Alpha enhancement of a transistor using base current feedback to the emitter

A circuit for enhancing the alpha of a transistor. A current supplying circuit passes the base current of the transistor through a first and second current mirror for providing current at the emitter of the transistor such that the apparent emitter current of the transistor is made substantially equal to the collector current of the transistor.

BACKGROUND OF THE INVENTION 
The present invention relates to bipolar transistors and, more 
particularly, to enhancing the alpha of a bipolar transistor. 
The alpha (a) of the transistor is defined to be the ratio of its collector 
current to its emitter current (I.sub.C /I.sub.E). Ideally, we would like 
a to be unity, but due to the base current, I.sub.E is larger than I.sub.C 
and, thus, a is less than unity. Furthermore, it is common to sense or 
measure the current in the emitter leg of a transistor and have the 
collector coupled to additional circuitry for transmitting this measured 
current. This is typically done since the circuitry coupled to the 
collector of a transistor may be high voltage while the circuitry coupled 
to the emitter is usually low voltage. Therefore, any difference in the 
emitter and collector currents of a transistor can present a problem when 
precision between the two is required. 
One method of enhancing the alpha of a transistor is well known as the 
Darlington method which involves taking the base current of a transistor 
to be alpha enhanced, and passing it through a driver transistor wherein 
their collectors are tied together. The objective of the Darlington method 
is to effectively add in the base current of the transistor to be enhanced 
to its collector current so that the collector and emitter currents are 
equal thereby making the alpha of the transistor to be enhanced equal to 
unity. However, a problem with the Darlington method is that since both 
transistors have their collectors tied together, if the transistor to be 
enhanced must be selected for high voltage, then so must the driver 
transistor and, thus, requiring two transistors with high breakdown 
voltages. The problem is further increased when the alpha enhancement 
circuitry, in this case the driver transistor, is desired to be 
incorporated on an integrated circuit which is used to drive an external 
transistor that is to be alpha enhanced. Since the collectors of the 
external and driver transistors are tied together, the enhancement 
circuitry on the integrated circuit must also be designed for high voltage 
which will require much die area on an integrated circuit. 
Hence, a need exists for an improved circuit to enhance the alpha of a 
transistor that is independent of the signal at the collector of the 
transistor to be enhanced so that the circuit can be designed for low 
voltage signals. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a circuit 
for enhancing the alpha of a transistor. 
Another object of the present invention is to provide an alpha enhancing 
circuit that is independent of the signal at the collector of the 
transistor to be alpha enhanced. 
In carrying out the above and other objects of the invention, there is 
provided a circuit for enhancing the alpha of a transistor by which the 
apparent emitter current of the transistor is made substantially equal to 
its collector current, comprising current supplying circuit responsive to 
an input voltage signal and coupled to the base of the transistor to be 
alpha enhanced for passing base current thereto; a first current mirror 
having an input coupled to the current supplying circuit for passing 
current that is substantially equal to the base current of the transistor 
to be enhanced; and a second current mirror having an input coupled to an 
output of the first current mirror and an output for providing current to 
the emitter of the transistor to be enhanced which is a function of the 
base current of the transistor to be enhanced multiplied by the product of 
the gains of the first and second current mirrors such that the alpha of 
the transistor is increased. 
The above and other objects, features and advantages of the present 
invention will be better understood from the following detailed 
description taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a detailed schematic diagram illustrating one 
embodiment of the present invention is shown comprising a current 
supplying circuit which includes transistor 10 having a collector coupled 
to an input of current mirror 12 which the latter has an output coupled to 
an input of current mirror 14 whose output is coupled to the emitter of 
transistor 16. The base of transistor 10, coupled to an input voltage 
signal, serves a similar role as did the base of transistor 16 such that 
any connection intended for the base of transistor 16 is now connected to 
the base of transistor 10. The emitter of transistor 10 is coupled to the 
base of transistor 16, while the emitter of transistor 16 is coupled to a 
first supply voltage terminal by resistor 18 for passing current I.sub.S. 
It is understood the emitter of transistor 16 can be coupled to any 
utilization circuit that is capable of passing current thereby replacing 
resistor 18. Finally, the collector of transistor 16 is coupled to 
utilization circuit 20 which is capable of passing current. 
