Opamp Quiz

DahakaNishoba · Post by **DahakaNishoba** » 21 Oct 2010, 22:28

Ok couple opamp questions
Im starting learning something more of the "inside" so stupid questions about simple things
Anyway
A - take a look at the TS/Sd1/90%of similar overdrives diode clipping section...between the two opamp pins there are 2/3 diodes (or more variants....)clipping
....a variable resistor (or a pot+fixed resistor)....gain control
But on a lot of similar circuits there is a small cap (often a 47/50pf not polarized) ....now the question is.....what the HELL that cap is about ?? Is only something optional or is really related to gain or clipping control?? Modifying the value or removing it affects deeply something? (I assume is not really related to tone but I really dunno)
B - assuming I have a spare tiny litle cute quad opamp just lying in the box and I wanna test a circuit for an opamp drive I'm drawing....there ANY downside to use a single 074 to do something like 1(buffer)2(clipping)3(boost)4(another buffer?)

RnFR · Post by **RnFR** » 21 Oct 2010, 23:20

A- in between those 2 pins is a negative feedback loop. the more resistance, the less feedback, the more gain. the diodes stop this at a certain point, conduct and add distortion. what that small cap does is let infinite neg feedback through for only the highest frequencies- hence the small cap. this reduces the gain for the frequencies that go through the cap, and reduce high end. you can see a similar kind of thing happening with a discrete circuit in the big muff. look at the small cap going from B to C. this is also a negative feedback loop.

B- you can use a quad opamp for anything that you use a single for. i believe it's just best for you to tie the unused points to Vb. i'll let someone else elaborate on that one.

i'm going to move this to the FAQ section.

DrNomis · Post by **DrNomis** » 21 Oct 2010, 23:39

The TLO74 is a good quad opamp IC,it uses jfets in the input stages to enable low bias currents,and also low noise,from what I can remember,these are equivalent to the LM324,and can be powered by single supplies or dual supplies,if you don't use all of the opamps,you can tie both inputs to ground and the output will sit at 0V,doing this stops the unused opamp from going into oscillation due to the very high gains involved,about at least 1,000,000 without negative feedback(open-circuit),if memory serves me right....

DahakaNishoba · Post by **DahakaNishoba** » 22 Oct 2010, 15:02

RnFR wrote: what that small cap does is let infinite neg feedback through for only the highest frequencies- hence the small cap. this reduces the gain for the frequencies that go through the cap, and reduce high end. you can see a similar kind of thing happening with a discrete circuit in the big muff. look at the small cap going from B to C. this is also a negative feedback loop.

so the cap is reducing the "top frequencies" gain, so changing the cap will result in a change of the value of frequencies that are affected by the gain increment ??
is it a good spot to experiment more values or is better to keep a small cap there and move forward to other kind of filters ?

i'm going to move this to the FAQ section.

sure sorry for misplacement