Op Amp Design

Hi All,

I'm looking to find the best way to create variable voltage output dependent on a variable voltage input. It's a special DC to DC.

I'd like to have 0-5V input track with a 1.5v - 7.5v output. Meaning when at 0V input the output will be 1.5v. When the input is 5v the output must be 7.5v.

Any thoughts on the best way of doing this with op amps? Or is there a better way?

What supply voltage(s) do you have?
How much current does the 1.5V to 7.5V output have to supply?

An opamp can do it with good linearity and accuracy, however, the stability of the DC offset is only as good as the opamp's voltage reference. If that reference is created by a voltage divider from the opamp's power supply, then any variation of the offset goes directly to the opamp output. If this is critical, then use a voltage reference IC to create the required offset at the input of the opamp.

Here's a simple way if the output load is only a few mA. But, as Mike points out, if stability and accuracy are important then a better voltage reference (for the 1.5V offset) would be needed. R2 and R3 can be trimmers to adjust the offset and gain respectively.

View attachment Offset&Scale.asc

I have +5V and +12V supply voltages available to me.

The output current for the 1.5v-7.5v is in the very low milliamps. It's only a control voltage for a backlighting of LED's.

Let me take a look at the circuit. Thanks!!!!

As for the DC offset I just thought that was the difference between the + and - terminals usually in very very low voltage difference. To account for this a offset voltage is used correct? Could someone explain a little more on that and how it applies to what I'm doing.

oops. I don't have LM324 for LTspice. How did you use it!?!

I'll just find one that's equiv in the library. Thanks again.

I think I got the circuit figured out.

1. First stage is just a buffer.

2. 2nd stage is a non-inverting op-amp amplifier where R2 and R1 are providing a 1.25V dc bias.(just a voltage divider)

3. The gain is set by 1 + R4/R3 ~=1.25V

4. Multiply 1.22 by 1.25 = 1.5 volts

I love this forum. Thanks for the help!!!!!

As for the DC offset I just thought that was the difference between the + and - terminals usually in very very low voltage difference

Click to expand...

Yes, that's the opamp input offset; but the offset I was referring to was the circuit output relative to the circuit input. Here, 'offset' = 'bias', provided by use of R1 and R2 (hence depends on the supply voltage). Your figuring out is correct. The buffer could be eliminated, but I threw it in for good measure as the LM324 is a quad op-amp. The Spice model for the LM324 is in the libraries downloadable from the Yahoo LTspice user-group.