sci.electronics.basics
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## Re: amplifier

 Subject: Re: amplifier default Wed, 30 Aug 2006 08:31:50 -0400 sci.electronics.basics
 ```On 29 Aug 2006 22:30:16 -0700, "Bill Bowden" wrote: >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 Emitter resistors are typically rather low - think 100 ohms or so. The current flowing through the transistor drops voltage across the resistor - a volt for the sake of discussion - that voltage moves the emitter away from zero while the base resistors are maintaining a more or less fixed voltage on the base. More current = greater drop on the emitter resistor = less "on" bias = less current. That is the DC theory - but it works with a signal imposed on the emitter current as well - the bias, sans capacitor, will try to track the signal - and will serve to decrease the amplification - bypass the resistor with a sufficiently large cap and the DC bias is maintained while the (higher frequency) signal is not affected. You seem to be trying to think in terms of a static switch - this is a transistor biased into its linear region to amplify a linear signal. We aren't concerned with it turning on quickly - it is never fully on or off (for class A operation). The cap isn't really charging and discharging - it is just providing a low impedance path for AC. (imposed on the DC) ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- ```
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