A constant-current source including a constant-current output circuit for supplying a constant current provided with one or more transistors with the bases biased with the same base potential, a first circuit which provides a first current signal for setting the strength of the constant current to be delivered from the constant-current output circuit, a second circuit which generates a second current signal and provides the same base potential in response to the second current signal, a third circuit which controls the second current signal to minimize any deviation of the second current signal from the first current signal, and a DC power supply for energizing at least the first, second and third circuits. The improvement is that the transconductance of the first circuit which represents the ratio of a change in the first current signal to a change in the output voltage of the DC power supply is equal to the transconductance of the second circuit which represents the ratio of a change in the second current signal to a change in the output voltage of the DC power supply.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a DC constant-current source, and in 
particular to a DC constant-current source capable of compensating for 
errors in the output current caused by changes in the output voltage of 
the DC power supply. 
2. Description of the Related Art 
Various types of circuits for constant-current source have been developed 
as needed. FIGS. 1 and 2 show circuits of first and second prior-art 
constant-current sources, respectively, which are of our present interest. 
The circuit shown in FIG. 1 is provided with DC power supply 2, 
output-current setting circuit 13, current regulating circuit 14 made up 
of pnp transistor Q.sub.4 and resistor R.sub.4, a current-difference 
amplifier made up of pnp transistor Q.sub.8 and resistor R.sub.8, and 
constant-current output circuit 5. 
Constant-current output circuit 5 (hereafter referred to as output circuit 
5) is made up of a plurality of pnp transistors Q.sub.16, ---, Q.sub.n-1, 
Q.sub.n of the same characteristics with the bases interconnected through 
a base line and the emitters connected to the positive electrode of DC 
power supply 2 through emitter resistors R.sub.16, ---, R.sub.n-1, R.sub.n 
of the same resistance. 
Output-current setting circuit 13, driven by DC power supply 2, generates a 
current signal I.sub.C2 (the collector current of transistor Q.sub.2). The 
current output of output circuit 5 is regulated to a value which 
corresponds to reference current I.sub.C2, as will be described below. 
Circuit 13 includes a series circuit composed of resistor R.sub.3A, 
temperature-compensated npn transistor Q.sub.1 and constant-voltage source 
1 connected in series between the positive and grounded negative 
electrodes of DC power supply 2. Constant-voltage source 1 supplies 
transistor Q.sub.1 with constant emitter potential V.sub.1 with respect to 
the ground potential. Transistor Q.sub.1 serves to provide base potential 
V.sub.B1 for biasing the base of transistor Q.sub.2, V.sub.B1 being 
V.sub.1 +V.sub.BE1 and V.sub.BE1 being the base-emitter voltage of 
transistor Q.sub.1. Resistor R.sub.3A is determined according to 
approximate equation R.sub.3A =(V.sub.2 -V.sub.1)/I.sub.3A, where V.sub.2 
and I.sub.3A represent the output voltage of DC power supply 2 and a 
prescribed current which flows through Resistor R.sub.3A. Npn transistor 
Q.sub.9 supplies a fraction of its current output to transistor Q.sub.1 as 
base current I.sub.B1 so as to minimize any deviation of collector current 
I.sub.C1 of transistor Q.sub.1 from current I.sub.3A, i.e. to minimize 
base current I.sub.B9 =I.sub.3A -I.sub.C1 of transistor Q.sub.9. This 
allows the deviation to be regulated to I.sub.3A /(f.multidot.h.sub.FE1 
.multidot.h.sub.FE9), an order of 10.sup.-4 .multidot.I.sub.3A, where 
h.sub.FE1 and h.sub.FE9 represent the current gains of transistor 1 and 9, 
respectively, and f denotes a fraction of the emitter current of 
transistor Q.sub.9 that is supplied to the base of transistor Q.sub.1. 
