[time-nuts] Input filter for data logger

[Bruce Griffiths]

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.griffiths at 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 <attila at kinali.ch> wrote:
> > 
> >         On Sun, 19 Nov 2017 16:10:54 -0500
> >         Vlad <time at 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
> > 
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> >     >