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Bypass and Coupling Capacitors

Author: Leonard Krugman

If the stabilizing resistor in the emitter lead is unbypassed, the amplifier gain is decreased. This is similar to the action of an unbypassed cathode resistor in a vacuum-tube amplifier. A value of 50 μf. works out well for the bypass capacitor in most audio frequency applications. The self bias resistances from collector to base may also be suitably bypassed to avoid a-c degeneration. In cascaded stages, the load is unusually low, and the a-c collector voltage is also low. In this case, the bypass capacitor can be omitted with only a slight loss in the stage gain.

The value of the coupling capacitor Cc must be large enough to pass the lowest frequency to be amplified. Usually a maximum drop of 3 db in gain is permitted. At this value, the reactance of the coupling capacitor is equal to the input resistance of the stage. Since the input

resistance is low for the grounded emitter and grounded base connections, relatively high capacity coupling condensers are required. For example: What is the minimum value of Cc necessary for coupling into a stage whose input resistance rt = 500 ohms, if a frequency response down to 100 cps is required? At 100 cps, transistor_basics_05-25.gif, andtransistor_basics_05-26.gif. Intypical circuits, the required value of the coupling capacitor varies from 1 to 10 μf.

Last Update: 2010-11-17