All about Op Amps...

[Brossman]

I've been wondering several things for a while now, and granted, I know the information is probably on the forums, albeit scattered.  However, I'd just like someplace we can talk all about Op Amps specifically - how they work; different kinds; opinions and testimonials; etc.

I myself have a few questions I'd like answered:

Why do 8-pin IC Op amps only seem to have 3-5 legs on a circuit schematic?  Are the rest grounded?  Either way, how are you supposed to count which pins are which when soldering your entire circuit together? (in other words, is there a specific order to which they're numbered/read/organized?)

Just a note - these questions stem from studying (and trying to understand) real tube overdrives, i.e. FrequencyCentral's PentaBooster/Caster (sp?)

Thanks for your input! I'll get back to y'all when I get off work, lol.

- Britt R.

[frequencycentral]

Here's a 4558 dual opamp data sheet:

http://www.chinaelite.com/pdf/jrc4558.pdf

See in the pin configuration that pin 4 is Vcc and pin 8 is Vcc+. in stompboxes Vcc would normally be grounded and Vcc+ would normally be connected to 9 volts, that leaves 3 pins for each of the two opamps in the package, + input, - input and output.

.....and tubes are very different to opamps, best dicussed as seperate issues!

[R.G.]

I've been wondering several things for a while now, and granted, I know the information is probably on the forums, albeit scattered.  However, I'd just like someplace we can talk all about Op Amps specifically - how they work; different kinds; opinions and testimonials; etc.

How they work is adequately covered in lots of tutorials on the web. It's probably easier if you read this one: http://focus.ti.com/lit/an/slod006b/slod006b.pdf and ask any questions you have left.

As to opinions and testimonials, those are much like noses - everyone has one, they're all similar but differ in detail, and some people are either inordinately proud of theirs, or alternatively hate them with a passion. Fact is, as long as you don't run them outside their linear region, one opamp is very much like another excepting for noise performance.

I myself have a few questions I'd like answered:
Why do 8-pin IC Op amps only seem to have 3-5 legs on a circuit schematic?  Are the rest grounded? 

Schematics are a "shorthand" representation of parts. Only the things which are electronically active are shown on the schematic. An ideal opamp has only three pins: + input, - input, and output. It's left to the reader to realize that of course, this is a real, not an idealized opamp, and it will require a power supply, or two. The three-pin drawings leave out the most positive power supply pin and the most negative power supply pin. Those are all of the pins normally used on modern, internally frequency compensated opamps.

Non- or under-compensated opamps do exist, and these usually have two more pins shown on the schematic. Likewise, there are sometimes "offset adjustment" pins shown, on the few opamps which have those.

Another idea you'll need is that packages are only loosely related to the IC function. A single opamp may only need five pins (+, -, out, and +/- power) but it's being sold in a standard 8-pin package. The 8 pin package is important to customers, as that is what fits in sockets and PCB outlines. If it's not in a standard package, the maker generally can't sell it. So all ICs are usually sold in standard packages. That same 8-pin package can hold two opamps if they share the power supply pins and use only three of the remaining pins each. There are the same *circuit* opamps sold four to a 14 pin package by sharing two power pins and using 3 pins each out of the remaining 12.

Either way, how are you supposed to count which pins are which when soldering your entire circuit together? (in other words, is there a specific order to which they're numbered/read/organized?)

That is a function of the **package** not the circuit. For Dual-Inline-Packages (DIPs) you find the end of the package with a notch or dent in it, and orient the package with pins down, notch away from you. The pin on the left side nearest the notch is pin 1; going down the left side is 2, 3, ... until you run out of pins, generally 4 (on an 8-pin), 7 (on a 14-pin), 8 (on a 16-pin) and so on. The next number is the pin nearest you on the bottom right. Then the numbers go up the right side. Google "DIP package pin numbering".

[Mark Hammer]

Similarly, there are some 14-pin dual op-amps that adopt a "standard" package with some common pinouts shared across part numbers, but some will make use of certain available pins for compensation purposes, while others will list them as "NC" (No Connection).

If one looks at IC power-amp chips, sometimes you'll see a sort of bent-over wing where the pins would be in the middle.  In other cases, the datasheet for the chip instructs the user to ground the middle 3 pins on each side, so you have pins 1 + 2 used, 3-5 grounded, and 6 + 7 used.  On the other side, it's the same thing, except 8 + 9 are used, 10-12 are grounded, and 13 + 14 are used.  Those middle pins are connected topgether internally and act as a way to direct heat buildup outside the chip.

In the case of BBDs, we see other interestng arrangements.  The venerable MN3008/3005/3208 and 3205 all come in the form factor of a 14-pin chip, but only 8 pin locations are used.  Since there is no real need to redirect heat buildup, unlike power-amp chips that deliberately use the middle 3 pins on each side, those pins are simply absent.

I mention this because it can sometimes lead to confusion when the pins are numbered with respect to their package location vs their pin sequence.  To whit, the 4th pin on the chip is in the same location that pin 7 would be...if it were a true 14-pin chip and not just the same size as one.  When the chip is rare and difficult to find (as MN3005s are), you want to make darn sure that the pin you think the schematic is referring to is the one it really IS referring to.  Likewise, if you post a request for debugging help here, and someone says to test the voltage at pin 8, you want to be sure your mutual signals don't get crossed and you're thinking they mean the pin in position 8, rather than the pin in position 14.