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.