PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]

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The Emitter Follower

Umar Sidik.BEng.MSc*

Director of Engineering

PT. Electronusa Mechanical System

*umar.sidik@engineer.com

1. Introduction

Figure 1. The emitter follower

0.7

While, the current output of transistor () is:

In the computation, the emitter follower can be described as following.

Figure 2. Real circuit of the emitter follower

This circuit involve as the internal resistance of emitter. Furthermore, and form a parallel

resistance. Furthermore, this circuit can be simplified to be following circuit.

PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]

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Figure 3. The equivalent circuit of the follower emitter

In this circuit, the input impedance is:

1||

For BC547, is 200, is 10kΩ, while is:

25Ω

Where is ( is 5 volt)

5 0.7

5 0.7

4.3

Then is:

25Ω4.3

5.81 Ω

Then, the input impedance is:

1||

200 15.81||10Ω

201 15.81 1

10,000Ω!

201 10,00058,100Ω 5.81

58,100Ω!

201 10,005.8158,100Ω !

PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]

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201 58,100Ω10,005.81!

2015.80Ω

1,17Ω

2. Simulation

We try with the following data:

= 10kΩ

"" = 5v

The, the result:

Figure 4. Simulation of the emitter follower

4.35

3. Result and Discussion

In the analytical work, is 4.3v due to is 0.7v, while in the simulation is 4.35v due to is

0.65v.