Abstract:
A base current-control circuit comprises a detector for detecting a load current of the output transistor and for enabling the circuit to generate a detected current proportional to the load current. A base current-control voltage generator generates a voltage as a function of the detected current, and a switch generates ON/OFF signals. A base current generator utilizes the voltage to generate a base current in response to the ON/OFF signals generated by the switch to drive the output transistor.

Description:
This application has priority rights based on South Korean application No. 1003/93 filed Jan. 27, 1993, which South Korean application is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field Of the Invention 
     The present invention relates to a base current-control circuit, of an output transistor. More particularly, this invention relates to a base current-control circuit which changes the base current of the output transistor as a function of the load current of the output transistor in order to maximize power consumption efficiency. 
     2. Description of the Prior Art 
     Typical electronic equipment for processing input signals generally has an output transistor for driving an external device. The output transistor is designed to carry large currents and supplies current from its collector to a load. The current supply from the collector is controlled by the base current. 
     FIG. 1. shows an output terminal of a typical piece of electronic equipment which comprises an output transistor Q out , a load R L , and a source of electric power V cc . When the input signal processed by the electronic equipment triggers a switching transistor Q SW , the switching transistor is alternately turned off and on. When the switching transistor Q SW  is turned on, the output transistor is turned on. When the switching transistor Q SW  is turned off, the output transistor is turned off. More specifically, when the switching transistor is turned on, a diode D 1  connecting a transistor base with the collector of the Q SW  transistor is also turned on, and a constant-voltage source loads a resistance R b  with a voltage V ref . As shown in FIG. 1, node A is at a voltage V A , which is equal to the total of V ref  and a diode voltage V D1 . At the same time, node B is at a voltage V B , which is equal to node voltage V A  minus the voltage (V BE , Q1) between the base and emitter of transistor Q 1 . Thus, V B  is equal to V ref  +V D1  -V BE ,Q1, and if V D1  is equal to V BE , Q1, V S  can be V ref . 
     The collector current of transistor Q 1 , which also functions as base current: I B  for output transistor Q out , is equal to the node voltage V B  divided by the load resistance across resistor R b  (i.e., V B  /R b ). This is the same as V ref  /R b , and I B  is constant. Therefore, I B  is determined by the resistance R b  and a constant voltage, and is unrelated to the magnitude of load resistance R L  across the output transistor Q out . Thus, regardless of the load current I o , an invariable base current I B  is utilized. As a result, excessive electric power is dissipated unnecessarily. However, it can be appreciated that if the base current I B  were controlled as a function of the magnitude of the load current I o , electric power would be used more efficiently. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a base current-control circuit of an output transistor, and more specifically, one which maximizes electric power consumption efficiency. 
     The base current-control circuit of the present invention controls a base current of the output transistor as a function of the load current of the output transistor. The base current-control circuit comprises a detector for detecting a load current of the output transistor and for enabling the circuit to generate a detected current proportional to the load current; a base current-control voltage generator for generating a voltage as a function of the detected current proportional to the load current;a switch for generating ON/OFF signals; and a base current generator for utilizing the voltage to generate a base current in response to the ON/OFF signals generated by the switch to drive the output transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating an output terminal of a typical piece of electronic equipment in prior art. 
     FIG. 2 is a block diagram illustrating the present invention, 
     FIG. 3 snows an embodiment of the present invention. 
     FIG. 4 is a graph showing the operational characteristics of the present invention in comparison to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The base current I B  of the output transistor of the present invention varies as a simple linear function of the load current I o . Thus, the load current, which is an independent variable, determines the base current. Otherwise put, the load current controls the base current. 
     Referring to FIGS. 2 and 3, the load current I o  of a driving terminal 8, which preferably includes output transistor Q out , is proportional to current I sense  as detected by a load current detector 1. A current-voltage converter 2 converts the detected current I sense  to a proportional voltage V sense . A constant-voltage source 4 outputs voltage V ref , and both V ref  and V sense  from the output of to a base current-control voltage generator 3. The base current-control voltage generator 3 outputs the base current-control voltage V c  (equal to V ref  +V sense ), which is then input to a switch 6. Thus, it can be appreciated that voltage generator 3 basically comprises constant-voltage source 4 and converter 2. When a signal from an output transistor ON/OFF controller 5 is then input to the switch 6, the base current-control voltage flows into a base current generator 7 through the switch. The base current generator 7 then inputs the controlled base current I B  to the output transistor of driving terminal 8. It can thus be appreciated that the base current I B  is controlled by the load current. 
