Occasionally it is desired to have a single-supply non-inverting op amp circuit with some fixed, arbitrary offset.
In general, this gives an interaction between the gain and offset voltages which makes it awkward to calculate the desired resistor values. And the calculations may generate non-standard resistor values.
Here's a circuit that allows easy calculation of the offset with the offset voltage determined by standard resistor values and the offset resistor value doesn't affect the gain.
The limitation is that the gain must be fixed at +1 for this simple relation to be valid.
The LTspice simulation of the circuit for a positive offset is below.
The offset is determined by the value of R6 and R1 where R1=R2.
The simulation shows offset values of +1V and +2V for R1=R2 values of 220kΩ and 100kΩ with a 0 to 4V ramp input.
As can be seen, the offset value changes with the value of R1=R2, but the gain stays constant at Av=+1.
If you want a more stable offset than provided by the supply voltage, replace V+ with a reference voltage at R2.
Below is the simulation for a negative offset.
The output shows offsets of -1V and -2V for R1 = R2 values of 120kΩ and 240kΩ.
In general, this gives an interaction between the gain and offset voltages which makes it awkward to calculate the desired resistor values. And the calculations may generate non-standard resistor values.
Here's a circuit that allows easy calculation of the offset with the offset voltage determined by standard resistor values and the offset resistor value doesn't affect the gain.
The limitation is that the gain must be fixed at +1 for this simple relation to be valid.
The LTspice simulation of the circuit for a positive offset is below.
The offset is determined by the value of R6 and R1 where R1=R2.
The simulation shows offset values of +1V and +2V for R1=R2 values of 220kΩ and 100kΩ with a 0 to 4V ramp input.
As can be seen, the offset value changes with the value of R1=R2, but the gain stays constant at Av=+1.
If you want a more stable offset than provided by the supply voltage, replace V+ with a reference voltage at R2.
Below is the simulation for a negative offset.
The output shows offsets of -1V and -2V for R1 = R2 values of 120kΩ and 240kΩ.
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upload_2017-4-2_14-1-34.png -
Diff Amp Single Supply with Pos Offset.asc
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Diff Amp Single Supply with Neg Offset.asc
