BJT Configurations 2
Three lesser known BJT configurations are the differential amplifier, current mirror and
the darlington amplifier. These circuits all require two transistors, which should be the
same type and have closely matched parameters for best performance,. For clarity, all
circuits are drawn without bias circuits. Note that the same circuits can also be made with
FET's and MOSFET's. If PNP transistors are used, power supply polarity should be reversed.
Differential Amplifier
The differential amplifier uses two matched transistors that share a common emitter connection.
The emitter connection is often connected via a single resistor but for best performance should
be fed via a constant current source as shown in the diagram. This circuit is also known as
a "long tailed pair". The circuit employs split supplies and input can be between either transistor
base and earth or applied simultaneously to both bases. Used in this manner the input signal will
be greatly amplified and appear at the collectors. If a balanced input signal is applied,
from say long cables, then any interference will be common on each wire. A signal with same
polarity applied to both differential inputs will not be amplified and this property is known
as "common mode rejection". The output can be taken from both collectors (balanced) or between
either collector and earth (unbalanced or single ended). Examples include DC amplifiers,
instrumentation amplifiers and audio circuits,
example
circuit.
Current Mirror
Sometimes also known as a programmable current mirror this unusual configuration uses two
closely matched transistors with their bases in parallel. The base current is supplied via
R1 and both transistors will have identical base currents. Because the transistors are matched,
and have the same forward current gain, then collector current I
2 will be the same
as I
1.
Darlington Amplifier
The darlington amplifier is a useful circuit and has the advantage of providing a very high
current gain, high input impedance and higher output power. (It is not necessary to use matched
transistors here) and often you see a smaller signal transistor driving a larger power transistor.
The current gain is approximately the product of both Q1 and Q2 forward current gains. One point
to note is that as Q1 emitter is connected to Q2 base the bias voltage required is V
be1
+ V
be2. This must be taken into account when designing bias circuits for the
darlington amplifier.