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Emitter-Follower Transistor Application Circuits

In this post we learn how to use a transistor emitter
follower configuration in practical electronic circuits, we study this
through a few different example application circuits.
An emitter follower is one of the standard transistor configurations
which is also referred to as common collector transistor configuration.
Let’s try to first understand what’s an emitter follower transistor and why it’s called a common collector transistor circuit.

What’s an Emitter Follower Transistor

As the name implies, in this type of transistor circuit the emitter
seems to be following something, to be precise the emitter voltage
follows the base voltage of the transistor which ultimately decides the
conduction pattren of the transistor.

We know that normally when the emitter of a transistor (BJT) is
connected to the ground rail or the zero supply rail, the base typically
requires around 0.6V to enable complete triggering of the device across
its collector to emitter. This operational mode of the transistor is
called the common emitter mode, and the 0.6V value is termed as forward
voltage value of the BJT. In this most popular form of configuration the
load is always found to be at the collector of the device.

This also means that as long as the base voltage of the BJT is 0.6V
higher than its emitter voltage, the device becomes forward biased or
gets triggered into conduction, or gets optimally saturated.

Now, in an emitter follower transistor configuration as shown below,
the load is connected at the emitter side of the transistor, that is
between the emitter and the ground rail.

Image result for homemade-circuits.com emitter follower transistor use
When this happens the emitter is not able to acquire a 0V potential, and the BJT is unable to turn ON with a regular 0.6V.
Suppose a 0.6V is applied to its base, due to the emitter load, the
transistor only just begins conducting which is not enough to trigger
the load.
As the base voltage is increased from 0.6V to 1.2V, the emitter
begins to conduct and allows a 0.6V to reach its emitter, now suppose
the base voltage is further increased to 2V….this prompts the emitter
voltage to reach around 1.6V.
From the above scenario we find that the emitter of the tramsistor is
always 0.6V behind the base voltage and this gives an impression that
the emitter is following the base, and hence the name.
The main features of an emitter follower transistor configuration can be studied as explained below:

  1. The emitter voltage is always around 0.6V lower than the base voltage.
  2. The emitter voltage can be varied by varying the base voltage accordingly.
  3. The emitter current is equivalent to the collector current. This
    makes the configuration rich in current if the collector is directly
    connected with the supply (+) rail.
  4. The load being attached between the emitter and the ground, the base
    is attributed with a high impedance feature, meaning the base being not
    vulnerable of getting connected to the ground rail through the emitter,
    does not require high resistance to safeguard itself, and is normally
    protected from high current.

How to use an Emitter Follower Transistor in a Circuit (Application Circuits)

An emitter follower configuration gives you the advantage of getting
an output that becomes controllable at the base of the transistor. And
therefore this can be implemented in various circuit applications
demanding a customized voltage controlled design.
The following few example circuits show how typically an emitter follower circuit can be used in circuits:
Simple Variable Power Supply:
The following simple high variable power supply exploits the emitter follower characteristic and successfully implements a neat 100V, 100 amp variable power supply which can be built and used by any new hobbyist quickly as a handy little bench power supply unit.

Adjustable Zener Diode:

Normally a zener diode comes with a fixed value which cannot be changed or altered as per a given circuit application need.
The following diagram which is actually a simple cell phone charger circuit is
designed using an emitter follower circuit configuration. Here, simply
by changing the indicated base zener diode with a 10K pot, the design
can be transformed into an effective adjustable zener diode circuit,
another cool emitter follower application circuit.

Hi Fi Power Amplifier:

Even wondered how amplifiers are able to replicate a sample music
into an amplified version without disturbing the waveform or the content
of the music signal? That becomes possible due to the many emitter
follower stages involved within an amplifier circuit.

Here’s a simple 100 watt amplifier circuit
where the output power devices can be seen configured in an source
follower design which is an mosfet equivalent of a BJT emitter follower.

There can be possibly many more such emitter follower application
circuits, I have just named the ones which were easily accessible to me
from this website, if you have more info on this, please feel free to
share through your valuable comments.

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