Transistor Q.sub.2 has an emitter grounded through resistor R.sub.2 and is 
biased with the same base potential as that of transistor Q.sub.1. This 
causes the emitter potential of transistor Q.sub.2 to equal that of 
transistor Q.sub.1, provided that the difference in the base-emitter 
voltages of the two transistors, .DELTA.B.sub.BE, is ignored. As a result, 
the emitter current I.sub.E2 of transistor Q.sub.2, thus collector current 
I.sub.C2, becomes approximately V.sub.1 /R.sub.2. In this way, collector 
current I.sub.C2, which is an output of output-current setting current 13, 
is set to a desired value by adjusting resistor R.sub.2. Transistor 
Q.sub.2 is also temperature-compensated so that a change in collector 
current I.sub.C2 caused by a temperature change in transistor Q.sub.1 will 
be compensated for. The advantage of output-current setting circuit 13 is 
that it is capable of establishing a current of a given strength with a 
smallsized circuitry. 
Transistor Q.sub.4 and emitter resistor R.sub.4 constitute an amplifier 
identical with each of the parallel amplifiers constituted by transistors 
Q.sub.16, Q.sub.17 ---, Q.sub.n and their emitter resistors R.sub.16, 
R.sub.17, ---, R.sub.n. The base of transistor Q.sub.4 is connected both 
to the bases of the group of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n 
and to the collector of transistor Q.sub.4 by way of transistor Q.sub.8 to 
constitute a current-mirror circuit, wherein transistor Q.sub.4 is the 
input transistor and the group of transistors Q.sub.16, ---, Q.sub.n-1 and 
Q.sub.n are the output transistors. The collector of transistor Q.sub.4 is 
also connected to the collector of transistor Q.sub.2 through a branch 
point where difference current I.sub.B8 =I.sub.C2 -I.sub.C4, which 
corresponds to the deviation of collector current I.sub.C4 of transistor 
Q.sub.4 from collector current I.sub.C2, is branched off. 
Transistor Q.sub.8, associated with resistor R.sub.8, provides a path of 
the base currents of the group of transistors Q.sub.16, ---, Q.sub.n-1, 
Q.sub.n and of transistor Q.sub.4. Transistor Q.sub.8 also acts to control 
emitter current I.sub.E4 of transistor Q.sub.4 so as to minimize 
difference current I.sub.B8 by the same operation as transistor 9. 
When the output current of output circuit 5 decreases, base potential 
V.sub.BG of the group of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n is 
raised. Since the base of transistor Q.sub.4 is voltage-biased by base 
potential V.sub.BG, the rise in base potential V.sub.BG causes a decrease 
in emitter current I.sub.E4 of transistor Q.sub.4, which results in an 
increase in base current I.sub.B8 of transistor Q.sub.8. Transistor 
Q.sub.8 acts to carry more collector current I.sub.C8, which causes base 
potential V.sub.BG to be lowered, whereby emitter current I.sub.E4 
increases to minimize base current I.sub.B8, i.e. to minimize the 
deviation of I.sub.C4 from I.sub.C2. Since emitter current I.sub.E4 is an 
input of the currentmirror circuit, the increase in I.sub.E4 causes the 
output current of the current-mirror circuit, i.e. output current I.sub.o 
of output circuit 5. Thus, output current I.sub.o is regulated to the 
value corresponding to collector current I.sub.C2. In this way, collector 
current I.sub.C2 serves as a reference current to be referred to by 
collector current I.sub.C4. 
Next, referring to FIG. 2, a second constant-current source of the prior 
art will be explained. The essential part of the constant-current source 
is identical with that of the first constant-current source shown in FIG. 
1. The difference is in output-current setting circuit 10. In 
constant-current setting circuit 10, reference current I.sub.r is 
established by applying a constant voltage V.sub.1 across resistor R.sub.2 
through negative feedback amplifier 11 of voltage gain 1 (a voltage 
follower) which serves as a buffer circuit. Reference current I.sub.r is 
determined from equation I.sub.r =V.sub.1 /R.sub.2, as is the case in the 
first constant-current source. 
The operation of the circuit shown in FIG. 2 to stabilize output current 
I.sub.o is similar to that shown in FIG. 1. 
A problem in the first constant-current source above has been that it is 
susceptible to changes in the output voltage of DC power supply 2. Let 
.DELTA.V.sub.2 be the change, and g.sub.m1, g.sub.m2 the transconductances 
of transistors Q.sub.1, Q.sub.2, respectively, then change .DELTA.I.sub.C2 
in collector current I.sub.C2 caused by .DELTA.V.sub.2 becomes 
(.DELTA.V.sub.2 /R.sub.3A) (g.sub.m2 /g.sub.ml), which entails a change in 
output current I.sub.o of the constant-current source. Further, another 
problem has been that, while transistor Q.sub.1 and Q.sub.2 are 
temperature-compensated, output-current setting circuit 13 as a whole is 
susceptible to temperature changes. 
