opamp voltage converter question

[Tom Del Rosso]

A simple level converter. It assumes the +4V supply can sink current, so
why not just have 2 resistors between +4 and +10 and use the opamp as a
buffer?


+10V
|
|
R
|
|
|----------
| |
| |
R |
| |
| |
GND |
|
|
|---- +

------- +7V out

+4V -----R------------- - |
| |
| |
|------2R-----|

 

[Tom Del Rosso]

There are three different ways to do this with just two resistors.

I know what you mean. This was just someone's idea of an example but he
should have thought of a better one. I wondered if there was any
justification for using 4 resistors.

 

[Tom Del Rosso]

The op-amp version has a -2 voltage gain for variations in the +4
line to the output. If you want that, you can't get it with passive
components.

Yes, I know you can't get gain with passive components.

I wasn't suggesting resistors with no opamp. I just wanted to know if the
4-resistor approach had any advantage over the obvious 2-resistor
equivalent.

In the OP I said 'use the opamp as a buffer', so do you mean that it would
have no gain (unity gain) with that approach?

 

[Tom Del Rosso]

If you use a pair of resistors to get 7V out of the 10V and the 4V,
then when the 4V changes by dV, the output will change by
significantly less than dV and in the same direction.

If you use the setup you show and the 4V changes by dV, then the
output will change by -2*dV. If what you need is to pick out and
amplify some signal that's riding on the 4V, then the pictured
circuit would be the way to go.
Thanks.


You don't say what the circuit is from or what you want to use it
for, so it's impossible to answer your "why not". My suggestion was
just that -- one possible reason "why not", which you then have take
and exercise your own brain cells to see if it makes sense or not.

It was somebody's example of a 'basic opamp circuit' so I wondered if there
was any reason to do that other than making an imaginary circuit.