The common-collector amplifier
Our next transistor configuration to
study is a bit simpler in terms of gain calculations. Called the
common-collector configuration, its schematic diagram looks like
this:
It is called the common-collector
configuration because (ignoring the power supply battery) both the
signal source and the load share the collector lead as a common
connection point:
It should be apparent that the load
resistor in the common-collector amplifier circuit receives both the
base and collector currents, being placed in series with the emitter.
Since the emitter lead of a transistor is the one handling the most
current (the sum of base and collector currents, since base and
collector currents always mesh together to form the emitter current), it
would be reasonable to presume that this amplifier will have a very
large current gain (maximum output current for minimum input current).
This presumption is indeed correct: the current gain for a
common-collector amplifier is quite large, larger than any other
transistor amplifier configuration. However, this is not necessarily
what sets it apart from other amplifier designs.
Let's proceed immediately to a SPICE
analysis of this amplifier circuit, and you will be able to immediately
see what is unique about this amplifier:
common-collector amplifier
vin 1 0
q1 2 1 3 mod1
v1 2 0 dc 15
rload 3 0 5k
.model mod1 npn
.dc vin 0 5 0.2
.plot dc v(3,0)
.end
type npn
is 1.00E-16
bf 100.000
nf 1.000
br 1.000
nr 1.000
vin v(3) 0.000E+00 2.000E+00 4.000E+00 6.000E+00
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
0.000E+00 7.500E-08 * . . .
2.000E-01 7.501E-08 * . . .
4.000E-01 2.704E-06 * . . .
6.000E-01 4.954E-03 * . . .
8.000E-01 1.221E-01 .* . . .
1.000E+00 2.989E-01 . * . . .
1.200E+00 4.863E-01 . * . . .
1.400E+00 6.777E-01 . * . . .
1.600E+00 8.712E-01 . * . . .
1.800E+00 1.066E+00 . * . . .
2.000E+00 1.262E+00 . * . . .
2.200E+00 1.458E+00 . * . . .
2.400E+00 1.655E+00 . * . . .
2.600E+00 1.852E+00 . *. . .
2.800E+00 2.049E+00 . * . .
3.000E+00 2.247E+00 . . * . .
3.200E+00 2.445E+00 . . * . .
3.400E+00 2.643E+00 . . * . .
3.600E+00 2.841E+00 . . * . .
3.800E+00 3.039E+00 . . * . .
4.000E+00 3.237E+00 . . * . .
4.200E+00 3.436E+00 . . * . .
4.400E+00 3.634E+00 . . * . .
4.600E+00 3.833E+00 . . *. .
4.800E+00 4.032E+00 . . * .
5.000E+00 4.230E+00 . . . * .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Unlike the common-emitter amplifier from
the previous section, the common-collector produces an output voltage in
direct rather than inverse proportion to the rising input
voltage. As the input voltage increases, so does the output voltage.
More than that, a close examination reveals that the output voltage is
nearly identical to the input voltage, lagging behind only about
0.77 volts.
This is the unique quality of the
common-collector amplifier: an output voltage that is nearly equal to
the input voltage. Examined from the perspective of output voltage
change for a given amount of input voltage change, this
amplifier has a voltage gain of almost exactly unity (1), or 0 dB. This
holds true for transistors of any β value, and for load resistors of any
resistance value.
It is simple to understand why the output
voltage of a common-collector amplifier is always nearly equal to the
input voltage. Referring back to the diode-regulating diode transistor
model, we see that the base current must go through the base-emitter PN
junction, which is equivalent to a normal rectifying diode. So long as
this junction is forward-biased (the transistor conducting current in
either its active or saturated modes), it will have a voltage drop of
approximately 0.7 volts, assuming silicon construction. This 0.7 volt
drop is largely irrespective of the actual magnitude of base current, so
we can regard it as being constant:
Given the voltage polarities across the
base-emitter PN junction and the load resistor, we see that they must
add together to equal the input voltage, in accordance with Kirchhoff's
Voltage Law. In other words, the load voltage will always be about 0.7
volts less than the input voltage for all conditions where the
transistor is conducting. Cutoff occurs at input voltages below 0.7
volts, and saturation at input voltages in excess of battery (supply)
voltage plus 0.7 volts.
Because of this behavior, the
common-collector amplifier circuit is also known as the
voltage-follower or emitter-follower amplifier, in reference
to the fact that the input and load voltages follow each other so
closely.
Applying the common-collector circuit to
the amplification of AC signals requires the same input "biasing" used
in the common-emitter circuit: a DC voltage must be added to the AC
input signal to keep the transistor in its active mode during the entire
cycle. When this is done, the result is a non-inverting amplifier:
common-collector amplifier
vin 1 4 sin(0 1.5 2000 0 0)
vbias 4 0 dc 2.3
q1 2 1 3 mod1
v1 2 0 dc 15
rload 3 0 5k
.model mod1 npn
.tran .02m .78m
.plot tran v(1,0) v(3,0)
.end
legend:
*: v(1)
+: v(3)
v(1) 0.000E+00 1.000E+00 2.000E+00 3.000E+00 4.000E+00
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
2.300E+00 . . + . * . .
2.673E+00 . . +. * . .
