Re: amplifier

John Popelish wrote:
chrajesh911@xxxxxxxxx wrote:
wat is the use of emitter resistor & bypass capacitor for any
amplifier?

The emitter resistor produces a negative feedback effect on current
passing through the transistor that helps stabilize the bias point for
gain variations (over temperature or with different devices). It also
reduces the AC gain. Bypassing that resistor with a capacitor
restores the gain at high frequencies (where the capacitive reactance
is much lower than the resistance).

I never fully understood the idea of increasing the gain with a bypass
cap across the emitter resistor. The problem you have is discharging
the cap after it has charged through the emitter circuit. Suppose the
emitter resistor is large at maybe 1 Megohm and the cap is 100uF. The
transistor can charge the cap very quickly as it turns on and therefore
the gain is increased at the collector due to the extra current. But
now the capacitor must discharge through the 1 Meg parallel emitter
resistor which takes RC time to fall 63% or about 100 seconds in this
case. If the frequency is high, the cap will not have time to discharge
much and cannot charge much on the next cycle.

So why does it work?

-Bill

.

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