Offset voltage on voltage follower

From: Tuurbo46 (alex_at_beale55.fsnet.co.uk)
Date: 08/20/04 

Date: Fri, 20 Aug 2004 13:16:41 +0100

Hi
I need a bit of help on a circuit im trying to build. I will try to explain
my problem below.

Circuit requirements:

*input to op-amp = 0 to 5v
*output from op-amp = 0.5v to 3.5v

Im am currently using a CA3140E opamp with a 0.5v offset in it, which is
quoted in the data ***.

Pin wirring on chip:

Pin 1: to left leg of 10k pot
pin 2: connected to pin 6 with no series resistor
pin 3: varible input voltage, 0 to 5v
pin 4: to middle leg of above 10k pot and connected to GND
pin 5: to right leg of above 10k pot
pin 6: connected to pin 2 with no series resistor, and then this voltage is
dropped across 2 resistors in series to produce the output voltage ratio

Im at the stage in the circuit, that when you increase the input voltage
from o to 5v, the output is alway 0.5v higher than the input which is
correct. At this point i drop the output voltage over a potential divider
and i get the correct output voltage ratio over this. The problem starts
when a 1k load resistor is taken from the potential divider. At this point
the voltage drops. When the load resistor is removed the circuit produces
the correct output voltage again.

I think the problem is the circuit cant provide enough corrent, but i cant
see how to change this.

Any ideas?

Or does anyone have any better circuit ideas?

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