The second stage amp outputs will - in the best circumstances - collapse
>to their negative rails. With real opamps perhaps they won't quite get
>there. Yes, not very useful, and hardly worth trying to pick devices.
>
> -Frank
And, I beleive, the degree to which they collapse depends on the value of R1/output current.
With regard to the other comments so far, 12mA (and more) at 3V can be done with something like a TS941. Although I was hoping to use something on hand.
As for "uselessness", wasn't that what they first said about the laser? It's just learning by tinkering here.
Why nothing meaningful? I didn't show the signal inputs on pin 2 of each amp, assuming it would be implied. It is just a 2Vpp audio frequency sinewave to both.
Hence the outputs are identical, but with a DC offset from each other governed by the load resistance.
Several experts here have said there is a problem with the circuit, but I would like to understand exactly why. It's an experiment, not a hi-fi system.
Where to start... The input opamp is a fairly common configuration to split the rails for the bias point of subsequent opamps, though often it's set up as a low-pass filter to reduce wideband noise. R3 does nothing good, and limits U1's output current (not a good thing).
U2 and U3 use U1's output for their positive and negative rail, respectively. This means the output of the upper rail cannot go below this and the lower cannot go above this. Therefore, you *must* have a DC current in the output. This isn't generally a good thing for an audio circuit. I guess there may be reasons for this but you haven't said what you're trying to do so this whole discussion is pretty silly.
The U3 is referenced to the negative rail so its output can't go below that. IOW, if your input is a sine wave with 0V DC bias it's going to clip. Ditto U2 and its negative rail (the output of U1). Both are going to clip on the negative half cycle. Even if there is a DC bias on the input signal, there is no DC bias where neither opamp is clipping.
Nope. At least one is always driven into the rails. Even if this were the case, why would you want such a thing? One opamp can do this, BTW. Just sum in a DC voltage.
If it's an experiment why ask here? Why not see for yourself that it does nothing good? If you're trying to accomplish something specific, give the complete picture and people will be glad to help (though it's probably better over in .basics).
You should know enough about op-amps to figure out basic operation. You can assume ideal operation and determine approximate behavior in a circuit. It appears that you are trying to achieve a sort of bridge circuit output that can swing between + and - rails from a virtual ground reference. But clearly the two output op-amps can only swing between their respective rails and
virtual ground. Thus the voltage across the load will always be positive. I doubt that is what you want. I can't see it being a learning experience to build this circuit, except maybe using a simulator. Usually, the only reason for building an actual prototype circuit is to observe behavior that is not as easily predicted or modeled, such as instability and oscillation. Or possibly behavior of op-amps under other than recommended conditions, which can cause latch-up and other possibly destructive operation.
You might want to check out some proven op-amp circuits and learn from them:
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