Discrete JFET-NPN Cascode Input OPAMP

[Guest]

I did a couple of additional tests with the only generator I have left, a sine wave generator made with a wiems bridge. Anyway. with load down to about 100 OHMs the circuit is stable down to unity gain from about 20Hz to above 100kHz using Tamas' feedback arrangement. The schematic has a couple of minor changes. http://users.adelphia.net/~thomasholley/JFET-NPN Cascode Input OPAMP5.gif

How do I measure slew rate? I will look at square wave as soon as I can come out of my test gear deficit.

Thanks for the criticism.

 

[Samuel Groner]

Unity gain stable? Not bad for a breadboarded circuit!

How do I measure slew rate?

Adjust the amp to the lowest stable gain (unity in this case). Connect a 100 pF cap to the output and solder the other end to a 1k resistor. Ground this resistor.

Now scope between the cap and the resistor while feeding a fast and large square wave through your amp. You'll get a positive and negative pulse for the according slew rate. Measure the voltage at the flat position of the pulse. Count 1 V/us per 100 mV reading, i.e. for this opamp you should get around 3 V, which equals 30 V/us.

Use a locally grounded probe, we deal with HF stuff right now.

Hope this is clear, otherwise I can draw something!
Samuel

 

[luvir]

Does anyone have the schematics previously linked?
The links seems not working yet.