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diyAudio Member
Join Date: Feb 2001
Location: earth
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Hi all.
Perhaps the experts here can offer some enlightenment. In Douglas Self's site, there is an article on the collector feedback pair , cfp. http://www.dself.demon.co.uk/cfp.htm He further writes that in the trimodal design page, "The main function of R25,26 is to define a suitable quiescent collector current for the drivers TR6,8, and to pull charge carriers from the output device base when that half of the circuit is in the process of turning off. The value is not normally critical, and may be in the region 47 - 100 Ohms, though I have only tested the circuit for the range 68 - 100. Lower values give faster output device turn-off, possibly at some expense in basic linearity; this is of course only relevant to the Class-B mode." R25 and R26 are the collector resistors for the first npn transistor of the cfp. The emitter and base of the power pnp transistor is connected across this collector resistor. My questions: i. what is the mechanism of charge storage in the pnp device and how does the collector resistor remove that charge? Where does the charge go to, the rail? ii. how does one balance the turn-off rate with the linearity requirement in class B? I read that someone called Bailey wrote something about charge removal in audio amps in the then Wireless World magazine in the 60's ,or was it 70's. Does anyone have a copy of that? Many thanks for any help. kind regards Yves |
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