CFP output stage - how to choose resistor - diyAudio
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Old 3rd July 2001, 01:49 AM   #1
diyAudio Member
Join Date: Feb 2001
Location: earth
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.

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

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
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