# Differential Amplifier

Discussion in 'Homework Help' started by hncelectronics123, Jun 4, 2008.

1. ### hncelectronics123 Thread Starter Member

May 23, 2008
10
0
Hello Guys
Any help on the folowing question would be much appreciated.

I have a wheatstone bridge with an output of 23.3mV and require a differential amplifier with a voltage gain of 30v to achieve an output of 4v.

I have been using multi-sim 9 to simulate this operation but keep on having no luck. I'm aware that the surrondung resistor values should be of a calculated value but this is where i'm stuck.

This may be of help
12.50v from the resistors without the strain guage.
The part where my strain guage is i get an output of 12.477v

As you can see the voltage voltage difference is 23mV

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Can you post your schematic as we can see which circuit to you use?

3. ### hncelectronics123 Thread Starter Member

May 23, 2008
10
0
Hey yeah sure.

Ignore the wheatstone bridge, on the (- input) of the amplifier i have 12.477v. and on the (+ input) of the amplifer i have 12.50v.
Also do i need the 2 x resistors inbetween the amplifier and the bridge circuit? Thanks

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63

Use this circuit as your differential amplifier and then connect the wheatstone bridge. I suggest you use the second circuit because it wont affect the operation of the wheatstone as much as the first circuit but the first circuit will be good if you use high values for the resistors on the op-amp.

The formula for the gain is :

Vout/Vin=Gain=V2*((1+R2/R1)/(1+R3/R4))-V1*R2/R1

where R1 is the top left resistor
R2 is the top right
R3 is the bottom left
R4 is the bottom right

5. ### hncelectronics123 Thread Starter Member

May 23, 2008
10
0
Hey thanks for the help.
Clarified the voltages on Multi sim and all checked out well. Cheers