Common collector

In electronics, a common collector amplifier (also known as an emitter

follower) is one of three basic single-stage bipolar junction transistor (BJT)
amplifier topologies, typically used as a voltage buffer.

In this circuit, the base terminal of the transistor serves as the input, the emitter
is the output, and the collector is common to both (for example, it may be tied to
ground reference or a power supply rail), hence its name. The analogous field-
effect transistor circuit is the common drain amplifier and the analogous tube
Figure 1: Basic NPN
circuit is the cathode follower.
common collector
circuit (neglecting
biasing details).
Basic circuit
The circuit can be explained by viewing the transistor as
being under the control of negative feedback. From this
viewpoint, a common-collector stage (Fig. 1) is an amplifier
with full series negative feedback. In this configuration (Fig.
2 with β = 1), the entire output voltage Vout is placed contrary
and in series with the input voltage Vin. Thus the two voltages
are subtracted according to Kirchhoff's voltage law (KVL) Figure 2: A negative-feedback amplifier
(the subtractor from the function block diagram is
implemented just by the input loop), and their difference Vdiff
= Vin − Vout is applied to the base–emitter junction. The transistor continuously monitors Vdiff and adjusts
its emitter voltage to equal Vin minus the mostly constant VBE (approximately one diode forward voltage
drop) by passing the collector current through the emitter resistor RE. As a result, the output voltage
follows the input voltage variations from VBE up to V+; hence the name "emitter follower".

Intuitively, this behavior can be also understood by realizing that VBE is very insensitive to bias changes,
so any change in base voltage is transmitted (to good approximation) directly to the emitter. It depends
slightly on various disturbances (transistor tolerances, temperature variations, load resistance, a collector
resistor if it is added, etc.), since the transistor reacts to these disturbances and restores the equilibrium. It
never saturates even if the input voltage reaches the positive rail.

The common-collector circuit can be shown mathematically to have a voltage gain of almost unity:

A small voltage change on the input terminal will be replicated at the output (depending slightly on the
transistor's gain and the value of the load resistance; see gain formula below). This circuit is useful
because it has a large input impedance
so it will not load down the previous circuit, and a small output impedance

so it can drive low-resistance loads.

Typically, the emitter resistor is significantly larger and can be removed from the
equation:

Figure 3: PNP
version of the
emitter-follower
circuit, all polarities

Applications are reversed.

The common collector amplifier's low output

impedance allows a source with a large output
impedance to drive a small load impedance without
changing its voltage. Thus this circuit finds
applications as a voltage buffer. In other words, the
circuit has current gain (which depends largely on the
hFE of the transistor) instead of voltage gain. A small
change to the input current results in much larger
change in the output current supplied to the output
load.

One aspect of buffer action is transformation of

impedances. For example, the Thévenin resistance of Figure 4: NPN voltage follower with current source
biasing suitable for integrated circuits
a combination of a voltage follower driven by a
voltage source with high Thévenin resistance is
reduced to only the output resistance of the voltage follower (a small resistance). That resistance
reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted
between a small load resistance and a driving stage presents a large load to the driving stage—an
advantage in coupling a voltage signal to a small load.

This configuration is commonly used in the output stages of class-B and class-AB amplifiers. The base
circuit is modified to operate the transistor in class-B or AB mode. In class-A mode, sometimes an active
current source is used instead of RE (Fig. 4) to improve linearity and/or efficiency.[1]

Characteristics
At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can
be derived. (Parameter and the parallel lines indicate components in parallel.)
 Approximate
Definition Expression Conditions
expression

Current gain

Voltage gain

Input resistance

Output
resistance

Where is the Thévenin equivalent source resistance.

Derivations
Figure 5 shows a low-frequency hybrid-pi model for
the circuit of Figure 3. Using Ohm's law, various
currents have been determined, and these results are
shown on the diagram. Applying Kirchhoff's current
law at the emitter one finds:

Define the following resistance values:

Figure 5: Small-signal circuit corresponding to

Figure 3 using the hybrid-pi model for the bipolar
transistor at frequencies low enough to ignore
bipolar device capacitances
Then collecting terms the voltage gain is found:

From this result, the gain approaches unity (as expected for a buffer amplifier) if the resistance ratio in
the denominator is small. This ratio decreases with larger values of current gain β and with larger values
of . The input resistance is found as
The transistor output resistance ordinarily is large
compared to the load , and therefore
dominates . From this result, the input resistance
of the amplifier is much larger than the output load
resistance for large current gain . That is,
placing the amplifier between the load and the source
presents a larger (high-resistive) load to the source
than direct coupling to , which results in less
signal attenuation in the source impedance as a
consequence of voltage division.

Figure 6 shows the small-signal circuit of Figure 5

with the input short-circuited and a test current placed
at its output. The output resistance is found using this Figure 6: Low-frequency small-signal circuit for
circuit as bipolar voltage follower with test current at output
for finding output resistance. Resistor
.

Using Ohm's law, various currents have been found, as indicated on the diagram. Collecting the terms for
the base current, the base current is found as

where is defined above. Using this value for base current, Ohm's law provides

Substituting for the base current, and collecting terms,

where || denotes a parallel connection, and is defined above. Because generally is a small resistance
when the current gain is large, dominates the output impedance, which therefore also is small. A
small output impedance means that the series combination of the original voltage source and the voltage
follower presents a Thévenin voltage source with a lower Thévenin resistance at its output node; that is,
the combination of voltage source with voltage follower makes a more ideal voltage source than the
original one.

See also

Electronics portal

Common base
Common emitter
Common gate
 Common drain
Common source
IC power-supply pin
Open collector
Two-port network

References
1. Rod Elliot: 20 Watt Class-A Power Amplifier (https://sound-au.com/project10.htm)

External links
R Victor Jones: Basic BJT Amplifier Configurations (https://web.archive.org/web/200709092
22435/http://people.seas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.ht
ml)
NPN Common Collector Amplifier (http://230nsc1.phy-astr.gsu.edu/hbase/electronic/npncc.h
tml) — HyperPhysics
Theodore Pavlic: ECE 327: Transistor Basics; part 6: npn Emitter Follower (http://www.tedpa
vlic.com/teaching/osu/ece327/lab1_bjt/lab1_bjt_transistor_basics.pdf)
Doug Gingrich: The common collector amplifier U of Alberta (https://web.archive.org/web/20
050405185240/http://www.phys.ualberta.ca/~gingrich/phys395/notes/node86.html)
Raymond Frey: Lab exercises U of Oregon (https://web.archive.org/web/20060919004917/h
ttp://zebu.uoregon.edu/~rayfrey/431/lab3_431.pdf)

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