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'[EE]: Voltage Un-Follower Op-Amp'
2003\05\17@233427 by Lim KK

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Hi,

I have configured my op-amp LM324 as a  voltage follower by typing the -ve
input to the output of 1 op-amp gate. When I tie the +ve input to ground,
the output is 0v. But when I tie the +ve input to Vcc(same as the +5v supply
to the LM324), the output is just 3.8v. Is there something I missed out?

Thanks Mark

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2003\05\17@234518 by Tom Messenger

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At 03:31 AM 5/18/03 +0000, you wrote:
>Hi,
>
>I have configured my op-amp LM324 as a  voltage follower by typing the -ve
>input to the output of 1 op-amp gate. When I tie the +ve input to ground,
>the output is 0v. But when I tie the +ve input to Vcc(same as the +5v supply
>to the LM324), the output is just 3.8v. Is there something I missed out?

Maybe.
Get the data sheet.
Look at the specification for  "output range" or "output swing".

Lm324's positive output swing will go to around 1.5 Volts less than the
positive rail.  If you need your buffer to go "rail-to-rail", then look for
a CMOS op-amp with rail-to-rail output swing.  There are many available
these days.

>
>Thanks Mark

No problem
Tom M.

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2003\05\18@002745 by Lim KK

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Thanks, I've look around but could not find one that uses Gnd and Vcc
instead of +/- Vcc. Is there any common and popular rail-to-rail opamp
(including those +/-Vcc ones)?

Thanks


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2003\05\18@004440 by Marcelo Puhl

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       LM358?

       TLC2274?

On 18 May 2003 at 4:26, Lim KK wrote:

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2003\05\18@012715 by Dwayne Reid

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At 03:31 AM 5/18/03 +0000, Lim KK wrote:
>Hi,
>
>I have configured my op-amp LM324 as a  voltage follower by typing the -ve
>input to the output of 1 op-amp gate. When I tie the +ve input to ground,
>the output is 0v. But when I tie the +ve input to Vcc(same as the +5v supply
>to the LM324), the output is just 3.8v. Is there something I missed out?

The LM324 input and output operating range is Gnd to (Vcc - 1.5V).  What
you are seeing is what I'd expect to see if you were operating the LM324
from the 5V rail.

The solution is simple: run the LM324 from the unregulated supply.

dwayne

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2003\05\18@084442 by fred jones

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Take a look at the MCP601 from, you guessed it, Microchip.  It uses +Vcc (to
5.5v) and gnd.  It's also rail to rail.
Good luck,
FJ

>Thanks, I've look around but could not find one that uses Gnd and Vcc
>instead of +/- Vcc. Is there any common and popular rail-to-rail opamp
>(including those +/-Vcc ones)?
>
>Thanks
>

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2003\05\18@105902 by Olin Lathrop

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> I have configured my op-amp LM324 as a  voltage follower by typing the
> -ve input to the output of 1 op-amp gate. When I tie the +ve input to
> ground, the output is 0v. But when I tie the +ve input to Vcc(same as
> the +5v supply to the LM324), the output is just 3.8v. Is there
> something I missed out?

Apparently.  The LM324 is not a rail to rail output opamp.  The specs
don't exactly say outright how close Vout can get to V+, but with 30V V+,
Vout is only guaranteed to be >= 27V with a 10Kohm load.


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2003\05\18@110114 by Olin Lathrop

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> Thanks, I've look around but could not find one that uses Gnd and Vcc
> instead of +/- Vcc.

Note that this is just a specsmanship issue.  The opamp just sees a
voltage between V+ and V- and doesn't care whether you think of V- as
being ground or not.  It really has to do with how low the common mode
input range extends, and how low the output voltage can go with respect to
V-.


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