The document analyzes the influence of capacitance in an emitter follower circuit. It presents analytical calculations for both DC and AC analyses. For the DC analysis, it calculates the Thevenin voltage, emitter voltage, and emitter current. For the AC analysis, it calculates various resistances, currents, and output voltage. Simulation results are presented and show differences ranging from 18-98% when compared to the analytical calculations. Increasing the capacitance decreases the frequency-dependent effects in the circuit.

1. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
Influence of Capacitance in The Emitter Follower (Part II)
Umar Sidik.BEng.MSc*
Director of Engineering
Electronusa Mechanical System
*umar.sidik@engineer.com
1. Introduction
Commonly, capacitor is used to pass the ac signal in the circuit. Furthermore, the characteristics of ac
signal in the circuit are influenced by capacitor. Theoretically, the capacitance of capacitor will
influence the time of charge and discharge, where the bigger capacitance requires longer time in
charge and discharge. In addition, this study uses the metalized ceramic plate capacitor for analysis.
2. Analytical Work
We have two analyzes consisting of:
a. Analysis of dc
b. Analysis of ac
2.1 Analysis of dc
(a) (b)
Figure 1. (a). The circuit of emitter follower
(b). The equivalent circuit in dc
First step, we have to calculate the Thevenin’s voltage in figure 1:
ܴଶ
்ܸு ൌ ൈ ܸ஼஼
ܴଵ ൅ ܴଶ
For this circuit, ܸ஼஼ is 5ܸ, then:
24݇Ω
்ܸு ൌ ൈ 5ܸ
10݇Ω ൅ 24݇Ω
24݇Ω
்ܸு ൌ ൈ 5ܸ
34݇Ω
்ܸு ൌ ሺ0.71ሻ ൈ 5ܸ
்ܸு ൌ 3.55ܸ
Actually, in this circuit ்ܸு ൌ ܸ஻ , so ܸ஻ ൌ 3.55ܸ.
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2. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
The second step, we have to calculate ܸா :
ܸா ൌ ܸ஻ െ ܸ஻ா
ܸா ൌ 3.55ܸ െ 0.7ܸ
ܸா ൌ 2.85ܸ
The third step, we have to calculate the ‫ܫ‬ா :
ܸா
‫ܫ‬ா ൌ
ܴா
2.85ܸ
‫ܫ‬ா ൌ
150Ω
‫ܫ‬ா ൌ 19݉‫ܣ‬
2.2 Analysis of ac
(a) (b)
Figure 2. (a). The circuit of emitter follower with the internal resistance of emitter
(b). The equivalent circuit in ac
The first step, we have to calculate ‫ݎ‬௘ in figure 2:
25ܸ݉
‫ݎ‬௘ ൌ
‫ܫ‬ா
25݉‫ݒ‬
‫ݎ‬௘ ൌ
19݉‫ܣ‬
‫ݎ‬௘ ൌ 1.32Ω
The second step, we have to calculate ‫ݎ‬௜௡ሺ௕௔௦௘ሻ :
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺߚ ൅ 1ሻ൫ሺܴଷ ൅ ܴସ ሻԡ‫ݎ‬௘ ൯
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ200 ൅ 1ሻ൫ሺ15݇Ω ൅ 8.2Ωሻԡ1.32Ω൯
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻ൫ሺ15,000Ω ൅ 8.2Ωሻԡ1.32Ω൯
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻሺ15,008.2Ωԡ1.32Ωሻ
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3. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
1 1
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻ ൬ ൅ ൰
15,008.2Ω 1.32Ω
1.32 15,008.2
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻ ൬ ൅ ൰
19,810.824Ω 19,810.824Ω
15,009.52
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻ ൬ ൰
19,810.824Ω
19,810.824Ω
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻ ൬ ൰
15,009.52
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ ሺ201ሻሺ1.32Ωሻ
‫ݎ‬௜௡ሺ௕௔௦௘ሻ ൌ 265.3Ω
The third step is to calculate ݅௕ :
‫ݒ‬௜௡
݅௕ ൌ
‫ݎ‬௜௡ሺ௕௔௦௘ሻ
1ܸ݉
݅௕ ൌ
265.3Ω
݅௕ ൌ 3.77݉‫ܣ‬
The fourth step is to calculate ݅௖ :
݅௖ ൌ ߚ݅௕
݅௖ ൌ ሺ200ሻሺ3.77݉‫ܣ‬ሻ
݅௖ ൌ 754݉‫ܣ‬
The last step is to calculate ‫ݒ‬௢௨௧ :
‫ݒ‬௢௨௧ ൌ ݅௖ ‫ݎ‬௢௨௧
‫ݒ‬௢௨௧ ൌ ሺ754݉‫ܣ‬ሻሺ1.32Ωሻ
‫ݒ‬௢௨௧ ൌ 995.1 ܸ݉
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4. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
3. Simulation Work
The simulation work classified into the analysis of dc and the analysis of ac.