A problem in the second constant-current source above has been that the 
buffer amplifier, i.e. negative feedback amplifier 11, requires a large 
size. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a constant-current 
source capable of compensating for changes in the output current of the 
constant current source caused by changes in the output voltage of the DC 
power supply. 
It is another object of the present invention to provide a small-sized 
constant-current source capable of compensating for changes in the output 
current of the constant current source caused by both changes in the 
output voltage of the DC power supply and changes in temperature of the 
circuit. 
In order to attain the first object above, the constant-current source 
according to the present invention includes a constant-current output 
circuit for supplying a constant current provided with one or more 
transistors with the bases biased with the same base potential, a first 
circuit which provides a first current signal for setting the strength of 
the constant current to be delivered from the constant-current output 
circuit, a second circuit which generates a second current signal and 
provides said same base potential in response to the second current 
signal, a third circuit which controls the second current signal to 
minimize any deviation of the second current signal from the first current 
signal, and a DC power supply for energizing at least the first, second 
and third circuits, wherein 
the transconductance of the first circuit which represents the ratio of a 
change in the first current signal to a change in the output voltage of 
the DC power supply is equal to the transconductance of the second circuit 
which represents the ratio of a change in the second current signal to a 
change in the output voltage of the DC power supply. 
Since the two transconductances equal each other, changes in the first and 
second current signals caused by an output-voltage change of the DC power 
supply are the same. Thus, the output voltage change does not exert any 
effect on controlling the second current signal by the third circuit, 
whereby the output current of the current output circuit will not be 
affected by the output voltage change of the DC power supply. 
The first circuit preferably comprises a first resistance connected to a 
first electrode of the DC power supply at one end thereof, a first 
transistor of a first conductivity type with its emitter connected to the 
other end of the first resistance and with its base circuit arranged so as 
to be insusceptible to any change in the output voltage of the DC power 
supply, a constant voltage source with the second electrode connected to 
the second electrode of the DC power supply, a second transistor of a 
second conductivity type with the emitter connected to a first electrode 
of the constant voltage source and the collector connected to the 
collector of the first transistor through a branch point where a 
difference current corresponding to a deviation of the collector current 
of the second transistor from the collector current of the first 
transistor is branched off, a regulation circuit which supplies a base 
current to the second transistor so as to minimize the deviation, a second 
resistance connected to the second electrode of the constant voltage 
source at one end thereof, and a third transistor of the second 
conductivity type with the emitter connected to the other end of the 
second resistance, the base connected to the base of the second transistor 
and the collector connected to the second circuit, the second circuit 
comprises a third resistance connected to the first electrode of the DC 
power supply, and a fourth transistor of the first conductivity type with 
the emitter connected to the other end of the third resistance, the base 
connected to the base of each transistor in the constant-current output 
circuit and the collector connected to the collector of the third 
transistor through a branch point where a difference current corresponding 
to the deviation of the collector current of the fourth transistor from 
the collector current of the third transistor is branched to be supplied 
to the third circuit, wherein the first resistance is determined such that 
the ratio of the first resistance to the third resistance equals the 
reciprocal of the ratio of the collector current of the first transistor 
to the collector current of the third transistor, and the first, second, 
third and fourth transistors have transconductances such that the ratio of 
the transconductance of the fourth transistor to that of the first 
transistor is equal to the ratio of the transconductance of the third 
transistor to that of the second transistor. 
