                            Amplifier Teaching Aid (GTZ, DED; 86 pages)  Preface  Introduction  Lesson 1 - Semiconductor Review  Lesson 2 - Bipolar Transistor  Bipolar Transistor II  First Evaluation  Lesson 4 - Transistor Fundamentals  Lesson 5 - Transistor Biasing  Lesson 6 - Transistor Biasing II  Second Evaluation  Lesson 7 - Small Signal Amplifier   Lesson Plan    Small signal amplifier    Coupling capacitor    Bypass capacitor    Amplifier analyzing method   Worksheet No. 7   Experiment No. 4  Lesson 8 - Small Signal Amplifier II  Lesson 9 - Small Signal Amplifier III  Lesson 10 - Large Signal Amplifier  Third Evaluation

#### Coupling capacitor

A capacitor is open at low frequencies and shorted at high frequencies. Capacitive reactance (XC) is inversely proportional to frequency (f) and to capacitance (C). For a coupling capacitor to work properly, it has to act like an ac short at the lowest frequency that the ac source can have. To realize that we can use the following rule:

XC < 10 R
Make the reactance at least 10 times smaller than the total resistance in series with the capacitor.

Ex: Calculate the capacitance of C1 for a proper ac transmittance. Frequency range: 20-20000 Hz Fig. 7-1: Use of coupling capacitor

Total resistance: 1KΩ + 500Ω = 1.5KΩ

XC <= 10 * R -- >  -- >  C = 53μF

The capacitor to choose should be bigger than 53μF. The next standard value is:

C = 56μF   