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From: Kevin McMurtrie
Subject: Re: Hot Op-Amp!
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Wed, 23 Oct 2002 07:56:24 GMT
NNTP-Posting-Date: Wed, 23 Oct 2002 00:56:24 PDT
In article <firstname.lastname@example.org>,
email@example.com (Rileyesi) wrote:
>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.
Do the math for what you have now. Vo=Vin * (1 + 0/0). There's no
solution and you're shorting the op-amp's output.
That should be R1=infinity, R2=0.
Vo=Vin * (1 + 0/infinity) == Vo=Vin * 1
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