[sdiy] Weird op amp features & rules of thumb

[JH.]

>* If the voltage difference on the input pins exceeds a magic number,
>the TL07x/TL08x do a really nasty phase inversion where the + acts
>like - and the - acts like +.

Yes.


>For typical negative feedback mode
>circuits where the pins try to stay at the same voltage, this probably
>isn't a problem,

It can well be a problem with noninverting amps if there's no limit of input 
voltage range.
That's why you sometimes divide-down, and then slightly amplify.

>So blindly plugging in a TL07x/08x for
>another op amp may be dangerous and should be carefully considered.

Absolutely.
Also, in some applications the nonlinear capacitance of these JFET input 
opamps
is said to cause distortion when driven from high impedance. I've never 
verified this myself, though.
It's probably more relevant in High End Audio reproduction than in synth 
circuits.

>BJT input amps need that little resistor from the + terminal to ground
>when you're setting up an inverter.

Only if offset volatage is an issue. Often, with AC coupling, it is not.
Note  that this resistor adds to the circuit noise, unless it's bypassed by 
a capacitor.

> Are my rules of thumb on target or off base?

If you find a vintage circuit that has TL07x / TL08x or similar BiFet amps 
in *some* places, but
other opamps in *other* places, it's almost a save bet there was a reason 
not to choose BiFet opamps in these other places. That reason *might* have 
simply been cost issues (at a time when BiFets were relatively expensive).
But then again, the opamp chosen for that other task may just have been the 
better technical choice, and "upgrading" with TL07x might actually be a step 
backwards.

Don't shy away when a data sheet talks of an "improoved 741".
This was a *compliment*, not a caveat, at a time, and in some respects (no 
phase reversal, stability, output drive) it still is. (Given that slew rate 
and noise of that specific amp are fit for the application).

JH.