Current mirror 12 includes transistor 27 having a collector coupled to its 
base by resistor 29 and to the input of current mirror 12, and an emitter 
coupled to a second supply voltage terminal by resistor 30. A transistor 
32 having a collector coupled to the output of current mirror 12, a base 
coupled to the collector of transistor 27, and an emitter coupled to the 
second supply voltage terminal by resistor 34. 
Current mirror 14 comprises transistor 36 having a collector coupled to its 
base by resistor 38 and to the input of current mirror 14, and an emitter 
coupled to a third supply voltage terminal by resistor 40. It is worth 
noting that a diode may be used to replace transistor 36 and resistor 38 
of current mirror 14 as well as transistor 27 and resistor 29 of current 
mirror 12. A transistor 42 has a collector coupled to the output of 
current mirror 14, a base coupled to the collector of transistor 36, and 
an emitter coupled to the third supply voltage terminal by resistor 44. 
The circuit further includes resistor 46 coupled between the output of 
current mirror 14 and the emitter of transistor 10 for providing a minimum 
quiescent operating current through transistor 10. 
In operation, a voltage is applied to the base of transistor 10 which 
defines a current through transistor 16 that is sensed or measured via 
resistor 18. It is desired to have the collector current of transistor 16 
substantially equal to its emitter current, but due to the base current of 
transistor 16, the emitter current of transistor 16 is larger than its 
collector current. However, transistor 10 and current mirrors 12 and 14 
provide a circuit for enhancing the alpha of transistor 16 such that the 
new or apparent emitter current of transistor 16, I.sub.S, which is 
measured via resistor 18, is less than the emitter current of transistor 
16, I.sub.E, and, thus, the apparent alpha of transistor 16, defined as 
the ratio of the collector current of transistor 16 to the current through 
resistor 18, I.sub.C /I.sub.S, is greater than the standard alpha of ratio 
I.sub.C /I.sub.E. Furthermore, since the collector of transistor 16 is not 
connected to the alpha enhancement circuit by any means, the alpha 
enhancement circuit can be designed for low voltage signals even if high 
voltage signals exist at the collector of transistor 16. 
Alpha enhancement of transistor 16 is accomplished by passing the base 
current of transistor 16 through transistor 10 such that the collector 
current of transistor 10 is substantially equal to the base current of 
transistor 16, neglecting a small error term comprising the base current 
of transistor 10. The collector current of transistor 10 is then passed or 
mirrored through current mirrors 12 and 14 such that the output of current 
mirror 14 sinks current from the emitter of transistor 16. By making the 
product of the gains of current mirrors 12 and 14 substantially equal to 
unity, the current sunk from the output of current mirror 14 is 
substantially equal to the base current of transistor 16, neglecting the 
small loss of current through resistor 46. Therefore, the apparent emitter 
current of transistor 16, I.sub.S, which is the emitter current of 
transistor 16 lessened by the base current of transistor 16, is 
substantially equal to the collector current of transistor 16 and, thus, 
the apparent alpha of transistor 16 is substantially equal to unity. It 
should be obvious that by increasing the product of the gains of current 
mirrors 12 and 14, the current loss through resistor 46 as well as the 
base current loss through transistor 10 can be compensated to yield an 
apparent alpha of precisely unity. Furthermore and in a similar manner, 
the apparent alpha of transistor 16 can be made greater than unity, if so 
desired. 
The method of alpha enhancement for transistor 16 may also be described by 
using mathematical equations. The general equation for the currents 
through transistor 16 of FIG. 1 is well known as: 
EQU I.sub.C =I.sub.E -I.sub.B (1) 
where I.sub.C, I.sub.E and I.sub.B are the currents through the collector, 
emitter and base of transistor 16, respectively. 
By applying Kirchoff's current law (KCL) at the emitter of transistor 16 of 
FIG. 1 and neglecting the base current of transistor 10 and the current 
through resistor 46, equation 2 can be obtained. 
EQU I.sub.E =I.sub.S +(K.times.I.sub.B) (2) 
where I.sub.E is the emitter current through transistor 16; 
I.sub.S is the current through resistor 18; 
I.sub.B is the base current of transistor 16; and 
K is the product of the gains of current mirrors 12 and 14. 
By substituting the expression for I.sub.E of equation 2 into equation 1, 
equation 3 is obtained. 
EQU I.sub.C =I.sub.S -I.sub.B .times.(1-K) (3) 
Furthermore, the apparent alpha of transistor 16 can be calculated by 
taking the ratio of I.sub.C /I.sub.S as shown in equation 4. 