In order to effect temperature-compensation of the ratio of the 
transconductance of the fourth transistor to that of the first transistor, 
and of the ratio of the transconductance of the third transistor to that 
of the second transistor, it is preferable that the current densities of 
the emitter currents carried by the first and fourth transistors be equal, 
and that the current densities of the emitter currents carried by the 
second and third transistors also be equal. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following description referring to 
the accompanying drawing which illustrates an example of a preferred 
embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to FIG. 3, an embodiment of the present invention will be 
explained below. Like the circuit shown in FIG. 1, the circuit of the 
constant-current source according to the present invention comprises DC 
power supply 2, output-current setting circuit 3, constant-current output 
circuit 5 (hereafter referred to as output circuit 5), current regulating 
circuit 4 made up of pnp transistor Q.sub.4 and emitter resistor R.sub.4, 
a current-difference amplifier made up of pnp transistor Q.sub.8 and 
resistor R.sub.8, and starter circuit 6. Among these, the current 
regulating circuit, the current-difference amplifier and output circuit 5 
are identical with those in the circuit shown in FIG. 1. Accordingly 
transistor Q.sub.4 and each of transistor Q.sub.16, ---, Q.sub.n-1, 
Q.sub.n have identical characteristics, and emitter resistor R.sub.4 and 
each of emitter resistors R.sub.16, ---, R.sub.n-1, R.sub.n have the same 
resistance, so that transistor Q.sub.4 and each of transistors Q.sub.16, 
---, Q.sub.n-1, Q.sub.n carry currents of the same current density, 
thereby constituting a current mirror circuit. 
The differences between output-current setting circuits 3 and 13 are that, 
in lieu of resistor R.sub.3A in output-current setting circuit 13, 
transistor Q.sub.3 and emittor resistor R.sub.3 are arranged in 
output-current setting circuit 3, that the ratio of resistance R.sub.3 to 
resistor R.sub.4 equals a reciprocal of the ratio of a prescribed value of 
emitter current I.sub.E3 of transistor Q.sub.3 to a prescribed value of 
emitter current I.sub.E6 of transistor Q.sub.6, and that both the ratio of 
emitter area S.sub.3 of transistor Q.sub.3 to emitter area S.sub.4 of 
transistor Q.sub.4 and the ratio of the emitter area S.sub.5 of transistor 
Q.sub.5 to emitter area S.sub.6 of transistor Q.sub.6 are equal to the 
ratio of emitter current I.sub.E3 to emitter current I.sub.E6. The base 
circuit of transistor 3 is arranged so that any output-voltage change of 
DC power supply 2 will not affect the base potential. In the present 
embodiment the base of transistor Q.sub.3 is connected to the base of 
transistor Q.sub.4. 
By the arrangement described above, substantially the same voltage as the 
voltage across resistor R.sub.4 is applied across resistor R.sub.3, 
causing the emitter potential of transistor Q.sub.3 with respect to the 
positive electrode of DC power supply 2 to be the same as the emitter 
potential of transistor Q.sub.4. Further, since collector currents 
I.sub.C5 and I.sub.C4 of transistors Q.sub.5 and Q.sub.4 are regulated to 
approach collector current I.sub.C3 and I.sub.C6 of transistor Q.sub.3 and 
Q.sub.6, respectively, the current densities of the emitter currents in 
transistors Q.sub.3, Q.sub.5 are substantially equal to those in 
transistors Q.sub.4, Q.sub.6 respectively, in the stable state of the 
constant-current source. 
As is well known in the art, when two transistors, say Q.sub.3 and Q.sub.4, 
in a monolithic IC carry emitter currents of the same current density, the 
difference between the base-emitter voltages, .DELTA.VBE=V.sub.BE3 
-V.sub.BE4, and its temperature coefficient .delta..DELTA.V.sub.BE 
/.delta.T vanishes. (This is because all factors except the emitter areas 
in the reverse saturation currents are equal in the transistors provided 
in a given monolithic IC, and thus the reverse saturation current is a 
function of a single emitter area.) Since the ratio of transconductance 
g.sub.m3 of transistor Q.sub.3 to the transconductance g.sub.m4 of 
transistor Q.sub.4 is 
##EQU1## 
and since V.sub.BE3 -V.sub.BE4 =0 under the equal current-density 
condition, it follows from equations (1) and (2) that 
##EQU2## 
As described above, since 
##EQU3## 
it follows that 
##EQU4## 
Augments similar to those setforth in equations (1), (2) and (4) hold in 
g.sub.m5 /g.sub.m6. Therefore equation (6) is temperature-compensated in 
the sense that equation (6) holds in the case that the temperature changes 
as well. 
