In this configuration the base of the transistor 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:
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:
and the small output impedance, so it can drive low-resistance loads:
Practically seeing, the emitter resistor could be significantly larger and can therefore be ignored in the above formula, which finally gives us the relationship:
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.
Example Application Circuits for BJT Common collector
Some of the classic examples of emitter follower or common collector transistor application circuits may be seen in the following examples