Re: Offset voltage on voltage follower

From: Robert C Monsen (rcsurname_at_comcast.net)
Date: 08/19/04 

Date: Thu, 19 Aug 2004 23:43:56 GMT

"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:8h3ai0trhsdiv616bpbm501qjq6cr2n0pk@4ax.com...
> On 19 Aug 2004 19:05:09 GMT, cfoley1064@aol.com (CFoley1064) wrote:
>
> >Summing Op Amp Level Shifter
> > Vin(0-5V)
> > ___
> > o-|___|-o---.
> > 2K | |
> > 3K.-. | .----------.
> > +5V | | | | VCC |
> > + | | | | + |
> > | '-' | | |\| |
> > .-. | | '---|-\ |
> >18K| | === | ___ | >---o
> > | | GND '-|___|--o---|+/ Output (0.5V to 3.5V)
> > '-' 33K | |/|
> > | ___ | VEE
> > o----------|___|--'
> > | 33K
> > .-.
> > 2K| |
> > | |
> > '-'
> > |
> > ===
> > GND
>
> This looks wrong. It boils down to:
>
> 34.2k
> 0-3V V1 O-----/\/\/----,
> |
> 0.5V V2 O-----/\/\/----+-----> V3, to opamp + input
> 34.8k
>
> That should yield:
>
> V3 = (V1*34.8k + V2*34.2k) / (34.2k+34.8k)
> = 247.83mV + 0.50435*V1
>
> at the + input. I'd tend to guess, then, that the output would vary
from about
> 245mV to about 1.76V.
>
> Not quite the desired 0.5 to 3.5V.
>
> Jon

First, scale the input to 1V using a voltage divider. Then, use a
non-inverting amplifier with a gain of 3 and a range of 0.5 to 3.5.

                 .-----------------.
                 | |
                .-. |
     Vin | | 2k |
    0-5V | | | |
       | I | '-' |
      .-. V | |
   4k | | | |
      | | | |\ |
      '-' o----|-\ |
       | | | >---------o--- Vout
       o---------|----|+/ 0.5 to 3.5V
       | | |/
       | |
      .-. .-.
   1k | | | | 1k
      | | | |
      '-' '-'
       | |
      GND Vx = -0.25V reference... :?

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Since Vin goes from 0 to 5, V+ goes from 0 to 1. Then, due to the
negative feedback, V- goes from 0 to 1.

At Vin = 0, we have V- = 0, so I = .25/1k = 250uA. Thus, Vout = 2k *
250u + 0V = 0.5.

At Vin = 5, V+ = 1, so I = 1.25/1k = 1.25mA. Thus, Vout = 1.25m * 2k +
1 = 3.5V

The problem, of course, is manufacturing the -1/4 V low impedance
reference.

Regards,
 Bob Monsen

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