3.020E+00 . . . + * .
3.322E+00 . . . + . * .
3.563E+00 . . . + . * .
3.723E+00 . . . +. * .
3.790E+00 . . . + * .
3.767E+00 . . . + * .
3.657E+00 . . . +. * .
3.452E+00 . . . + . * .
3.177E+00 . . . + . * .
2.850E+00 . . .+ * . .
2.488E+00 . . + . * . .
2.113E+00 . . + . * . .
1.750E+00 . + * . . .
1.419E+00 . + . * . . .
1.148E+00 . + . * . . .
9.493E-01 . + *. . . .
8.311E-01 .+ * . . . .
8.050E-01 .+ * . . . .
8.797E-01 . + * . . . .
1.039E+00 . + .* . . .
1.275E+00 . + . * . . .
1.579E+00 . + . * . . .
1.929E+00 . . + *. . .
2.300E+00 . . + . * . .
2.673E+00 . . +. * . .
3.019E+00 . . . + * .
3.322E+00 . . . + . * .
3.564E+00 . . . + . * .
3.722E+00 . . . +. * .
3.790E+00 . . . + * .
3.768E+00 . . . + * .
3.657E+00 . . . +. * .
3.451E+00 . . . + . * .
3.178E+00 . . . + . * .
2.851E+00 . . .+ * . .
2.488E+00 . . + . * . .
2.113E+00 . . + . * . .
1.748E+00 . + * . . .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Here's another view of the circuit, this
time with oscilloscopes connected to several points of interest:
Since this amplifier configuration
doesn't provide any voltage gain (in fact, in practice it actually has a
voltage gain of slightly less than 1), its only amplifying factor
is current. The common-emitter amplifier configuration examined in the
previous section had a current gain equal to the β of the transistor,
being that the input current went through the base and the output (load)
current went through the collector, and β by definition is the ratio
between the collector and emitter currents. In the common-collector
configuration, though, the load is situated in series with the emitter,
and thus its current is equal to the emitter current. With the emitter
carrying collector current and base current, the load in this
type of amplifier has all the current of the collector running through
it plus the input current of the base. This yields a current gain
of β plus 1:
Once again, PNP transistors are just as
valid to use in the common-collector configuration as NPN transistors.
The gain calculations are all the same, as is the non-inverting behavior
of the amplifier. The only difference is in voltage polarities and
current directions:
A popular application of the
common-collector amplifier is for regulated DC power supplies, where an
unregulated (varying) source of DC voltage is clipped at a specified
level to supply regulated (steady) voltage to a load. Of course, zener
diodes already provide this function of voltage regulation:
However, when used in this direct
fashion, the amount of current that may be supplied to the load is
usually quite limited. In essence, this circuit regulates voltage across
the load by keeping current through the series resistor at a high enough
level to drop all the excess power source voltage across it, the zener
diode drawing more or less current as necessary to keep the voltage
across itself steady. For high-current loads, an plain zener diode
voltage regulator would have to be capable of shunting a lot of current
through the diode in order to be effective at regulating load voltage in
the event of large load resistance or voltage source changes.
One popular way to increase the
current-handling ability of a regulator circuit like this is to use a
common-collector transistor to amplify current to the load, so that the
zener diode circuit only has to handle the amount of current necessary
to drive the base of the transistor:
There's really only one caveat to this
approach: the load voltage will be approximately 0.7 volts less than the
zener diode voltage, due to the transistor's 0.7 volt base-emitter drop.
However, since this 0.7 volt difference is fairly constant over a wide
range of load currents, a zener diode with a 0.7 volt higher rating can
be chosen for the application.
Sometimes the high current gain of a
single-transistor, common-collector configuration isn't enough for a
particular application. If this is the case, multiple transistors may be
staged together in a popular configuration known as a Darlington pair,
just an extension of the common-collector concept:
Darlington pairs essentially place one
transistor as the common-collector load for another transistor, thus
multiplying their individual current gains. Base current through the
upper-left transistor is amplified through that transistor's emitter,
which is directly connected to the base of the lower-right transistor,
where the current is again amplified. The overall current gain is as
follows:
Voltage gain is still nearly equal to 1
if the entire assembly is connected to a load in common-collector
fashion, although the load voltage will be a full 1.4 volts less than
the input voltage:
Darlington pairs may be purchased as
discrete units (two transistors in the same package), or may be built up
from a pair of individual transistors. Of course, if even more current
gain is desired than what may be obtained with a pair, Darlington
triplet or quadruplet assemblies may be constructed.
- REVIEW:
- Common-collector
transistor amplifiers are so-called because the input and output
voltage points share the collector lead of the transistor in common
with each other, not considering any power supplies.
- The output voltage on a
common-collector amplifier will be in phase with the input voltage,
making the common-collector a non-inverting amplifier circuit.
- The current gain of a common-collector
amplifier is equal to β plus 1. The voltage gain is approximately
equal to 1 (in practice, just a little bit less).
- A Darlington pair is a pair of
transistors "piggybacked" on one another so that the emitter of one
feeds current to the base of the other in common-collector form. The
result is an overall current gain equal to the product
(multiplication) of their individual common-collector current gains (β
plus 1).
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