As the base voltage is increased from 0. From the above scenario we find that the emitter of the tramsistor is always 0. The main features of an emitter follower transistor configuration can be studied as explained below:. In contrast to the collector voltage response, the emitter voltage is in phase with the input base signal Vi. Meaning both input and the output signals tend to replicate their positive and negative peak levels, simultaneously.
As understood earlier, the output Vo appears to be "following" the input signals levels Vi, through an in-phase relationship, and this represents its name emitter follower. The emitter-follower configuration is mainly used for impedance-matching applications, due to its high impedance characteristics at the input and a low impedance at the output.
This appears to be the direct opposite of the classic fixed-bias configuration. The outcome of the circuit is quite similar to as that acquired from a transformer, in which the load is matched to the source impedance for achieving highest levels of power transfer through the network. Zo : The output impedance can be best defined by first evaluating the equation for the current Ib :. Now, if we build a network using the above derived equation, presents us with the following configuration:.
Therefore, the output impedance could be determined by setting the input voltage Vi to zero and. Since, RE is normally much bigger than re , the following approximation is mostly taken into account:.
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:. The following simple high variable power supply exploits the emitter follower characteristic and successfully implements a neat V, amp variable power supply which can be built and used by any new hobbyist quickly as a handy little bench power supply unit.
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. The following figure shows the circuit diagram of an Emitter Follower. The input signal voltage applied between base and emitter, develops an output voltage V o across R E , which is in the emitter section. As the output voltage developed across R L is proportional to the emitter current, this emitter follower circuit is a current feedback circuit.
As the Emitter Follower circuit is a prominent one, let us try to get the equation for the voltage gain of an emitter follower circuit. If an AC equivalent circuit of the above circuit is drawn, it would look like the below one, as the emitter by pass capacitor is absent.
In order to find the voltage gain of the amplifier, the above figure can be replaced by the following figure. Note that input voltage is applied across the ac resistance of the emitter circuit i. Assuming the emitter diode to be ideal, the output voltage V out will be.
In practice, the voltage gain of an emitter follower is between 0. The emitter follower circuit which was just discussed lacks to meet the requirements of the circuit current gain A i and the input impedance Z i. In order to achieve some increase in the overall values of circuit current gain and input impedance, two transistors are connected as shown in the following circuit diagram, which is known as Darlington configuration.
As shown in the above figure, the emitter of the first transistor is connected to the base of the second transistor. The collector terminals of both the transistors are connected together. The following information is adapted from the lab materials of an electronics course at Pomona College.
Most signals and voltage sources are imperfect. Generally, when we try to draw current from a voltage source, the voltage decreases. This decrease in voltage is called sag. Sag can cause significant problems in multi-stage circuits where later stages depend on receiving a stable voltage. While there are many reasons why signals and voltage sources sag, we can usually explain sag with a simple model involving a perfect voltage source and a resistor.
This model is shown in the circuit diagram above. We call the resistor in the model the Thevenin Resistance Rth. The bigger the Thevenin Resistance, the more a voltage source sags as current is drawn. The Emitter-Follower circuit will reduce the Thevenin Resistance of a voltage supply or signal by a factor of Be sure to check the documentation for your transistor as the order of pins can vary. Use the 3. Attach your sagging signal to the base of the transistor. Attach the load or the next stage your circuit to the emitter of the transistor.
When appropriate voltages are applied to the base and collector of an NPN transistor, the transistor adjusts its internal current flow until it meets the following conditions:. The first rule explains why the output signal of an Emitter-Follower follows the input.
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