# Vary voltage in synchrony with other voltage?

#### cupoftea

Hi,
We have a voltage which will vary from 2v to 5v
When it does this, we want another voltage to go from 1.76V to 0.7V.
(just signal voltages)
(linearly)
Is the attached the lowest component way for this?
LTspice and jpeg attached

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This would be approach, attached.

Regards, Dana.

#### Attachments

• sloa030a.pdf
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• cupoftea

Simple Math leads to simple design.
In = 2 to 5V
Out=1.76V to 0.7V
Gain = (1.76-0.70) / (2 - 5)= 1.06/-3 = -0.35333
Offset= Mean (out-in) = {(1.76+0.7)-(2+5)}/2 = -2.27V
Using a Differential Amplifier with negative gain = -0.3533 and offset of -2.27 using +Vref= 1.82V = (1+Av-)*5V

Choose R values and tolerances to meet your expectations. (design specs)

Last edited:
• cupoftea and KerimF

Is the attached the lowest component way for this?
A single transistor can do the job although to adjust values is a trial-and-error task.
NPN inverts the slope of the incoming signal.
Is your power supply 5V? It can be dropped via zener/led/diode string. Is the attached the lowest component way for this?

Not quite.
How about with four fewer op amps, six fewer resistors, and only one voltage: If interested, here is a procedure for determining the component values for the circuit in post #5.
This is basically a variation of the procedure posted in post #3.

• The inverting gain must equal -0.35333 as D. A. calculated in post #3 (the closest value of -0.3549 generated here by the values of R1 and R2).
• This output for a 2V input would then be 2 * -0.3549 = -.7099V (without any added offset)
• Since +2V input should give +1.76V at the output, a positive output offset of (+1.76V + 0.7099V) = +2.47V is thus needed.
• The non-inverting gain at the (+) input is (1+ 0.3549) =1.3549 so a voltage of (+2.47V / 1.3533) = +1.823V is required at the (+) input, which is generated by the voltage divider R3 and R4 from the +5V supply.

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