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Capacitor parallel with Emitter resistor

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huji

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emitter resistor

Hi,
Can anybody give me a detail explaination on Common Emitter esp on the capacitor parallel with RE. Why is it place there and why does it increase the stability? How to determine the value of the capacitor.. Thanks
 

The use of bypass capacitor is that no appreciable AC voltage will be dropped from emitter to ground to "feed back" to the input and suppress voltage gain. but the dc travel through the feedback resistor, dropping a DC voltage between emitter and ground which lowers the DC voltage gain which stabilizes the amplifier's DC response, preventing thermal runaway.
Because the reactance of this capacitor (XC) to be as low as possible, bypass capacitor should be sized relatively large
 

The gain of a common emitter stage is approx. equal to:
` Zc/Ze
` Where
` Zc = the collector impedance (Cllector resistor in parallel with the
` load impedance)
` Ze = the emitter impedance
` With no capacitor, Ze = Re (approximately)
` With a capacitor, Ze = Re in parallel with the capcaitive reactance, so
` the lower the capacitive reactance at the frequency of interest,
` the higher the gain.
`
' Keep in mind that the lower the value of Zc, the lower the value of the
` input impedance to the amplifier stage. The input imepdance is approx
` equal to Ze X Beta, where Beta is the current gain of the transistor.
 

You can also get the information from :
 

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