So you need an op amp…and you can’t use just any generic op amp because of certain requirements. For example, a low supply voltage (
Has to run on a 3V “single supply” and we’ll have our DC offset at 1.5V
Input offset voltage (Vos) must be
Since we need to handle such a large swing, it should be rail-to-rail to within .2V of the rails to give us space.
1000 gain at 20KHz means the gain bandwidth (GBW) must be > 20MHz (in reality it can be much lower because the tape probably isnt good enough to record past 12KHz)
Since our input signal is 10uV then (at worst) we want to have less than 5% noise (which isn’t so good but we’re willing to have poor quality audio). 5% of 10uV = 500nV. 500nV/sqrt(20000) = 3.5nVrtHz noise figure.
Package should be SOIC for easy soldering
Low power is nice but not -that- necessary, maybe
Low price! Must be under $2 at quantity 100, the lower the better
Stuff we don’t care about:
Slew rate (at 10mV and 20KHz, it can be really slow)
The OP37, for comparison, has 12MHz GBW, requires 8V power supply, not rail-to-rail, Vos = 0.03mV, and noise figure of 3.2nVrtHz. So: great offset, great noise, so-so bandwidth and incompatible power requirements.
Lets go to TI and see what they have to offer. Select >=16MHz GBW, rail-to-rail and 8-SOIC package. The only chip they’ve got is the OPA350.
Now lets try National. Their system is a little tougher to use: click on 1mA offset first, which will pare down the options to 5 items, none of which are SOIC. You’ll notice national doesnt have a very good selection of low noise, ~20MHz op amps.
Next, we go to Analog Devices. Enter in package = SOIC, Vcc-Vee = 3V, noise STMicro (click on “low noise”) they don’t have anything that runs on 3V.
Linear Technologies has a bunch of incredibly low noise op amps, at extraordinary bandwidths, but they’re rather expensive, at least $3 which is outside of our budget.
Maxim has a few good options (sort by noise, then compare all of the ones 20MHz GBW, that leaves the MAX4488.
There are a couple other manufacturers but we’ve covered the most common. So let’s stop here. The OPA350 is clearly not a good choice, it’s the most expensive, noisiest and most power-hungry. So we’ll just ignore that, leaving the AD8655 and MAX4488. Now its just a comparison between price, power and noise. Since our noise figure is already higher than we’d like, I’ll place priority on that: the price difference is pretty small and power isn’t a huge priority.
Finally, we have chosen the AD8655. Yay Analog Devices!
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This is a nice example and I suspect you’re aware of the following but I wanted to mention a couple things for the potential edification of other readers:
1. The Art of Electronics also contains some good information on op-amp selection.
2. In a somewhat demanding amplifier design such as the one in your example, it is sometimes also worth considering a two-stage design. Sometimes two op-amps of somewhat lesser specs (purchased as a dual monolithic device) cascaded appropriately work as well as and cost less than a single fancier one. You generally wouldn’t be able to get out of whatever noise requirement you have (for the first stage anyways) but the requirement for gain-bandwidth product of each op-amp is reduced appreciably.
I would like to mention that the AD8655 actually has a thermal noise floor of 2.7nV/rtHz.