Electronic – How to amplify and offset the voltage in an opamp


I need to amplify a voltage, which comes from a DAC and is 0-2.5V and scale it 0-25V in an operational amplifier. After amplified, I need to offset it -1.25V, so the final voltage is -1.25 to 23.75V. In other words, being an operational amplifier, I want to do the y=ax+b operation. with a=10, and b=-1.25V

I thought of 2 different ways of doing this.

  1. The first idea I had is to use a second amplifier as a summing amplifier, with a fixed -1.25V voltage.

  2. The second idea is to use a 1.2V zener in series with the output, along with a biasing resistor to -V.

Why do I want this? I want to set the ADJ pin in an LM338 voltage regulator from an MCU. LM338 out is ADJ+1.25V. If ADJ is -1.25V then the output is 0V.

Best Answer

You can use a differential amplifier

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As you can see from the shown equation VR is just an offset and doesn't get amplified so set the resistors for a gain of 10 , connect GND to V1 and -1.25 to VR

The reply here shows an alternative solution that doesn't need a 1.25v reference but it needs a negative rail voltage to supply the opamp