of the maximum value, conditions then exist for the opening of the comparison transistor 6 or 6&#39; and, hence, a control of the transistors 9 or 9&#39; operarting in class C. The current of the latter passes through the transistor 4 or 4&#39; which is then saturated.