Current mirror circuit and method for providing zero temperature coefficient trimmable current ratios

A resistively trimmed current mirror circuit including a diode coupled in parallel to the base-emitter conduction path of a transistor and to which a reference current is supplied to produce a proportional output current at the collector of the transistor. A trimmable resistor is connected in the collector-emitter conduction path of the transistor which is adjusted to vary the output current for a given reference current to trim the ratio between the two currents. By making the reference current a thermal current, i.e., a current whose magnitude is proportional to thermal voltage and inversely proportional to resistance, the trimmed ratio remains temperature independent after being trimmed to a desired value.

BACKGROUND OF THE INVENTION 
This invention relates to current sources and, more particularly, to a 
current mirror circuit arrangement for providing an output current that is 
proportional to an input current supplied to the current mirror circuit 
and which the ratio of the two currents is both trimmable and temperature 
independent. 
Monolithic integrated current mirror circuits are well known to those 
skilled in the art. A simple current mirror circuit that is known includes 
an output transistor whose collector provides the output current and whose 
emitter is returned to a reference potential and a diode whose 
anode-cathode is connected in parallel with the base-emitter junction of 
the transistor. The input current is supplied to the anode of the diode. 
The diode is typically formed by an additional transistor which has its 
collector interconnected to both its base and the base of the output 
transistor. The ratio of the output current I.sub.O to the input current 
I.sub.T can be set to a desired value by area ratioing the emitters of the 
two transistors as is understood. 
It is sometimes desirable to trim the two currents to a desired ratio while 
maintaining the trimmed ratio independent of temperature variations. One 
method to trim the current ratio in the above described current mirror 
circuit is to trim the emitter areas of the two transistors with respect 
to one another. Although emitter area trimming results in a ratio that is 
independent of temperature, it is not a practical method to be used. 
Another method for adjusting the current ratio of the current mirror 
circuit is to provide resistive trimming using trimmable resistors coupled 
respectively between ground reference and the diode as well as the emitter 
of the output transistor. By trimming one or the other or both resistors, 
the value of I.sub.O can be adjusted with respect to a given input current 
I.sub.T. However, the current density of the output transistor is changed 
relative to that of the diode-connected transistor as one or the other 
resistor is trimmed. This produces a current ratio having some temperature 
coefficient (TC) other than zero which may not be desirable. 
Hence, a need exists for a method for providing trimmable current ratios in 
such current mirror circuits wherein the adjusted ratio is independent to 
temperature variations, i.e., a zero TC. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved current mirror circuit. 
It is another object of the invention to provide a method for use with a 
current mirror circuit of provide trimmable zero TC current ratios. 
Yet another object of the present is to provide an integrated current 
mirror circuit in which the current ratio is trimmable and independent of 
temperature. 
In accordance with the above and other objects there is provided a current 
mirror circuit comprising two parallel circuit paths including a diode 
connected in the first circuit path and a transistor having its 
collector-emitter connected in the second circuit path and its base 
coupled to the diode and a current supply providing a thermal current to 
the diode. The current mirror circuit includes trimmable resistive 
elements in one or the other or both of the circuit paths the value of 
which may be trimmed to change the ratio of the currents flowing in the 
two circuit paths wherein the resulting ratio is both a constant and 
temperature independent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the sole FIGURE there is illustrated current mirror 
circuit 10 of the preferred embodiment. Current mirror circuit 10 includes 
NPN transistor 12 whose base or control electrode is coupled at circuit 
node 14 to diode 16. Diode 16, which as understood, may be formed by a NPN 
transistor having its collector shorted to its base is coupled between 
node 14 and common terminal 19 via resistor 18 in a first current circuit 
path to earth reference potential. The emitter or first main electrode of 
transistor 12 is connected via resistor 20 to common terminal 19 while the 
collector or second main electrode of transistor 12 is coupled to output 
node 22. The collector-emitter conduction path of transistor 12 comprises 
a second current circuit path and provides an output current I.sub.O 
therefrom. It is understood that output node 22 is connected to some load 
utilization means. A current supply 24 coupled between a power supply 
conductor 26 and node 14 sources a reference input current to node 14 to 
forward bias diode 16. 
