help on the classic differential amplifier LM 339 (opamp)

[Morten]

Hi, I am using the LM339 in a circuit like shown below. The LM339 is a
single supply opamp.

_____R________
| |
| ------- |
V1 --R-------|- - |
| -------------- Vout
V2 --R-------|+ -
| -------
|
R
|
gnd


Opamp supply: +12 V and ground.
V1=0
V2=6V
R=100 kohm

With these values I get Vout = 0V!!! . I expected vout to be equal V2
because of unity gain. What am I doing wrong. Is it the LM339 that cant be
used in the circuit?

 

[Anthony Fremont]

"Morten" <2583060m001@mail1.stofanet.dk> wrote

Hi, I am using the LM339 in a circuit like shown below. The LM339 is a
single supply opamp.
With these values I get Vout = 0V!!! . I expected vout to be equal V2
because of unity gain. What am I doing wrong. Is it the LM339 that
cant be
used in the circuit?

First off, don't post ascii art unless it's readable using a fixed font.
Secondly the 339 is not an op-amp per se, it's a comparator; there is a
difference. Finally the output of the 339 is open-collector, you need a
pullup resistor on it. Read the datasheet.