Oops I meat to say: Thats a MOSFET variant of a fairly standard JFET-BJT feedback amplifier.
Bruce > On 20 November 2017 at 11:47 Bruce Griffiths <bruce.griffi...@xtra.co.nz> > wrote: > > > Hoi Attila > > That's a fairly standard JFET BJT negative feedback amp that's not > usually unstable. > > The unity gain version has been employed as the input stage of various > high impedance oscilloscope preamps. > > Bruce > > > > > > On 20 November 2017 at 11:21 Attila Kinali <att...@kinali.ch> wrote: > > > > On Sun, 19 Nov 2017 16:10:54 -0500 > > Vlad <t...@patoka.org> wrote: > > > > > > > > > > Here is my schematic: > > > > > > http://www.patoka.ca/OCXO/LOGGER/IMG_20171119_155907272.jpg > > > > > > > > > > Ok.. I am surprised, this doesn't oscillate. > > > > You have a two stage amplifier, where the second stage > > has a negative feedback path into the first stage. > > > > When a pulse comes in, the jfet will turn on and conduct > > current through its drain and source resistors. When the > > current reaches something around 6-8mA the pnp will start > > conducting. But the collector current of the pnp goes into > > the source resistor of the jfet. This will increase the > > voltage on the source, thus decreasing the gate-source > > voltage, thus turn the jfet off, which in turn will turn > > the pnp off, which in then will stop conducting, thus > > no current into the source resistor, thus the jfet will > > start conducting again... I guess you get it. > > > > > > > > > > I did some simple tests for this. In it seems it was OK up to > > > 10Mhz. > > > > > > > > > > > > > > But guessing from what you showed, I would say that > > > > your amplifier > > > > circuit isn't stable and has some gain peaking at > > > > around 10MHz. > > > > There are two ways to proceed: Either optimize your > > > > circuit or > > > > simplify it using modern components to the input signal > > > > you expect. > > > > > > > > > > > > > > The main purpose for this circuit is to protect the MCU input > > > and make > > > some sine to square conversion. > > > > > > > > > > Use a biased 74AC04. That's the easiest. And you will have very > > little noise degradation. > > > > I would think that the MCU can probably take more abuse than the > > 74AC. Modern ASICs have quite a bit of protection circuits on > > their inputs. I am not sure whether the 74-families have seen > > upgrades on their protection circuits in the last 30-40 years. > > > > Attila Kinali > > > > -- > > You know, the very powerful and the very stupid have one thing in > > common. > > They don't alters their views to fit the facts, they alter the > > facts to > > fit the views, which can be uncomfortable if you happen to be one > > of the > > facts that needs altering. -- The Doctor > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
