Current mirrors
An interesting and often-used circuit
applying the bipolar junction transistor is the so-called current
mirror, which serves as a simple current regulator, supplying nearly
constant current to a load over a wide range of load resistances.
We know that in a transistor operating in
its active mode, collector current is equal to base current multiplied
by the ratio β. We also know that the ratio between collector current
and emitter current is called α. Because collector current is equal to
base current multiplied by β, and emitter current is the sum of the base
and collector currents, α should be mathematically derivable from β. If
you do the algebra, you'll find that α = β/(β+1) for any transistor.
We've seen already how maintaining a
constant base current through an active transistor results in the
regulation of collector current, according to the β ratio. Well, the α
ratio works similarly: if emitter current is held constant, collector
current will remain at a stable, regulated value so long as the
transistor has enough collector-to-emitter voltage drop to maintain it
in its active mode. Therefore, if we have a way of holding emitter
current constant through a transistor, the transistor will work to
regulate collector current at a constant value.
Remember that the base-emitter junction
of a BJT is nothing more than a PN junction, just like a diode, and that
the "diode equation" specifies how much current will go through a PN
junction given forward voltage drop and junction temperature:
If both junction voltage and temperature
are held constant, then the PN junction current will likewise be
constant. Following this rationale, if we were to hold the base-emitter
voltage of a transistor constant, then its emitter current should
likewise be constant, given a constant temperature:
This constant emitter current, multiplied
by a constant α ratio, gives a constant collector current through Rload,
provided that there is enough battery voltage to keep the transistor in
its active mode for any change in Rload's resistance.
Maintaining a constant voltage across the
transistor's base-emitter junction is easy: use a forward-biased diode
to establish a constant voltage of approximately 0.7 volts, and connect
it in parallel with the base-emitter junction:
Now, here's where it gets interesting.
The voltage dropped across the diode probably won't be 0.7 volts
exactly. The exact amount of forward voltage dropped across it depends
on the current through the diode, and the diode's temperature, all in
accordance with the diode equation. If diode current is increased (say,
by reducing the resistance of Rbias), its voltage drop will
increase slightly, increasing the voltage drop across the transistor's
base-emitter junction, which will increase the emitter current by the
same proportion, assuming the diode's PN junction and the transistor's
base-emitter junction are well-matched to each other. In other words,
transistor emitter current will closely equal diode current at any given
time. If you change the diode current by changing the resistance value
of Rbias, then the transistor's emitter current will follow
suit, because the emitter current is described by the same equation as
the diode's, and both PN junctions experience the same voltage drop.
Remember, the transistor's collector
current is almost equal to its emitter current, as the α ratio of a
typical transistor is almost unity (1). If we have control over the
transistor's emitter current by setting diode current with a simple
resistor adjustment, then we likewise have control over the transistor's
collector current. In other words, collector current mimics, or
mirrors, diode current.
Current through resistor Rload
is therefore a function of current set by the bias resistor, the two
being nearly equal. This is the function of the current mirror circuit:
to regulate current through the load resistor by conveniently adjusting
the value of Rbias. It is very easy to create a set amount of
diode current, as current through the diode is described by a simple
equation: power supply voltage minus diode voltage (almost a constant
value), divided by the resistance of Rbias.
To better match the characteristics of
the two PN junctions (the diode junction and the transistor base-emitter
junction), a transistor may be used in place of a regular diode, like
this:
Because temperature is a factor in the
"diode equation," and we want the two PN junctions to behave identically
under all operating conditions, we should maintain the two transistors
at exactly the same temperature. This is easily done using discrete
components by gluing the two transistor cases back-to-back. If the
transistors are manufactured together on a single chip of silicon (as a
so-called integrated circuit, or IC), the designers should
locate the two transistors very close to one another to facilitate heat
transfer between them.
The current mirror circuit shown with two
NPN transistors is sometimes called a current-sinking type,
because the regulating transistor conducts current to the load from
ground ("sinking" current), rather than from the positive side of
the battery ("sourcing" current). If we wish to have a grounded
load, and a current sourcing mirror circuit, we could use PNP
transistors like this:
- REVIEW:
- A current mirror is a
transistor circuit that regulates current through a load resistance,
the regulation point being set by a simple resistor adjustment.
- Transistors in a current mirror
circuit must be maintained at the same temperature for precise
operation. When using discrete transistors, you may glue their cases
together to help accomplish this.
- Current mirror circuits may be found
in two basic varieties: the current sinking configuration,
where the regulating transistor connects the load to ground; and the
current sourcing configuration, where the regulating transistor
connects the load to the positive terminal of the DC power supply.
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