I cannot get the expected gain from my LM358. Input signal is a 1 volt100Hz wave fed via a 3K3 resistor to the non-inverting input. Feedbackis from output to inverting input via 1.270M and then 750K frominverting input to earth. Power supply is 14VDC.Can someone please give it quick thought and tell me what doesn't lookright? I keep trying with no success.Also, the gain increases dramatically when I put a 104 cap across the750K, but oscillation appears.Pardon the simple question, I am just at student level, but there isno one on duty this time of year to ask.Eric Marlow

What's the DC bias at IN+?-- Regards, Joerg"gmail" domain blocked because of excessive spam.Use another domain or send PM.

Why no gain like this?

Dec 28, 2010 21 Replies

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Jamie 15 years ago

Vg = (750k+1.270)/750k = 2.693

So with your 1 Volt in you should be getting ~2.69 output..

If you are getting less, apply a DC of 1 Volt in and then if you do get this gain correctly, you maybe running into a BandPass issue with that large loop back R you're using..

There is a small amount of Cap in an Op-amp to help reduce the chance of oscillation, something that you get with comparators.

Also, you may want to look for "Miller Effect"

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Get down to ~page 30 and see the second on miller effect..

Jamie.

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Joerg 15 years ago

[...]

I am not _that_ old :-)

It was mostly CMOS stuff, CD-series. Nice thing is that the quiescent current isn't too high when mid-biased, as long as you kept the supply around 4-5V. Of course, there wasn't any bandwidth to write home about.

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.

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