Created by Created December 10, 2012 Last modified June 11, 2013 Tags

### Summary

A very simple model of a typical optocoupler using an
opamp as a transimpedance amplifier.

Simulate > DC Sweep > Run DC Sweep

### Description

A very simple model of a typical optocoupler using an opamp as a transimpedance amplifier..

More complex models take account of the non-linearity of the LED optical power o/p vs current characteristics and may have detailed modelling of rise and fall times and propagation delay.

Note that, although D1 provides a load that represents an LED, in terms of voltage drop, resistance and junction capacitance etc., the LED model has no optical parameters. The only coupling to the receiver is via the behavioural current source Ictr1 monitoring the current through D1.

Ictr1 is the (LED current)*(the datasheet CTR value)/(Q1_hFE)

In this example CTR=60%

D2 has no optical effect. It is present purely to model the reverse leakage (dark current) and junction capacitance effects of a photodiode.

Note that the transimpedance: V(out)/I(in) set by:

V(out) = -I(in)*Rfb

is a bit too high and the opamp output hits the -5V rail before the LED current reaches its maximum.

Cfb is to compensate for the photodiode capacitance.

Simulate > DC Sweep > Run DC Sweep

https://www.circuitlab.com/circuit/bdxks2/optocoupler-01/

https://www.circuitlab.com/circuit/95w75k/optocoupler-02/