Suppose that due to an output voltage change of DC power supply 2, 
V.sub.BE3 and V.sub.BE4 change by .DELTA.V.sub.BE3 and .DELTA.V.sub.BE4, 
respectively. Since under the equal current-density condition, 
EQU .DELTA.(V.sub.BE3 -V.sub.BE4)=.DELTA.V.sub.BE3 -.DELTA.V.sub.BE4 =0, and 
(7) 
since 
##EQU5## 
Similarly, with regard to transistors Q.sub.5 and Q.sub.6 
EQU .DELTA.I.sub.C6 =(g.sub.m6 /g.sub.m5) .DELTA.I.sub.C5 =(g.sub.m6 /g.sub.m5) 
.DELTA.I.sub.C3 (10) 
From equations (9), (10) and (6) it follows that 
EQU .DELTA.I.sub.B8 =.DELTA.(I.sub.C6 -I.sub.C4)=0. (3) 
Thus, a change in the output voltage in DC power supply 2 does not exert 
any effect on base current I.sub.B8 of transistor Q.sub.8. Consequently, 
the base currents of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n, and 
thus the output current of the constant-current source are not subject to 
any adverse effect caused by any output change of the DC power supply. 
It should be appreciated that, since the temperature coefficients of both 
sides of equation (6) vanish under the equal current-density condition, as 
described above, the circuit shown in FIG. 3 is temperature-compensated, 
and that this circuit can be realized in a small size. 
Starter circuit 6 comprises resistor R.sub.6, diodes D.sub.1 and D.sub.2 
connected in series between the electrodes of DC power supply 2 and npn 
transistor Q.sub.7 with the base connected between diodes D.sub.1 and 
D.sub.2, and with the emitter and collector connected with the emitter and 
collecter of transistor Q.sub.6, respectively. 
At start-up time, when the base potential of transistor Q.sub.7 rises above 
that of transistor Q.sub.6, transistor Q.sub.7 turns on, whereby 
collector-emitter voltage V.sub.CE4 of transistor Q.sub.4 is established. 
Collector-emitter voltage V.sub.CE4 allows the emitter-base junctions in 
transistors Q.sub.4 and Q.sub.8 to be forwardly biased in series, whereby 
the base potentials of transistors Q.sub.4 and Q.sub.3 are established, 
allowing transistor Q.sub.3 to turn on. The turn-on of transistor Q.sub.3 
allows the base-emitter junctions in transistors Q.sub.9 and Q.sub.5 to be 
forwardly biased in series, whereby the base potentials of transistors 
Q.sub.5 and Q.sub.6 are established. When the base potential of transistor 
Q.sub.6 rises above that of transistor Q.sub.7, transistor Q.sub.7 is cut 
off, and the whole circuit of the constant-current source starts to 
operate. After startup, transistor Q.sub.8 acts so as to minimize I.sub.C6 
-I.sub.C4. Since transistor Q.sub.4 and the group of transistors Q.sub.16, 
---, Q.sub.n-1, Q.sub.n constitute a current mirror circuit, current 
output I.sub.o of output circuit 5 is regulated so that the collector 
current of each of transistors Q.sub.16, ---, Q.sub.n-1, Q.sub.n equals 
collector current I.sub.C6, the reference current. 
In the above embodiment, the base of transistor Q.sub.3 is connected to 
that of transistor Q.sub.4 in order to make clear the basic concept of the 
present invention. However, it is not always necessary to do so. The thing 
to be noted is that the base circuit of transistor Q.sub.3 is arranged so 
as not to be directly affected by any change in the output voltage of DC 
power supply 2. For example, transistor Q.sub.3 may be collector-to-base 
shorted, or diode-connected. 
Further, in the case that it is required to compensate for changes in the 
output current due to changes only in the output voltage of the DC power 
supply, any circuit will do in which the transconductance which represents 
the ratio of the change in the output of the output-current setting 
circuit to the change in the output voltage of the DC power supply equals 
the transconductance which represents the ratio of the change in the 
output of the current regulating circuit to the change in the output 
voltage of the DC power supply. 
It is to be understood that although characteristics and advantages of the 
present invention have been set forth in the foregoing description, the 
disclosure is illustrative only, and changes may be made in arrangement of 
parts within the scope of the appended claims.