3.1 Analysis of dc
Figure 3. ்ܸு in simulation
In the simulation ்ܸு is 3ܸ (figure 3), while in the analytical work ்ܸு is 3.55ܸ. The different of the
analytical work and the simulation work is:
்ܸுሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ்ܸுሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
்ܸுሺ௔௡௔௟௬௧௜௖௔௟ሻ
3.55ܸ െ 3ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
3.55ܸ
0.55ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
3ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 18.33%
Figure 4. ܸா in simulation
In the simulation ܸா is 2.25ܸ (figure 4), while in the analytical work ܸா is 2.85ܸ. the different of the
analytical work and the simulation work is:
ܸாሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ܸாሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
ܸாሺ௔௡௔௟௬௧௜௖௔௟ሻ
2.85ܸ െ 2.25ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
2.85ܸ
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5. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
0.6ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
2.85ܸ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 21.05%
Figure 5. ‫ܫ‬ா in simulation
In the simulation ‫ܫ‬ா is 15݉‫ ,ܣ‬while in the analytical work ‫ܫ‬ா is 19݉‫ .ܣ‬The difference is:
‫ܫ‬ாሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ‫ܫ‬ாሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
‫ܫ‬ாሺ௔௡௔௟௬௧௜௖௔௟ሻ
19݉‫ ܣ‬െ 15݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
19݉‫ܣ‬
4݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
19݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 21.05%
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6. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
3.2 Analysis of ac
(a) (b) (c)
(e) (f) (g)
Figure 6. ݅௕ in different the circuit’s capacitance
(a). ‫ ܥ‬ൌ 1݊‫ܨ‬ (b). ‫ ܥ‬ൌ 10݊‫ܨ‬ (c). ‫ ܥ‬ൌ 22݊‫ܨ‬
(d). ‫ ܥ‬ൌ 1.8‫ܨ݌‬ (e). ‫ ܥ‬ൌ 120‫ܨ݌‬ (f). ‫ ܥ‬ൌ 820‫ܨ݌‬
In the simulation, ݅௕ is 0.07݉‫ ,ܣ‬while in the analytical ݅௕ is 3.77݉‫ .ܣ‬the difference is:
݅௕ሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ݅௕ሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
݅௕ሺ௔௡௔௟௬௧௜௖௔௟ሻ
3.77݉‫ ܣ‬െ 0.07݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
3.77݉‫ܣ‬
3.70݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
3.77݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 98.14%
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7. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
(a) (b) (c)
(d) (e) (f)
Figure 7. ݅௖ in different the circuit’s capacitance
(a). ‫ ܥ‬ൌ 1݊‫ܨ‬ (b). ‫ ܥ‬ൌ 10݊‫ܨ‬ (c). ‫ ܥ‬ൌ 22݊‫ܨ‬
(d). ‫ ܥ‬ൌ 1.8‫ܨ݌‬ (e). ‫ ܥ‬ൌ 120‫ܨ݌‬ (f). ‫ ܥ‬ൌ 820‫ܨ݌‬
In the simulation, ݅௖ is 15݉‫ ,ܣ‬while in the analytical ݅௕ is 754݉‫ .ܣ‬the difference is:
݅௕ሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ݅௕ሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
݅௕ሺ௔௡௔௟௬௧௜௖௔௟ሻ
754݉‫ ܣ‬െ 15݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
754݉‫ܣ‬
739݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
754݉‫ܣ‬
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 98.01%
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8. Electronusa Mechanical System [Research Center for Electronic and Mechanical]
(a) (b) (c)
(d) (e) (f)
Figure 8. ‫ݒ‬௢௨௧ in different the circuit’s capacitance
(a). ‫ ܥ‬ൌ 1݊‫ܨ‬ (b). ‫ ܥ‬ൌ 10݊‫ܨ‬ (c). ‫ ܥ‬ൌ 22݊‫ܨ‬
(d). ‫ ܥ‬ൌ 1.8‫ܨ݌‬ (e). ‫ ܥ‬ൌ 120‫ܨ݌‬ (f). ‫ ܥ‬ൌ 820‫ܨ݌‬
In the simulation, ‫ݒ‬௢௨௧ is 0ܸ݉, while in the analytical ‫ݒ‬௢௨௧ is 995.1ܸ݉. the difference is:
‫ݒ‬௢௨௧ሺ௔௡௔௟௬௧௜௖௔௟ሻ െ ‫ݒ‬௢௨௧ሺ௦௜௠௨௟௔௧௜௢௡ሻ
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
‫ݒ‬௢௨௧ሺ௔௡௔௟௬௧௜௖௔௟ሻ
995.1ܸ݉ െ 0ܸ݉
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
995.1ܸ݉
995.1ܸ݉
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ ൈ 100%
995.1ܸ݉
ሺ%ሻௗ௜௙௙௘௥௘௡௖௘ ൌ 100%
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