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.
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 biasing analysis is otherwise similar for one transistor. In practice, these two transistors are placed in a single transistor housing and the three terminals are taken out of the housing as shown in the following figure.
This three terminal device can be called as Darling ton transistor. The darling ton transistor acts like a single transistor that has high current gain and high input impedance.
Since the characteristics of the Darling ton amplifier are basically the same as those of the emitter follower, the two circuits are used for similar applications. Till now we have discussed amplifiers based on positive feedback. The negative feedback in transistor circuits is helpful in the working of oscillators. The topic of oscillators is entirely covered in Oscillators tutorial. Previous Page.
Next Page. The emitter follower is widely used as a buffer amplifier to reduce the loading on the previous stage and provide a lower impedance output for any following circuits. The electronic circuit design for the stage is also very straightforward and easy to accomplish. Transistor common collector circuit configuration Looking at the circuit it can be seen that although the emitter voltage follows that of the base, in DC terms it is actually less than that of the base by a voltage equal to the PN junction drop between the base and emitter.
Emitter follower transistor amplifier characteristics summary The table below gives a summary of the major characteristics of the common collector, emitter follower transistor amplifier. Emitter follower input resistance DC coupled emitter follower, common collector circuit The simplest way of connecting an emitter follower is to directly couple the input as shown below.
Directly coupled emitter follower circuit Choose transistor: As with other forms of transistor circuit, the transistor should be chosen to meet the anticipated requirements.
Emitter resistor value: The voltage on the emitter is easy to define. It is simply that appearing at the previous stage. Say for example this is half the rail voltage, then the voltage on the emitter Q1 will be 0. Simply calculate the value of the resistor for the current required. AC coupled emitter follower, common collector circuit It is not always possible to directly couple the emitter follower, common collector buffer.
AC coupled emitter follower circuit The emitter follower can be designed and electronic component values determined using the design flow below as a basis: Choose transistor: As before, the transistor type should be chosen according to the anticipated performance requirements. Select emitter resistor: Choosing an emitter voltage of about half the supply voltage to give the most even range before the onset of any clipping, determine the current required from the impedance of the following stage.
Determine the base voltage: The base voltage is simply the emitter voltage plus the base emitter junction voltage - this is 0.
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