As so far described, current mirror 10 is conventional in structure and 
operation. Without considering resistors 18 and 20, as diode 16 is forward 
biased the current I.sub.T flows therethrough and transistor 12 is 
rendered conductive to provide the collector current I.sub.O. I.sub.O is 
proportional to I.sub.T depending on the ratio of the emitter areas of the 
two transistors. For example, if the emitter area of transistor 12 is made 
N times the emitter area of diode-connected transistor 16, where N is any 
positive number, I.sub.O will be approximately equal to NI.sub.T. 
As previously mentioned, it is often desirable to trim the two currents 
I.sub.O and I.sub.T to a desired ratio while maintaining the ratio 
independent of temperature. Trimmable resistors 18 and 20 provide a 
convenient means for adjusting this ratio. For instance, by trimming 
resistor 20 the value of I.sub.O is adjusted for a given I.sub.T. There 
are many known methods for trimming these resistors. If, for instance, 
current mirror circuit 10 is fabricated in integrated circuit form, 
resistors 18 and 20 may be thin film metal resistors the value of which 
can be adjusted by laser trim techniques familiar to those skilled in the 
art. As the value of resistor 20 is trimmed, for example, the value of 
I.sub.O is adjusted for a given I.sub.T which sets the desired ratio of 
the two currents. Although the described trim technique has been used in 
the past the resultant trimmed ratio is not constant with temperature due 
to the fact that the current density of transistor 20 is changed relative 
to that of diode 16. 
As will be described hereinafter in detail, it was discovered that the 
ratio of the two currents could be adjusted and still have a zero TC by 
making current reference I.sub.T a thermal current, i.e., a current of the 
form: 
EQU I.sub.T =(kT/qR)1n K (1) 
where: 
k is Boltzmann's constant 
q is the charge of an electron 
R is a resistance of a given resistivity and TC; 
T is absolute temperature; and 
K is a constant. 
Prior art current sources providing thermal currents of the above described 
form are well known. For example, U.S. Pat. No. 4,435,678 discloses such a 
current source. 
If I.sub.T is a thermal current of the form of equation 1, it can be shown 
the ratio of I.sub.O to I.sub.T is a constant and can be adjusted by 
trimming either resistor 18 or 20 such that the resultant ratio is 
independent of temperature. Thus, with the current of equation 1 supplied 
to current mirror circuit 10, the following current mirror ratio can be 
expressed: 
EQU I.sub.T /I.sub.O =(R2/R)1n KI/1n[(I.sub.T /I.sub.O)NK.sup.(R1/R) ](2) 
where: 
R2 is the resistance of resistor 20 
R1 is the resistance of resistor 18; and 
R, R1, and R2 are all of the same resistivity material and have the same 
TC. 
Because (R2/R)1n K is always a constant, C1, and NK.sup.(R1/R) is always a 
constant, C2. Then: 
EQU (I.sub.T /I.sub.O)1n[C2(I.sub.T /I.sub.O)]=C1 (3) 
Since, the ratio of I.sub.T to I.sub.O must be a constant to satisfy 
equations 2 and 3, the ratio must then be independent of temperature. 
The current ratio I.sub.T /I.sub.O or its inverse can be trimmed by 
adjusting R, N, R1, R2 or K and still remain temperature independent. 
However, in practice, R1 and R2 are most conveniently trimmed to adjust 
the current ratio. 
It is further understood that multiple current ratios can be provided by 
using multiple transistors connected to node 14 in the similar fashion as 
transistor 12, i.e., having their bases connected to node 14 and the 
collector-emitter conduction paths coupled in series with a trimmable 
resistor to common terminal 19. 
Hence, what has been described is a method of providing resistive trimming 
in a current mirror circuit to adjust the ratio of output current to the 
input current thereof while maintaining the ratio independent of 
temperature by driving the current mirror circuit with a thermal current.