EQU a=I.sub.C /I.sub.S =1+I.sub.B /I.sub.S .times.(K-1) (4) 
Equation 4 as well as equation 3 again reveal as aforedescribed that for a 
value of K substantially equal to unity, the apparent alpha of transistor 
16 is substantially equal to unity. Furthermore, by ascertaining K greater 
than unity, the apparent alpha of transistor 16 can be made greater than 
unity if so desired. 
Referring to FIG. 2, a detailed schematic of a second embodiment of the 
present invention shows how to perform alpha enhancement of PNP transistor 
28. The components of FIG. 2 similar to those of FIG. 1 have been 
designated by the same reference numerals. A current supplying circuit 
comprising transistor 22 has its collector coupled to the input of current 
mirror 24 which the latter has an output coupled to an input of current 
mirror 26 whose output is coupled to the emitter of transistor 28. The 
base of transistor 22, coupled to an input voltage signal, serves a 
similar role as did the base of transistor 28 such that any connection 
intended for the base of transistor 28 is now connected to the base of 
transistor 22. The emitter of transistor 22 is coupled to the base of 
transistor 28 while the emitter of transistor 28 is coupled to a first 
supply voltage terminal by resistor 18 for passing current I.sub.S. 
Finally, the collector of transistor 28 is coupled to utilization circuit 
20 which is capable of passing current. 
Current mirror 24 comprises transistor 48 having a collector coupled to its 
base by resistor 50 and to the input of current mirror 24, and an emitter 
coupled to a second supply voltage terminal by resistor 52. Transistor 54 
has a collector coupled to the output of current mirror 24, a base coupled 
to the collector of transistor 48, and an emitter coupled to the second 
supply voltage terminal by resistor 56. 
Current mirror 26 comprises transistor 58 having a collector coupled to its 
base by resistor 60 and to the input of current mirror 26, and an emitter 
coupled to a third supply voltage terminal by resistor 62. Similarly, a 
diode may be used to replace transistor 58 and resistor 60 of current 
mirror 26 as well as transistor 48 and resistor 50 of current mirror 24. 
Transistor 64 has a collector coupled to the output of current mirror 26, 
a base coupled to the collector of transistor 58, and an emitter coupled 
to the third supply voltage terminal by resistor 68. The circuit further 
includes resistor 70 coupled between the output of current mirror 26 and 
the emitter of transistor 22 for providing a minimum quiescent operating 
current through transistor 22. 
The operation of the circuit shown in FIG. 2 is very similar to the circuit 
shown in FIG. 1. As before, a voltage is applied to the base of transistor 
22 which defines a current through transistor 28 that is sensed or 
measured via resistor 18. Transistor 22 and current mirrors 24 and 26 
provide a circuit for enhancing the alpha of transistor 28 such that the 
new or apparent emitter current of transistor 28, I.sub.S, is less than 
the emitter current of transistor 28, I.sub.E, and, thus, the apparent 
alpha (I.sub.C /I.sub.S) of transistor 28 is increased. 
Alpha enhancement of transistor 28 is accomplished by passing the base 
current of transistor 28 through transistor 22 such that the collector 
current of transistor 22 is substantially equal to the base current of 
transistor 28, neglecting a small error term comprising the base current 
of transistor 22. The collector current of transistor 22 is then passed 
through current mirrors 24 and 26 such that the output of current mirror 
26 sources current to the emitter of transistor 28. By making the product 
of the gains of current mirrors 24 and 26 substantially equal to unity, 
the current sourced from the output of current mirror 26 is substantially 
equal to the base current of transistor 28, neglecting the small loss of 
current through resistor 70. Therefore, the apparent emitter current of 
transistor 28, I.sub.S, which is the emitter current of transistor 28, 
I.sub.E lessened by its base current, is substantially equal to the 
collector current of transistor 28 thereby ascertaining the apparent alpha 
of transistor 28 substantially equal to unity. 
Furthermore, the mathematical approach utilizing equations 1 and 2 still 
apply for the circuit shown in FIG. 2, therefore equations 3 and 4 can 
also be derived for the circuit of FIG. 2 in the aforedescribed manner. 
By now it should be appreciated that there has been provided a novel 
circuit that successfully enhances the alpha of an NPN or PNP transistor 
independent of the signal at the collector of the NPN or PNP transistor.