[sdiy] How to add stability cap to voltage follower?

[Theo]

Going to simplify a bit, please forgive ;)

The cap and resistor in the feedback loop of the voltage follower don't do
anything because there is no current going.
Not sure if you need 1x amplification or nor but...

Start with a voltage follower.
Add a feedback resistor of 10k and an other 10k resistor from - input to
GND.
Now you have a noninverting amp with 2x amplification.
BTW 1) add a (22pF ?) cap in parallel to the feedback resistor to taste.
BTW 2) TL07x and some other popular opamps seem to "like" feedback resistors
around 10k. At least that is what they told us at school. But other values
work too.

To get back to a 1x amp you  simply divide the input voltage by two.
In other words two resistors in series to GND  and
were the resistors meet you hook up the + input of the opamp.
Now you have a second place to put a small cap to add stability,
parallel  to the resistor from + input to GND.
BTW <kicking a open door> the sum of the divider resistors is your input
resistance.

A 1k series resistor behind the output like you did with the inverting amp
is still a good idea.

HTH
Theo

----- Original Message -----
From: Scott Juskiw <scott at tellun.com>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Monday, January 20, 2003 2:22 AM
Subject: [sdiy] How to add stability cap to voltage follower?


> I was wondering if someone can help me with a minor design issue.
>
> I'm familiar with the technique of adding a cap in parallel with the
> feedback resistor in an inverting amplifier to add stability. But how
> do you add the same kind of stabilizing cap to a non-inverting
> amplifier?
>
> The circuit in question is a state variable filter and I want all the
> outputs to be non-inverting. I started out with a simple inverting
> amplifier, gain of -1, with Rin=100K, Rfeedback = 100K, and 1K
> hanging off the output. To this I added a 22 pF cap in parallel with
> Rfeedback. But now I realize that I need a non-inverting amp. OK, a
> voltage follower is simple enough to build. But I'm not sure how to
> hook up a stabilizing cap to that. I can put Rfeedback back in and
> add the cap in parallel with that as I had with the inverting
> amplifier. It doesn't seem to hurt anything, but is it doing good? Or
> should I not worry about adding the cap?
>
>