Transistor Base Drive Regulator

The invention relates to the regulation of a switching transistor and more particularly to preventing the switching transistor from going into saturation while in the on or conductive state.

The most popular method of regulating conduction of switching transistors is the clamp diode method. This method utilizes a diode between the base and collector elements of the switching transistor. This method has found success even though an excess amount of base drive power is wasted by shunting excess base drive power through the transistor.

Another circuit for regulating base drive is disclosed in NASA report No. NASA TN 0-6511 dated December, 1971. This method is similar to the instant invention except a plurality of series diodes are employed between the emitter of the drive regulator and the base of the output transistor. This scheme provides a minimum of 2.1 volts on the collector of the output transistor which is three times greater than that level required for 0 collector to base voltage. This results in three times the conduction dissipation of the output transistor and is therefore undesirable in terms of circuit efficiency.

Other methods of regulating base drive power employ complicated feed back control circuits requiring a substantial number of additional electrical components for their operation. This increases economic cost, weight and susceptibility to failure.

There would be immediate acceptance by industry of a means for regulating base drive to a switching transistor that was more efficient, comprised of a limited number of components and could be produced at a reduced economic cost over the prior art devices.

Accordingly, an object of the instant invention is to provide a new and improved circuit for controlling the base drive to a transistor operating in a switching mode.

Another object of the instant invention is to provide a system for maintaining the collector voltage level of a transistor operating in a switching mode from dropping to a level less than its base voltage level.

Still another object of the instant invention is to provide a feedback circuit wherein a transistor operating in a switching mode controls its own base drive voltage level.

Still a further object of the instant invention is to provide minimum conduction dissipation of an output transistor operating in a switching mode.

Briefly, these and other objects of the instant invention are attained by the use of a regulator transistor in series between the signal voltage and the base of a transistor operating in a switching mode and connecting the collector of the switching transistor to the base element of the regulator transistor through a diode enabling the switching transistor to limit its own conductivity.

The circuit is simple and inexpensive to construct and provides for effective base drive control of a transistor operating in a switching mode .

The above and other specific features of the instant invention will be readily apparent as the description continues while being read in conjunction with the appended drawings.

FIG. 1 is a schematic of the circuit of the instant invention as applied to an NPN type transistor.

FIG. 2 is a schematic of the circuit of the instant invention as applied to a PNP type transistor.

FIG. 3 is a schematic showing of a second embodiment of the invention shown in FIG. 1.

FIG. 4 is a schematic showing of a second embodiment of the invention of FIG. 2.

The same reference numerals are used throughout the drawings to depict the same element or part.

Referring now to FIG. 1, the circuit 10 of the instant invention is shown utilizing a series regulator transistor 12 and switching transistor 14 of the NPN type.

The positive going signal voltage used for controlling the switching of transistor 14 is applied to the collector element 16 and through a resistor 18 to the base 20 of transistor 12. The value of the resistor 18 is determined by the manufacturer's requirements, i.e., for an MJE 200 transistor manufactured by Motorola, a resistor is used that divides the signal voltage input level to a voltage of a ratio of approximately 10 to 1 on the collector and base elements so as to provide normal operation of the transistor 12. Other type transistors and those made by other manufacturers require various different operating voltage ratios. The emitter element 22 of transistor 12 is connected to the base element 24 of transistor 14. The collector element 26 of transistor 14 is connected to the cathode element 28 of a diode 30. The anode element 32 of the same diode 30 is connected to the base element of transistor 12. The collector element 26 of transistor 14 is connected to a source of collector voltage and the emitter 27 is connected to ground or to a point common with the negative potential of the input signal and collector voltages.

Under certain operating conditions, namely, when the base drive is at a low level, it is desirable to reduce the gain of transistor 12, a resistor 19 is connected between the collector 16, and emitter 22. The value of the resistor is determined by the transistor 12. Generally the resistor will supply a voltage to the base of transistor 14 of approximately 0.1 volt below the voltage between the base 20 and emitter 22 of transistor 12.

The regulator transistor 12, the diode 30 and the switching transistor 14 are constructed of material having the same or similar electrical characteristics so that when current flows through diode 30, the voltage level at the base 20 of transistor 12 is substantially equal to twice the voltage at the base of the switching transistor 14 so that the voltage on the collector element 26 of the switching transistor 14 is at least equal to or greater than its base drive voltage level.

The embodiment shown utilizes an MJE 200 as transistor 12, an MR 818 as diode 30 and a BU 108 as transistor 14. The devices of the preferred embodiment are constructed of silicon type material. It should be noted that although the transistor and diode of the preferred embodiment utilize silicon any known semi-conductor material could be employed so long as the characteristics of the material provide the voltage levels on the collector and base of the switching transistor as hereinbefore specified.

FIG. 2 is a showing of the instant invention employing a PNP type transistors 34, 36. The only difference being that the input signal is negative going, the collector voltage for transistor 36 is at a high negative potential and the emitter 27 is connected to a high positive potential.

The resistance value of resistor R, shown in the Figs., is determined from the manufacturers's operating specifications. Typically, the resistive value of resistor R is determined by transistor requirements and circuit characteristics and is generally known in the art.

The input signal is applied to the conductor 16, 16' of transistor 12, 34 and the base 20, 20' through resistor 18, 18' causing current to flow through resistor 18, 18' and into base 20, 20' of transistor 12, 34. At this instant transistor 12, 34 conducts between its collector 16, 16' and emitter 22, 22'. This conduction supplies a base voltage level to its respective transistor 14, 36 turning it on. When this base voltage is applied to transistor 14, 36, current flows between the collector element 26, 26' and the emitter element 27, 27' reducing the normally high voltage potential on the collector 26, 26' as conduction of the transistor increases. When the collector voltage reaches a level less than the voltage level at base 24, 24' of transistor 14, 36 the diode 30 conducts, its back bias voltage now being removed, as the voltage level of the collector 26, 26' of transistor 14, 36' drops below its base voltage level, dividing the current path between base emitter junction of transistor 12, 34 into two substantially equal resistive paths. The diode 30 and the transistors are constructed of material having the same or similar electrical characteristics. The current flow through two substantially equal resistive paths provides a voltage level substantially equal at the collector 26, and base elements 24, 24' of transistor 14, 36. The reduction of voltage at the base element 20, 20' of transistor 12, 34 reduces the base drive to transistor 14, 36, thus reducing the dissipation of drive power while maintaining the switching transistor in an on state. The maintaining of a voltage level on the collector element 26, 26' of switching transistor 14, 36 equal to or greater than the voltage level in its base element 24, 24' prevents the transistor 14, 36 from saturating thereby improving the transistor high speed switching capabilities by controlling the transistor saturation level.

The resistor 19 may be required where a low level of input voltage is supplied to the regulator transistor 12, 34 so as to maintain a level of voltage on the base 24, 24' of the output transistor 14, 36 at all times.

Many changes may be made in details of the instant invention, in the method and material of fabrication, in the configuration and assemblage of the constituent elements, without departing from the spirit and scope of the appended claims, which changes are intended to be embraced therewithin.