# How to properly Bias a Opto Coupler

I had some problems setting up a Opto Coupler today. The data sheet is here. I set it up putting a 470 ohm resistor to ground from the cathode (pin 2). I am understand the selection of this resistor as it is used to set the current through the LED to about 10mA when he ANODE(pin 1) is given 5 Volts. What I don't understand is why a 270 Ohm resistor(as shown in the test circuit) is needed between pin 4 and 6. From testing I know that with the 270 resistor I get a range of about .2 to 5 volts on the output, and without the outputs range is 0.08 to 1.4 volts. I know its something to do with the biasing the circuit and the gains from the internal transistor... Wondering if someone could point me towards a source that actually explains how to bias a opto coupler... its been a few years since I played with transistors.

The purpose of 270 $\Omega$ on the test circuit is simply to provide a defined amount of current for the output logic to sink during the test. Whatever pull-up value you use is up to you, just don't exceed the sinking capability of the output stage.