Re: Need explantion of part of schematic
- From: Bob Monsen <rcsurname@xxxxxxxxxx>
- Date: Thu, 07 Apr 2005 21:10:10 -0700
On Thu, 07 Apr 2005 17:00:48 -0700, Lessie wrote:
> The schematic concerned is at the end of the microchip
> document on microchip serial programming
> http://ww1.microchip.com/downloads/en/AppNotes/91013b.pdf
>
> The circuit generates VPP for the programming phase. D1 is
> the reference of 12.7V and Q2 forms a negative feedback
> with the opamp. This part I do understand. However, Q2 is
> strange, what does it do? It seems that it will be in a
> cut out state because base will be higher than emitter by
> 0.6V due to Q2 ?
It's a bizzare way to draw the circuit. However, if you notice, the
emitters are both connected to the same node, which is also the feedback.
Thus, it's just a follower. Why they feel they need low
impedance for Vpp is beyond me, though. Usually, Vpp draws almost no
current, so the opamp by itself should work fine. Whatever.
----
Regards,
Bob Monsen
.
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