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"Bill Bowden" <wrongaddress@xxxxxxx> wrote in message
news:1156915815.971190.91800@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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?
You're thinking essentially DC. Think instead about the
impedance of the capacitor at the frequencies of the
signal in question. If it's audio at, say, 2KHz for
example, your 100uF capacitor would have an impedance
of 0.8 Ohms. Compare that with your 1 Meg that's
setting the bias point.
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