Switching amplifiers are commonly used for controlling the energization of load devices such as a reversible DC motor. In a known arrangement, four high current switching transistors are connected in a bridge circuit with a power supply connected across one pair of diagonally opposite terminals and the load device connected across the other pair of diagnonally opposite terminals. When a first pair of diagonally opposite transistors is switched on and the second diagonally opposite pair is switched off, voltage is applied to the load device in one direction and vice versa. The degree of energization of the load device is controlled by rapidly switching the transistors on and off according to a controlled program.
In many applications of switching amplifiers with high current transistors in a bridge circuit, a large base drive current is required. An example of such a requirement is found in automotive applications such as controlling the energization of an electric power steering servo motor. In such applications, it is important to minimize the waste of energy and accordingly a highly efficient switching amplifier is desired. Improved efficiency can be obtained by minimizing the power losses in the base drive circuit itself and also by providing a base drive current which is varied directly with the output current of the high current transistors.
Heretofore, switching amplifiers have been used in which the base of the high current transistor is connected to the junction of a voltage divider comprising a resistor and a switching transistor connected across the voltage source. The load device is serially connected with the collector-to-emitter circuit of the high current transistor across the voltage source. The base drive current for the high current transistor is applied through the resistor when the switching transistor is turned off and hence the load device is energized. When the switching transistor is turned on it shunts the base-to-emitter circuit of the high current transistor which is turned off and the load device is deenergized. A disadvantage with this base drive circuit is that the resistor is continuously drawing current and a significant amount of power is wasted. Another disadvantage is that the base drive current varies with power supply voltage.
A switching amplifier having a base drive circuit of improved efficiency, relative to that described above, is disclosed in U.S. Pat. No. 4,224,535. The switching amplifier of this patent comprises a high current transistor and a load device connected across the voltage source with the load device in the collector circuit. Base drive current is provided by a circuit including a base current regulating transistor, an inductor and a base current shunting transistor in series across the voltage source with the base of the high current transistor being connected to the junction of the inductor and the base current shunting transistor. A diode is connected across the inductor and the base current shunting transistor to provide a current path for the inductor current when the base current regulating transistor is turned off. In this circuit, base drive current for the high current transistor is supplied through the inductor when the base current shunting transistor is turned off; when the base current shunting transistor is turned on, it diverts the base drive current and the high current transistor is turned off. The inductor is of low resistance and the power loss therein is minimized. The inductor stores energy when the base current regulating transistor is turned on and it releases energy to maintain current flow for base drive when the base current rgulating transistor is turned off.
A general object of this invention is to provide a bridge circuit switching amplifier with a base drive circuit which overcomes certain disadvantages of the prior art.