LM3900 substitute?

Is an LM324 a good replacement for an LM3900? I couldn't locate one of the latter so relayed my circuit to use an LM324 instead, well it doesn't work, i checked my output from the first stage and it sits at 10V although the input is a 4V p-p sine wave from my generator, i am building the circuit here:

https://www.aaroncake.net/circuits/3chspec.asp

I understand the LM3900 is no longer available, so what would be a good substitute?

cheers

Neil.

Hi,

The LM3900 was a very different kind of op amp. It operates on the difference in input current rather than the difference in input voltages.
It can not be replaced with a LM324 op amp.
From what i read, the NTE992 is a replacement. The LM359 is a high speed version.

It would be possible however to change the design a little to use a more typical op amp. From the LM3900 data sheet it looks like the three amps with two caps each are just bandpass filters. Redesigning them for other op amps probably wouldnt be much of a problem really.
In fact, grounding the non inverting terminals of the op amps instead of running them to +12 through a resistor would probably allow operation with the LM324 without any further modification. The Q of each bandpass would be a little higher though so you'd have to try it and modify the values slightly if it doesnt seem to work the way you want it to.

We can call the input resistor R1 and the resistor to ground R2 and the feedback resistor R3.
With a Q of 9.95 we can keep the input resistor R1 at 10k for all three op amps and this comes close to the original design.
The passband gain is about 20, so G=20.
The feedback resistor R3 is then:
R3=G*2*R1
and R2 is then:
R2=Q/((2*Q^2-G)*2*pi*f*C)

It looks like the original design kept all the resistors the same and just swapped out the caps for each different channel.
This may not be the ideal setup, but it might still work.

MrAl said:

In fact, grounding the non inverting terminals of the op amps instead of running them to +12 through a resistor would probably allow operation with the LM324 without any further modification.

Click to expand...

The circuit uses a single polarity supply so grounding the non-inverting input of the opamps would cause their outputs to be grounded and they would work like rectifiers.
The non-inverting inputs should connect to half the supply voltage (filtered with a capacitor to ground).

audioguru said:

The circuit uses a single polarity supply so grounding the non-inverting input of the opamps would cause their outputs to be grounded and they would work like rectifiers.
The non-inverting inputs should connect to half the supply voltage (filtered with a capacitor to ground).

Click to expand...

Hello,


Yes, i forgot to mention the negative power supply, or as you noted, splitting the supply but then the supply voltage probably has to be increased also.

ok thanks guys, i'm going to scrap my board and start again with one of the replacement chips that MrAl mentioned, unless of course the pin out is the same, i'll have to go check it first.

Neil.

The LM3900 is not a normal OP amp. See: https://www.electro-tech-online.com/custompdfs/2011/11/MT-034.pdf

See the AD8001 which is a current part at least for singles.

One of the cool things about the LM3900 is that the input voltage can exceed the supply rails without a lot of fancy stuff. Thus you could look for the presence of line to speaker levels very easily and even filter the hiss. That's what a specific timer of mine does.

Hi,

I'd be careful about using an AD8001 for a replacement. It's very different than the LM3900. For one thing, the input bias would be different.
I probably have few AD8001's around, but i dont think i have any LM3900's around anymore. Back in the 80's i did

The point I guess I was trying to make is that current mode OP amps are still made, but the current feedback LM3900 is like the LM741 of voltage feedback OP amps. Very early products.

Hi again,

Yeah i remember these from back in the 1980's. I am surprised to see them in circuits today. Maybe this was a very old design.

The old circuit used the LM3900 and it also used Japanese LED driver ICs that also are probably not available.

Jameco has the LM3900's: https://www.jameco.com/webapp/wcs/s...amecoall&ddkey=http:StoreCatalogDrillDownView

www.Futurlec.com has both parts.

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KeepItSimpleStupid said:

The point I guess I was trying to make is that current mode OP amps are still made, but the current feedback LM3900 is like the LM741 of voltage feedback OP amps. Very early products.

Click to expand...

I think MrAl's point is that a current differencing amp and a current feedback amp are two totally different animals.
LM3900 is not a current feedback amp. The terminology is really ambiguous, but the input structures of the two are totally different.

I seem to have blown out on at least two things here, i'm gonna scrap that project, build something else i need then come back to it using a different circuit.

thanks for your time guys.

Neil.