"The beta is relatively low when the transistor is off" Condition, transistor off (no base drive), Beta = Ic / Ib ... With Ic =10 uA (leakage) and Ib = 0 Ic / Ib => undefined RF Jim ----- Original Message ----- From: "Peter L. Peres" To: Sent: Tuesday, November 05, 2002 3:09 PM Subject: Re: [EE]: Norp12 > On Tue, 5 Nov 2002, Michael Rigby-Jones wrote: > > *>Surely the transistor will only just conduct enough to keep 0.6 volts on the > *>base, the transistor will not be able to be entirely switched off until the > *>cap is charged. This would effectively cause the relay to switch off very > *>slowly. I certainly don't see how this is going to cause an oscillation as > *>the cap is effectively providing NEGATIVE feedback at anything over DC. > > Correct but I have to add again that relays do not switch slowly. As I > said before the magnetic force generated by the coil depends on the > inverse square of the gap and relays have a well known 'catch' and > 'release' voltage, far apart from each other. Just try to make an ordinary > relay switch 'slowly'. Use a benchtop regulated psu as accurate as you > wish. > > The capacitor on CB is equivalent to a capacitor across BE where Cbe ~= > Ccb * beta I think. This is most useful since it causes negative feedback > in ac there where the transistor has most gain (which in this circuit > amplifies noise). The beta is relatively low when the transistor is off or > saturated and there the capacitor plays a smaller role. So entering and > leaving saturation and off state is less slowed down than with a Cbe cap. > Also a 1uF plastic cap will have significantly lower leakage than a 100uF > electrolytic. > > Peter > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.