The Emitter Follower

1. PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
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
ܸா ൌ ܸ஻ െ 0.7ܸ
While, the current output of transistor (‫ܫ‬ா ) is:
ܸா
‫ܫ‬ா ൌ
ܴா
In the computation, the emitter follower can be described as following.
Figure 2. Real circuit of the emitter follower
This circuit involve ‫ݎ‬௘ as the internal resistance of emitter. Furthermore, ‫ݎ‬௘ and ܴா form a parallel
resistance. Furthermore, this circuit can be simplified to be following circuit.
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2. PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
Figure 3. The equivalent circuit of the follower emitter
In this circuit, the input impedance is:
ܴ௜௕ ൌ ሺߚ ൅ 1ሻሺ‫ݎ‬௘ ||ܴா ሻ
For BC547, ߚ is 200, ܴா is 10kΩ, while ‫ݎ‬௘ is:
25Ω
‫ݎ‬௘ ൌ
‫ܫ‬ா
Where ܸா is (ܸ஼஼ is 5 volt)
ܸ஼஼ ൌ ܸ஻ா ൅ ܸா
5‫ ݒ‬ൌ 0.7‫ ݒ‬൅ ܸா
ܸா ൌ 5‫ ݒ‬െ 0.7‫ݒ‬
ܸா ൌ 4.3‫ݒ‬
Then ‫ݎ‬௘ is:
25Ω
‫ݎ‬௘ ൌ
4.3‫ݒ‬
‫ݎ‬௘ ൌ 5.81 Ω
Then, the input impedance is:
ܴ௜௕ ൌ ሺߚ ൅ 1ሻሺ‫ݎ‬௘ ||ܴா ሻ
ܴ௜௕ ൌ ሺ200 ൅ 1ሻሺ5.81||10݇Ωሻ
1 1
ܴ௜௕ ൌ ሺ201ሻ ൬ ൅ ൰
5.81 10,000Ω
10,000 5.81
ܴ௜௕ ൌ ሺ201ሻ ൬ ൅ ൰
58,100Ω 58,100Ω
10,005.81
ܴ௜௕ ൌ ሺ201ሻ ൬ ൰
58,100Ω
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3. PT. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
58,100Ω
ܴ௜௕ ൌ ሺ201ሻ ൬ ൰
10,005.81
ܴ௜௕ ൌ ሺ201ሻሺ5.80Ωሻ
ܴ௜௕ ൌ 1,17݇Ω
2. Simulation
We try with the following data:
ܴா = 10kΩ
ܸ = 5v
௖௖
The, the result:
Figure 4. Simulation of the emitter follower
ܸா ൌ 4.35‫ݒ‬
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.
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