sci.electronics.basics
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## Re: amplifier

 Subject: Re: amplifier "Bob Myers" Wed, 30 Aug 2006 20:13:38 GMT sci.electronics.basics
 ```"Bill Bowden" wrote in message news:1156915815.971190.91800@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > 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. Sorry, but you're thinking of this in the wrong terms. You're really thinking about how the capacitor behaves in DC terms, when the important consideration here is the impedance seen at various frequencies. At sufficiently high frequencies (depending on the capacitor value and that of the emitter resistor, etc.), the cap is "bypassing" the emitter resistor - it is essentially a short to ground for AC at those frequencies. Work through the gain calculations (this is assuming that you're taking the output from the collector, of course - a typical CE amp configuration) and you'll see that impedance in the emitter-to-ground leg actually REDUCES the gain. > 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. That's the whole point - a large enough cap CANNOT change its charge state rapidly enough to respond to rapidly-changing AC currents in this manner. That is really what makes it a low impedance to AC - the fact that you CAN'T change the voltage across the cap very quickly. Bob M. ```
 Current Thread Re: amplifier, (continued) Re: amplifier, Michael Black Re: amplifier, default Re: amplifier, John Popelish Re: amplifier, Eeyore Re: amplifier, jpopelish Re: amplifier, Bill Bowden Re: amplifier, Greg Neill Re: amplifier, default Re: amplifier, John Popelish Re: amplifier, John O'Flaherty Re: amplifier, Bob Myers <= Re: amplifier, Bill Bowden Re: amplifier, Bob Myers Re: amplifier, jasen Re: amplifier, Jamie