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4QD-TEC: Electronics Circuits Reference Archive
Current sources and
mirrors

There are many useful circuit elements: current sources and current mirrors
are two of them. Sources and mirrors are so closely related that it is sometimes
difficult to differentiate between them. So what? - labelling things is a
function of language not of electronics!

Circuit 1

This is about the simplest current source possible. It is
essentially an emitter follower where the transistor's Vbe is used to define the
current through R2. If R2 is chosen so that the current through it is much more
than the the transistor's base current, then ib can be ignored and
the current through R3 (the load) is Vb/R2.

The same circuit is also used as a Vbe multiplier - when R3 is
connected B-C of the transistor. As iR2 = iR3 the voltage
between c-e is the transistor is Vbe x (R2+R3) / R2

Circuit 2

This is a very standard current source. Vin
causes a current to flow through R1 into the diode. So a voltage is developed
across the diode. This voltage is also present across the base-emitter junction
of the transistor, so a current must flow in the transistor. The magnitude of
the transistor current depends on the voltage developed across the diode and
this is a function of its geometry. So the ratio of input to output current is
not easy to calculate. However it will be pretty consistent with identical
diodes and transistors because modern production causes very little spread in
device parameters. The base-emitter junction of a transistor behaves just like a
diode.

Circuit 3

Instead of a diode, you can use an LED. An LED (depending
on colour, intensity etc) drops about 2v so you can calculate the output current
as shown.

Circuit 4

This is a commonly used source: Vin drives
current through R1 into the base of the second transistor, so current flows into
the transistor's collector, through it and out of its emitter. This current must
flow through R2. If the current gets too high, the first transistor turns on and
robs the second transistor of base current, so its collector current can never
exceed the value shown. This is an excellent way of either making a current
source or of limiting the available current to a defined maximum value.

Circuit 5

This is a true current mirror: feed a current
(i1) into the first transistor (usually by driving it via a resistor
from a suitable voltage) and a 'mirror' current (i2) flows in the
second transistor. Provided the two transistors are matched and of high gain,
input and mirror current will match quite closely. In fact, with modern
transistors, if you use a -C gain rating (450-900) matching of any two
transistors is probably adequate for all usual needs.

Look at the equivalent circuit of virtually any analogue IC and you will
notice this type of current source/mirror (it may be used for either) but it
works just as well with discrete transistors.

One source of errors is if the voltage across the output transistor is high
enough to cause the transistor to heat: the mirror's performance fails because
the two transistors are now no longer well matched!

This circuit can also be used with resistors in both emitters - in this case
the mirror ratio will depend on the ratio of these resistors. An example of this.

Circuit 6

This is for the purists: circuit 4 gives an error due to
the base bias current. Here is the commonest way of reducing this error. It
gives significantly improved performance.

Circuit 7

This is another way of reducing the error but this circuit
is not quite so commonly known. If you calculate the formulae using practical
transistors (beta = 500 or so) both 5 and 6 give very close matching - but 6 is
slightly better. An example of
this.

However there is a problem which sometimes occurs on current mirrors: if the
output current is flowing from a significantly high voltage (Vc), then there may
be enough power (Vc x i²) in the output transistor of figure 5 to cause the
transistor to get warm: heating destroys the balance between the two transistors
and so destroys the mirror's accuracy. In circuit 6 any heating is in the third
transistor (at the top) and it is not part of the mirror so the mirror ratio is
not affected by heating in this transistor. See 'Useful facts' below.

Circuit 8

Just to prove that I can use ICs, here is a current source
(or a voltage to current converter) using an IC. You can use a single transistor
(the PNP one) but the transistor's base current slightly alters the ratio. You
can of course use an FET (zero gate current) but these are relatively expensive.
Here is a complimentary long tailed pair biased to cancel out base current
errors. Very good - provided you have a suitable bias point for Vb!

Circuit 9

One of the problems with circuits 2,3,4,5 and 6 is heating
in the output transistor - a problem which was solved in circuit 7. Here's
another way of avoiding heating causing an error - this circuit uses a cascode
arrangement so that the current is defined in the bottom transistor, which has
very little voltage across it. The top transistor has any high voltage across
it, but heating in the top transistor doesn't affect the current, as it's the
bottom transistor that is defining it.

Useful facts

There are two very useful figures that I remember about silicon diodes are to
do with the change in voltage/current. If you increase the voltage across a
diode by 60 millivolts, then the current through the diode increase by a factor
of 10 - or vice versa. Try measuring the diode's drop when it has 100 microamps
and 1 milliamp flowing: you will find the 60mV figure quite accurate.

The second figure is to do with the change in diode drop with temperature:
increase the temperature by 1 degree K and the diode's voltage drops by 2
millivolts.

These figures hold for diodes and transistor base-emitter junctions.

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Page's Author: Richard Torrens

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