Inverting Attenuator Circuits

[cochenob]

I've been reading that opamp circuits with gains less than unity
(between 0 and 1) can become unstable. (i.e. - simply making the input
resistance larger than the feedback resistance doesn't cut it).

Why is this?

In addition, I've found a few circuits that claim to rectify any
instability, but don't have any associated math to back it up.

Anyone have any good references that can settle this?

---
BC

 

[Mark]

Less gain = more feedback.

If the op amp itself is not designed for that much feedback, it can
become unstable. It has to do with the way yht wgain and phase of the
op amp reduce at higher frquencies

Goggle key words:

op amp unity gain stable gain margin phase margin

 

[Tim Wescott]

cochenob wrote:

I've been reading that opamp circuits with gains less than unity
(between 0 and 1) can become unstable. (i.e. - simply making the input
resistance larger than the feedback resistance doesn't cut it).

Why is this?

In addition, I've found a few circuits that claim to rectify any
instability, but don't have any associated math to back it up.

Anyone have any good references that can settle this?

The gain of an op-amp circuit is set by the gain of the op-amp itself

and the gain of the feedback network. For lower amplifier gains the
feedback network must have more gain to "hold back" the amplifier more.
No op-amp has perfect, infinite gain, however. They don't even look
like perfect integrators. So there will be some setting of the feedback
network gain for which the op-amp circuit will go unstable.

Any good book on circuit design should clear this up.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[John Larkin]

On 14 Feb 2005 12:55:49 -0800, "cochenob" <cochenob@hotmail.com>
wrote:

I've been reading that opamp circuits with gains less than unity
(between 0 and 1) can become unstable. (i.e. - simply making the input
resistance larger than the feedback resistance doesn't cut it).

Why is this?

In addition, I've found a few circuits that claim to rectify any
instability, but don't have any associated math to back it up.

Anyone have any good references that can settle this?

The vast majority of opamps are "unity gain stable" which means they
are stable with Rf = 0, ie as a voltage follower or as an inverter
with any gain. A few opamps are undercompensated, and are clearly
identified on their datasheets as "stable for gains > 5" or some such.

Extremely high impedance feedback networks can cause instability, but
that's a different issue.

John

 

[John Larkin]

On Tue, 15 Feb 2005 00:29:21 -0800, "bg" <bg@nospam.com> wrote:

Less than unity gain requires more compensation than
unity gain and is not usually specified in a spec sheet.

The vast majority of opamps are internally compensated to be stable at
any gain, including the follower configuration which has 100% negative
feedback.

John