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TCA0372 1A opamp stability probs?

Started by BobG June 8, 2011
Anyone used a TCA0372 1A output opamp? Seems prone to oscillate?  Does
it need some minimum gain to be stable? I'm running it from +-15V, two
equal resistor inverters in series, input is 10.00V reference, trying
to make a +-10V pot rail supply. Its squirrelly. Would a BFC across
the feedback R help?
On Jun 8, 12:02=A0pm, BobG <bobgard...@aol.com> wrote:
> Anyone used a TCA0372 1A output opamp? Seems prone to oscillate? =A0Does > it need some minimum gain to be stable? I'm running it from +-15V, two > equal resistor inverters in series, input is 10.00V reference, trying > to make a +-10V pot rail supply. Its squirrelly. Would a BFC across > the feedback R help?
Wow, an amp out of a DIP! I've never used it. Do you have a capacitor on the output? I've made the mistake several times of using an opamp for a power supply and I then hang 0.1uF cap on it.... "Why's this thing oscillating?" A BFC in the feedback may help. There are other 'tricks' to help an opamp drive a capacitor. (If that's the problem.) George H. Can you really get an amp out off it without it burning up?
On Jun 8, 4:41=A0pm, George Herold <gher...@teachspin.com> wrote:
> Can you really get an amp out off it without it burning up?
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D It gets hotternhell, but I havent left any skin on it yet, so I guess I'm shuttin it off fast enough. I used to use an opamp with an LT1010 buffer, but I thought I'd try something new, but its a whole new set of weird problems.
On Wed, 8 Jun 2011 13:41:51 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Jun 8, 12:02=A0pm, BobG <bobgard...@aol.com> wrote: >> Anyone used a TCA0372 1A output opamp? Seems prone to oscillate? =A0Does >> it need some minimum gain to be stable? I'm running it from +-15V, two >> equal resistor inverters in series, input is 10.00V reference, trying >> to make a +-10V pot rail supply. Its squirrelly. Would a BFC across >> the feedback R help? > >Wow, an amp out of a DIP! I've never used it. Do you have a >capacitor on the output? I've made the mistake several times of using >an opamp for a power supply and I then hang 0.1uF cap on it.... "Why's >this thing oscillating?" > >A BFC in the feedback may help. There are other 'tricks' to help an >opamp drive a capacitor. (If that's the problem.)
The usual trick is to use a resistor (100 ohms or so) in series with the output, and connect the feedback resistor *after* that (not directly at the output pin). A small capacitor (10-50 pF) is used between the output pin and inverting input. This scheme works great for driving (say) long cables using "normal" op-amps. I think it would be a bad choice as a power supply, though, since the series resistor would be wasting power (and reducing output range). Best regards, Bob Masta DAQARTA v6.02 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!
On Jun 8, 8:59=A0pm, BobG <bobgard...@aol.com> wrote:
> On Jun 8, 4:41=A0pm, George Herold <gher...@teachspin.com> wrote:> Can yo=
u really get an amp out off it without it burning up?
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D
> It gets hotternhell, but I havent left any skin on it yet, so I guess > I'm shuttin it off fast enough. I used to use an opamp with an LT1010 > buffer, but I thought I'd try something new, but its a whole new set > of weird problems.
I couldn't find a maximum power disipation number on the spec sheet. Just a footnote saying don't let the die get above 150 C. (And then you get to work through the thermal resistances...) But something like 1 Watt looks like a max power depending on the package. How much current are your pots drawing? I like the OPA544T as a power opamp, but it's not cheap. George H.
Thanx for the replies guys. Nice to see some electrical chat
occaisionally. The +-10V opamps are just driving the rails of a couple
of 5K pots in a servo.... I'll try the R in the feedback loop fix.
Bob Masta you de man. 100ohms in the feedback loop seems to make it
stop singing.
On Thu, 9 Jun 2011 16:45:36 -0700 (PDT), BobG
<bobgardner@aol.com> wrote:

>Bob Masta you de man. 100ohms in the feedback loop seems to make it >stop singing.
Thanks, but it wasn't original with me! I think I saw that many years ago in a Walt Jung book or article. I've used it for driving long cables, but always with the feedback cap as well (from the original article). Maybe your setup has enough parasitic C to do the job? May only take a few pF. I no longer remember the math to pick the values (if I ever knew it), but "rule of thumb" seems to work well! Best regards, Bob Masta DAQARTA v6.02 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI Science with your sound card!
On Wed, 8 Jun 2011 17:59:41 -0700 (PDT), BobG <bobgardner@aol.com>
wrote:

>On Jun 8, 4:41&#2013266080;pm, George Herold <gher...@teachspin.com> wrote: >> Can you really get an amp out off it without it burning up? >========================================================= >It gets hotternhell, but I havent left any skin on it yet, so I guess >I'm shuttin it off fast enough. I used to use an opamp with an LT1010 >buffer, but I thought I'd try something new, but its a whole new set >of weird problems.
I ran into something like that with a Pentawatt TO220 op amp. It needed a cap and resistor on the output (a "snubber") or would pick up every radio station around and try to drive the motor I was using with MHZ noise. It also ran hot. Driving me crazy until I put a speaker across the motor and heard an AM station. .22 and 10 ohm in series from each output to ground (using 2 push-pull to run the motor bi directionally off one supply)
default wrote:

> On Wed, 8 Jun 2011 17:59:41 -0700 (PDT), BobG <bobgardner@aol.com> > wrote: > > >>On Jun 8, 4:41 pm, George Herold <gher...@teachspin.com> wrote: >> >>>Can you really get an amp out off it without it burning up? >> >>========================================================= >>It gets hotternhell, but I havent left any skin on it yet, so I guess >>I'm shuttin it off fast enough. I used to use an opamp with an LT1010 >>buffer, but I thought I'd try something new, but its a whole new set >>of weird problems. > > > I ran into something like that with a Pentawatt TO220 op amp. > > It needed a cap and resistor on the output (a "snubber") or would pick > up every radio station around and try to drive the motor I was using > with MHZ noise. It also ran hot. > > Driving me crazy until I put a speaker across the motor and heard an > AM station. .22 and 10 ohm in series from each output to ground > (using 2 push-pull to run the motor bi directionally off one supply)
Lets see... At 15k Hz, well assume Xc reactance to be ~48.25 Z = sqrt(Xc^2+10^2)= Sqrt(2328+100) = 49.27 give or take a bit.. SO, lets see the Z at lets say 100khz Xc = 7.29 Z = sqrt(7.29^+10^) = sqrt(52+100) = 12; So, we can see that at 100Khz, the load Z would be 12 ohms and at 15khz , 49. So it is assumed that since 10 ohms is enough load to stop oscillation the cap in series will render the load as an open circuit at low freqs. Inductive loads like speakers and such tend to have some H-Z at radio freqs.. that little series Z match helps with that with out places a too low of a Z on the output. At least it stops a lot of op-amp and amp circuitry from oscillating, since the output will look like a low Z at the attempted oscillating freq.. There is a name for this type of circuit in audio land, I just can't remember it at the moment. Jamie