Abstract:
In a power amplifier circuit the ends of the DC source for the class A stage are isolated from the amplifier transistors by diode. A resistor may also be connected across the class A amplifier diodes.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a power amplifier in which a class B amplifier is employed as a current source in order to improve the efficiency of the class A amplifier, and which is so designed that an unnecessarily high voltage is not applied to the class A amplifier when a problem occurs in the class B amplifier. 
     In order to improve the efficiency of a class A amplifier, a circuit as shown in FIG. 1 has been proposed. In FIG. 1, reference characters A 1  and A 2  designate drive stages. The drive stage A 1  operates to drive transistors Q 1  and Q 2  in a class A amplifier. The output of the drive stage A 1  is applied to the bases of the transistors Q 1  and Q 2 , the emitters of which are connected to an output terminal OUT, and the output terminal OUT is grounded through a load R L . 
     The collector of the transistor Q 1  is connected to a DC source +E 2 , and the collector of the transistor Q 1  is connected to a DC source -E 2 . The connecting point of the DC sources +E 2  and -E 2  is connected through a feedback circuit β to an input terminal of the drive stage A 2 . 
     An input IN is applied to the input terminals of the drive stages A 1  and A 2 . The output of the drive stage A 2  is applied to the bases of transistors Q 3  and Q 4  which form a class B amplifier. The emitter of the transistor Q 3  is connected to a DC source +E 1 , and the emitter of the transistor Q 4  is connected to a DC source -E 1 . 
     The DC sources +E 1  and -E 1  are connected in series with each other, and the connecting point of the DC sources +E 1  and -E 1  is grounded. The collector of the transistor Q 3  is connected to the DC source +E 2  ; i.e., it is connected to the collector of the transistor Q 1 . The collector of the transistor Q 4  is connected to the DC source -E 2  ; i.e., it is connected to the collector of the transistor Q 2 . The voltages of the DC sources +E 2  and -E 2  are lower than those of the DC sources +E 1  and -E 1 , respectively. In practice, alternating current is rectified and is then smoothed with capacitors C 1  and C 2  as shown in FIG. 2, to obtain the DC sources +E 2  and -E 2 . 
     In FIG. 2, reference character T 2  designates the secondary winding of a power transformer. Both ends of the secondary winding T 2  are connected through diodes D 1  and D 2  to the capacitors C 1  and C 2 , respectively. The capacitors C 1  and C 2  are connected in series with each other, and the connecting point of the capacitors C 1  and C 2  is connected to the center tap of the winding T 2 . 
     When a problem occurs in the circuit in FIG. 1: such as when the emitter and collector of one of the transistors Q 3  or Q 4  of the class B amplifier are short-circuited, the connecting point a of the DC sources +E 2  and -E 2  is brought into contact with the other elements or the class B amplifier oscillates, then for instance a current I 1  flows to charge the capacitor C 1 . 
     Since the rated voltage of the capacitor C 1  is low, the capacitor C 1  is over-charged in such a case. The same thing can be said to occur on the negative side. 
     Thus, when, in the case where the class A amplifier employs the class B amplifier as a current source, a problem occurs in the class B amplifier, then an over-voltage is applied to the capacitors C 1  and C 2  forming the DC source of the class A amplifier, thus adversely affecting the class A amplifier. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to eliminate the above-described difficulty. More specifically, an object of the invention is to provide a power amplifier in which, even when a problem occurs in the class B amplifier, the smoothing capacitors or the like in a DC source for the class A amplifier are not affected thereby. 
     Briefly, this object is achieved by isolating the class A amplifier transistors from their respective DC source connections by diodes so that when the normal class A amplifier is forced to operate in a class B mode, the DC sources will be essentially isolated, and feedback to the class B stage input will be discontinued. A resistor may optionally be connected across the class A amplifier transistors for further protection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a circuit diagram of a conventional power amplifier; 
     FIG. 2 is a circuit diagram showing a part of a power sorce for a class A amplifier in the conventional power amplifier and in a first example of a power amplifier according to this invention; 
     FIG. 3 is a circuit diagram showing the first example of the power amplifier according to this invention; 
     FIGS. 4(a) through (d) are diagrams showing waveforms at various circuit points in the circuit of FIG. 3 when the power amplifier operates normally; 
     FIGS. 5(a) through (d) are diagrams showing waveforms at the various circuit point in the circuit of FIG. 3 when the operation of the class A amplifier is shifted into a class B amplifier operation; and 
     FIG. 6 is a circuit diagram showing a second example of the power amplifier according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of a power amplifier according to the invention will be described with reference to FIGS. 3 through 6. FIG. 3 is a circuit diagram showing the arrangement of a first example of a power amplifier of this invention. For simplification in description, in FIG. 3, those components which have been previously described with reference to FIG. 1 are designated by the same reference characters or numerals, and the following description is primarily directed to the components in FIG. 3 which are different from those in FIG. 1. 
