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Emitter-Follower Transistor - Application Circuits and Working Concept

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. Transistor emitter follower also called transistor common collector, or BJT common collector 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.

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 the BJT pinouts Connected

In this configuration the base of the BJT is wired for receiving the input trigger supply, the emitter lead is connected as the output, and the collector is hooked up with the positive supply, such that the collector becomes a common terminal across the base trigger supply Vbb and the actual Vdd positive supply.

This common connection gives it the name as common collector.

The common collector BJT configuration is also called the emitter follower circuit due to the simple reason that the emitter voltage follows the base voltage with reference to the ground, meaning the emitter lead initiates a voltage only when the base voltage is able to cross the 0.6V mark.

Therefore, if for example the base voltage is 6V, then the emitter voltage will be 5.4V, because the emitter has to provide a 0.6V drop or leverage to the base voltage for enabling the transistor to conduct, and hence the name emitter follower.

In simple terms, the emitter voltage will be always less by a factor of around 0.6V than the base voltage because unless this biasing drop is maintained the transistor will never conduct. Which  in turn means no voltage can appear at the emitter terminal, therefore the emitter voltage constantly follows the base voltage adjusting itself by a difference of around -0.6V.

How a BJT Common Collector or an Emitter Follower Works

Let's assume we apply 0.6V at the base of a BJT in a common collector circuit. This will produce zero voltage at the emitter, because the transistor is just not fully in the conducting state.

Now suppose this voltage is slowly increase to 1V, this may allow the emitter lead to produce a voltage that may be around 0.4V, similarly as this base voltage is increased to 1.6V will make the emitter to follow up to around 1V....this shows how the emitter keeps following the base with a difference of around 0.6V, which is the typical or the optimal biasing level of any BJT.

A common collector transistor circuit will exhibit a unity voltage Gain, which means the voltage gain for this config is not too impressive, rather just on par with the input.

Mathematically the above may be expressed as:

 {A_mathrm{v}} = {v_mathrm{out} over v_mathrm{in}} approx 1

PNP version of the emitter follower circuit, all polarities are reversed.

Even the smallest of the voltage deviations at the base of a common collector transistor is duplicated across the emitter lead, which to an extent is dependent on the gain (Hfe) of the transistor and the resistance of the load attached).

The main benefit of this circuit is its high input impedance feature, which allows the circuit to perform efficiently regardless of the input current or the load resistance, meaning even huge loads can be efficiently operated with inputs having minimal current.

That's why a common collector is used as a buffer, meaning a stage which efficiently integrates high load operations from a relatively weak current source (example a TTL or Arduino source)

The high input impedance is expressed with the formula:

 r_mathrm{in} approx beta_0 R_mathrm{E}

and the small output impedance, so it can drive low-resistance loads:

 r_mathrm{out} approx {R_mathrm{E}} | {R_mathrm{source} over beta_0}

Practically seeing, the emitter resistor could be significantly larger and can therefore be ignored in the above formula, which finally gives us the relationship:

 r_mathrm{out} approx {R_mathrm{source} over beta_0}

What about Current

The current gain for a common collector transistor configuration high, because the collector being directly connected with the positive line is able to pass the full required amount of current to the attached load via the emitter lead.

Therefore if you are wondering how much current an emitter follower would be able to provide to the load, rest assured that won't be an issue as the load would be always driven with an optimal current from this configuration.

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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