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moom_ommo 27th December 2007 11:42 AM

Searching for an op-amp with specific characteristics

I was looking for something to replace the 709 with. After reading on several websites that it's "irreplaceable", I did some digging and found out that I need only the following characteristics:

External frequency compensation
Bad CMRR (around 70dB perhaps... lower is better).

The thing is that I would need it for an art project where I need to treat objects as sources for sound - something like the (in)famous Kraakdoos (but please let's not get into that). That's why I'm looking for an op-amp which can be influenced easily...

AndrewT 27th December 2007 12:03 PM

buy any slow, cheap opamp that is NOT unity gain stable.

If your circuit needs unity gain stability then buy the 741, or a really cheap equivalent.

moom_ommo 27th December 2007 12:34 PM

The problem is that I'm not an engineer. I need to know how to find not unity gain stable op-amps...

(I don't even know what's unity gain)

741 is a good idea but if I could find 741 I could also find 709... and here, in Romania, I can't... I would need something more common...

Sorry for being such a hard nut to crack :(

moom_ommo 27th December 2007 12:55 PM

I found the 741 (namely UA741CN-ST/R) but it's specified in his data sheet that "no frequency compensation required". Can I make sound of it without frequency compensation to influence the output? I was counting on the feedback and external frequency compensation, you see...

Also it has two additional terminals - Offset null 1 and Offset null 2. Could somebody tell me what are these for?

(sorry for the two consecutive posts but I lack the ability to edit. thank you for your understanding...)

AndrewT 27th December 2007 01:09 PM

the datasheet will tell you if the opamp is unity gain stable or tell you to fit compensation for low gain values.

Go download a 5534 datasheet and see the 22pF compensation that must be fitted for low gain settings.

moom_ommo 27th December 2007 01:22 PM

5534 is great! Sounds like what I need! Many many thanks, Andrew.

The only thing that leaves me quite unsure is

"The op amps are internally compensated for gain equal to, or higher than, three. The frequency response can be optimized with an external
compensation capacitor for various applications (unity gain amplifier,
capacitive load, slew rate, low overshoot, etc.)"

The line frequency response can be optimized with an external compensation capacitor means that it works like the external frequency compensation on the old 709?

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