     As is apparent from comparison of FIG. 3 with FIG. 1, the circuit in FIG. 3 is different from that in FIG. 1 in the provision of diodes D 3  and D 4 . The diode D 3  is connected between the positive terminal of the DC source +E 2  and the collector of the transistor Q 1  in the class A amplifier, which is connected to the collector of the transistor Q 3  in the class B amplifier. 
     The diode D 4  is connected between the negative terminal of the DC source -E 2  and the collector of the transistor Q 2  in the class A amplifier, which is connected to the collector of the transistor Q 4  in the class B amplifier. 
     The operation of the power amplifier thus arranged will be described with reference to waveform diagrams in FIGS. 4 and 5. The waveform diagram of FIG. 4 is for the case where the power amplifier is operating normally, and the waveform diagram of FIG. 5 is for the case where the load becomes abnormally heavy. 
     In the normal operation, an input is applied to the drive stages A 1  and A 2 , the outputs of which are applied to the transistors Q 1  and Q 2  in the class A amplifier and the transistors Q 3  and Q 4  in the class B amplifier, respectively. Thus, the transistors Q 1  and Q 2  and the transistors Q 3  and Q 4  operate according to the amplitudes of the outputs of the drive stages A 1  and A 2 . 
     In this operation, a current I B  as shown in FIG. 4(a) flows in the collector of the transistor Q 3  in the class B amplifier, a current I A  as shown in FIG. 4(b) flows in the collector of the transistor Q 1  in the class A amplifier, and a current I C  as shown in FIG. 4(c) flows in the diode D 3 . A voltage waveform at the connecting point a of the collectors of the transistors Q 1  and Q 3  is as indicated by reference character a in FIG. 4(d). A voltage waveform at the connecting point b of the collectors of the transistors Q 2  and Q 4  is as indicated by reference character b in FIG. 4(d). A voltage at the connecting point c or at the output terminal OUT is as indicated by reference character c in FIG. 4(d). 
     As the load is increased and the class A amplifier is caused to operate in a class B amplification mode, the waveforms at the aforementioned various circuit points become as shown in FIGS. 5(a) through (d), wherein FIGS. 5(a) through (d) correspond to FIGS. 4(a) through (d), respectively. 
     When the class A amplifier is caused to operate in class B amplification mode, then no currents are supplied to the class A amplifier from the DC sources +E 2  and -E 2  ; that is, the class A amplifier is effectively disconnected from the DC sources at the diodes D 3  and D 4 . Thus, the application of feedback to the class B amplifier is temporarily suspended. Therefore, with a large positive signal the transistor Q 3  is rendered completely conductive, and with a large negative signal the transistor Q 4  is rendered completely conductive. Thus, the collector current I B  of the transistor Q 3  is as shown in FIG. 5(a). 
     On the other hand, with the large positive signal the transistors Q 4  and Q 2  are rendered non-conductive, and with the large negative signal the transistors Q 3  and Q 1  are rendered non-conductive. Therefore, the currents I B , I A  and I C  mentioned above become as shown in FIGS. 5(a), (b) and (c), respectively. The voltage at the connecting point c or at the output terminal OUT is as indicated by reference character c in FIG. 5(d). 
     As is apparent from the above description, even when a problem occurs in the class B amplifier, no over-voltage is applied to the capacitors C 1  and C 2  (FIG. 2) forming the DC sources +E 2  and -E 2 , and accordingly the capacitors C 1  and C 2  will never be damaged. 
     FIG. 6 is a circuit diagram showing a second example of the power amplifier according to the invention. The circuit shown in FIG. 6 can be obtained by connecting a resistor R between the connecting points a and b in the circuit of FIG. 3. Owing to the provision of the resistor R, a current I R  flows in the resistor R at all times even when the class A amplifier is caused to operate in a class B amplification mode. Therefore, a potential slightly lower than 2E 2  is maintained between the connecting points a and b. 
     In other words, the voltages at the connecting points a and b follow voltage variation at the connecting point c. Thus, there is substantially no possibility that when the connecting points a and b are released from the circuit in the example shown in FIG. 2, the transistors Q 3  and Q 4  will be rendered conductive to thus increasing the collector loss. 
     In summary, the circuit in FIG. 6 is advantageous in that the class A amplifier is protected from the adverse effect of a problem occurring in the class B amplifier, and lowering of the efficiency of the class B amplifier can be prevented. 
     As is clear from the above description, according to the invention, the diodes are inserted between the DC sources of the class A amplifier and the collectors of the transistors in the class B amplifier so that, when the operation of the class A amplifier is shifted into a class B amplifier operation as the load is increased, the supply of current from the class B amplifier to the DC sources of the class A amplifier is prevented. Thus, the capacitors forming the DC sources of the class A amplifier can be protected from damage.