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Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
I'm currently using the NE5534 as an amplifier for a 40kHz signal. Everything works to expectation, except the DC offset, which is excessive (~2V). Sure, I could nullify this with a potentiometer, but I need to build three of these amplifiers, and I don't feel like going through the trouble of constantly calibrating them. Also, putting a capacitor on the output risks oscillation and instability.
However, I noticed that a lot of dual op amp ICs have no offset balance inputs, so I thought a dual version of the NE5534, the NE5532, might be self- or pre-compensated. So I looked at their schematics:
NE5534:
NE5532:
The problem is that I can't see any major differences between the two schematics, so I don't know if the NE5532 is actually internally compensated or not.
Is there some critical op amp building block I don't know about? Or are there any suggestions for other amplifiers that do not have serious offset?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
so I thought a dual version of the NE5534, the NE5532, might be self- or pre-compensated.
No. If you leave the offset balance inputs open on a 5534, it behaves like 50% of a 5532. Many opamps, especially lowish noise ones like this, get used for audio. This means AC coupled, so DC offset is irrelevant.
The 5534 is a very old design now and there are better ones out there, with untrimmed DC offsets in the uV.
If low offset with high gain is an issue, then you need to check your source DC impedance. If it's high resistance, then bias and offset current in the amplifier are as important as offset voltage. A much lower current amplifier may be what you need. Maxim do a large range of amplifiers, and push samples on amateurs, rather than having to be extracted like teeth from the other amplifier vendors.
You say 40kHz signal. Can you AC couple either the input or the feedback network, or is the level of the DC important as well?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Indeed, the 5534 has high offset and bias currents as op-amps go. It's designed for low noise over DC precision. It's normally used in AC coupled audio circuits where DC offsets don't matter within reason. It might be old, but it's still one of the best audio op-amps, especially in terms of value for money.
You may want to scale the impedances in your circuit down by a factor of 10, or if you can't do that, use a more DC-friendly op-amp. Modern bipolar op-amps have bias cancellation schemes, superbeta front ends, and so on, all offering big DC improvements over the 5534's plain long-tailed pair. See the OP27 for an example.
And of course there are FET op-amps like the TL07x, that have practically no bias and offset current, but poor offset voltage. These are the ones to use in real high impedance circuits, where the benefit due to lack of offset current outweighs the problem of high offset voltage.
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Amplifier Difference