     In FIG. 3 it can be seen that transistor Q S  is set up in parallel with output transistor Q out  in order to detect the load current from the driving terminal 8. The output transistor Q out  and transistor Q S  for detecting the load current are both of the PNP type. 
     The detecting current I sense  is determined by the ratio of the emitter areas between the transistor Q S  and the output transistor Q out . That is, when the emitter area of Q S  is divided by the emitter area of Q out , the result is ,equal to a constant K, and I sense  is equal to K×I o . Since K is fixed, I sense  changes proportionally to I o . 
     V be ,QS, which is the voltage between the base and the emitter of the transistor Q S , is equal to V be ,Qout, which is the voltage between the base and the emitter of the output transistor Q out . 
     Thus, the following conditions are met: ##EQU1## 
     Here V T  is the transistor thermal voltage, I S  is a saturation current, and K is equal to the emitter area of Q S  divided by the emitter area of Q out . Therefore, the collector current of I C ,GS of transistor Q s  is equal to K×I c ,Q.sbsb.out. K has a range between 1/100 to 1/1000. 
     Current-voltage converter 2 converts the detected load current I sense  to an equivalent voltage. In the disclosed embodiment, resistor Rs operates as the converter. The detected load current I sense  flows into the resistor R S , which causes a voltage drop V sense . The size of the voltage drop is proportional to the size of the inflow current. That is, the detected voltage V sense  is equal to I sense  ×R S . 
     Referring to FIG. 2, the base current-control voltage generator 3 receives the detected voltage V sense  and reference voltage V ref , and then outputs the base current-control voltage V c , which is applied to node C. Reference voltage V ref  in series with resistor R S , is added to the voltage across resistance R S  to form the total voltage at node C. In the absence of a load, reference voltage V ref  is the base current-control voltage V c  of the output transistor. 
     As shown in the circuit, V ref  is fixed, and since V c  =V ref  +V sense , it is also true that V c  =V ref  +I S  ×R S  and that V c  =V ref  +K×I o  ×R S . It can be appreciated, therefore, that. V c  is a simple linear function of I o . Base current-control voltage V c  varies in proportion to I sense . 
     Referring to FIG. 2, it can be seen that base current-control voltage V c  is input to switch 6. The input signal is output from the output transistor ON/OFF controller 5 forming part of the electronic equipment. The switching transistor Q sw  turns ON or OFF in accordance with these signals. When the switching transistor is turned on, base current-control voltage V c  flows into NPN type transistor Q 1 , which functions as a buffer, and the base current-control voltage appears across resistor R b  connected to the emitter of Q 1 . Thus, base current shows I B  can be expressed as V c  /R b , or alternatively, equation 1 as follows. ##EQU2## 
     The base current generator 7 of FIG. 2 can be embodied in the transistor Q 1  as shown in FIG. 3. A collector current of the transistor Q 1 , which is equal to the base current I B  of the output transistor, is controlled by I o , as expressed by equation 1. The voltage at node B is the sum of V ref  and K×I o  ×R S . 
     FIG. 4 is a graph which shows the operational characteristics of the circuit of the present invention in comparison with the prior art. The vertical and horizontal axes plot the magnitude of the base current I B  versus the load current I o . In the prior art, as shown by graph line A, the base current I B  is constant regardless of the load current I o . In contrast, in the present invention, and as expressed by equation 1, the graph line B indicates that the base current I B  is dependent upon the load current I O . 
     In sum the output current is related to the load, which receives driving power from a suitable amount of base current I B . Thus, if the base current in the prior art and the present invention are I B1  and I B2  respectively, for voltage V cc  and load current I o , the power consumption of the present invention can be reduced by as much as (I B1  -I B2 )×V cc .