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I have build the following circuit on a stripboard

enter image description here

Now the problem is, even if I do not have the BPW34 diode attached my output shows a nice sqaure wave as you can see here enter image description here

But I have no idea where to start how to analyze where this comes from or why this happens. So I am very grateful for any ideas how to approach this problem. One addiational thing: I did not use the 330k Resisitors instead I used 300k resistors. Edit: The transistor is a BF245B.

Here you can see my stripboard. enter image description here

enter image description here

This blue thing in the upper picture is a capacitor which is not connected. And remember: The BPW diode is missing but when it is soldered on the board and made lightproof it also starts oscillating with the same frequency and amplitude.

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  • \$\begingroup\$ That could quite easily pickup stuff like the AC hum from power lines if the wiring is too long. What does your strip board look like? (post a picture) \$\endgroup\$ – JRE Mar 2 '18 at 12:14
  • \$\begingroup\$ Where did the design come from? Is it from a reputable source? What power supply did you use? Did you use the specified op-amp? Where are the supply decouplers for the op-amp? How awful might your strip board be when you have an AC gain of about 900? \$\endgroup\$ – Andy aka Mar 2 '18 at 14:15
  • \$\begingroup\$ Why the radiation symbol? \$\endgroup\$ – Finbarr Mar 2 '18 at 14:20
  • \$\begingroup\$ @JRE This was also my idea so I shielded the whole circuit with aluminum foil and grounded the foil but there was no difference. I did not know that the circuit can pickup AC hum \$\endgroup\$ – Alex Mar 2 '18 at 15:14
  • \$\begingroup\$ @Andyaka Yes from here link. I googled and some people have this problem with oscillation. Power suppy was a 9V battery and my lab power supply but there wasn't any difference and yes I used the specified op-amp. \$\endgroup\$ – Alex Mar 2 '18 at 15:15
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Look at the original PCB design from Elektor: -

enter image description here

It's a double sided board with a full coverage ground plane on the component side and a partial ground plane on the solder side. These ground planes are probably quite fundamental in achieving low noise and stability given that the circuit has an AC gain of 900.

You'd have to be at the top of your game to make this work consistently on strip board or just plain lucky.

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  • \$\begingroup\$ Ok thank you. If there are no other good ways to shield a perfboard i think I cannot make it work. At least I tried :-D \$\endgroup\$ – Alex Mar 2 '18 at 16:11
  • \$\begingroup\$ Try minimising the tracks on pins 2 and 6. It might help. \$\endgroup\$ – Andy aka Mar 2 '18 at 16:14
  • \$\begingroup\$ Try manhattan style construction ... \$\endgroup\$ – rackandboneman Mar 20 '18 at 10:41
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It works. I only use a 10M\Omega Resistor from diode to ground instead of 20M\Omega so I think the Fet was biased wrong. I also put the whole stripboard into a metal case. enter image description here

Look at this beautiful picture :-) If someone could enlighten me why only using a 10M\Omega from diode to ground works but 20M\Omega does not work....this would be great.

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