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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Hot Op-Amp!
Date: Wed, 23 Oct 2002 03:20:23 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 20:20:23 PDT
> I made a PCB that has four TI (Burr Brown, actually) OPA547 op amps. One of
> them is getting REALLY hot while the other three are not. They are all fed
> from the same power supply (+12/-12). They are all configured for
> non-inverting operation and the three cool ones are set for a gain of 2 while
> the hot one has a gain of 1. The test I ran was with no inputs on all four op
> The way I am getting this is by using two feedback resistors. R1 is from the
> inverting input to ground and R2 is from the output to the inverting input.
> The gain formula is Vo=Vin*(1+R2/R1). On the hot op amp, I am using a jumper
> wire for R1 and R2 (as was suggested from this newsgroups, or maybe
> sci.electronics.basics??) Maybe I need a resistor at R1 even though I want 0
> ohms for R2??? On the three that have a gain of two, I am using R1=R2=3.01k
> ohms. If I do need an R1 resistor, what value?? I'd like to use the 3.01 k
> just for commonality of parts.
> Also, I know that running an op amp where the output is at the midpoint of the
> power supply is the worst case for heat disipation. My inputs will not go less
> than ground, so I intend to change to a +12/-5 V power supply which should help
> in the heat problem...right??
> Any thoughts would be appreciated.
You really need R1 to be open. In that case, R2 can usually be a short
unless input bias current is large. BTW, what is the load connected to
the output? I'm not familiar with the OPA547. Is it stable with unity
gain feedback? There are just a few internally compensated opamps that
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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