R2 can be increased in value to increase the output voltage in this configuration, the tradeoff being some increase in the distortion. If you have a high impedance load (>1.5KΩ) and are satisfied with an output voltage of about 1/2 the input (depending upon the opto transfer gain), then U5 may not be needed and you can use the voltage across R2 as the output. Op amp U5 is a buffer/non-inverting amplifier that amplifies the voltage across R2 to convert the opto isolator output current back to a voltage.
.jpg "optical isolator 4n25")
The opto is operated Class A with about 1 mA DC bias current.
Optical isolator 4n25 driver#
Op amp U3 is configured as a current driver for the opto input, generating a current proportional to the voltage input. (Ocom is the isolated output common and Rsim is just to allow the simulation to run with an isolated output ground). This eliminates the non-linear LED input voltage to current relationship, with the opto isolator output current typically being quite linear with respect to the input current, especially for low current levels.īelow is the LTspice simulation of such a circuit using the 4N25, a common and representative general purpose opto isolator.
There are special purpose (and relatively expensive) optical isolators available with separate feedback connections designed for high analog linearity and fidelity but, for non-critical AC applications, a standard opto isolator may be adequate if it is driven by a current source instead of a voltage.
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