[circuit design] open collector OC output of triode

What is open collector ？

Transistor's OC（Open Collector） Output is also called open collector gate , As the name suggests, it is collector The pole remains open .

The principle of open collector

OC Circuits and OD Circuit similarity , Mainly used to realize Line and 、 Line or The logical function of , Sometimes it is also used as current output .

① When input Input is 0,Q1 end ,Q2 the R2 Pick up 5V Conduction , Output 0V.
② When input Input is 1,Q1 Conduction ,Q2 end ,C Point hanging in the air , It's a high resistance state .

It can be seen that , after Q1 Output Vo1 Will be with Vin Reverse phase , and Q2 Of OC Output Vo2 Will be with Vo1 Reverse phase . Quadratic inversion , Final Vo2 And Vin The phase of remains the same .

The functions completed by this circuit are familiar , Similar switch .

The logical meaning of open collector

If you want to complete basic logical functions , Need to be in output Connect the high level through the pull-up resistor . The pull-up resistance value is usually 1K-10K.

How big is the pull-up resistance ？ If the maximum current of the triode is 100mA, Large pull-up resistance makes the current as small as possible . If it flows 1mA, You can choose 5.1K resistance .

In the above circuit , triode OC The output is connected to the high level through the pull-up resistor , Grounding through load .

① When input When you are high ,Q1 Conduction ,LED Extinguish ;
② When input When it's low ,Q1 end ,LED Lighten up .

Open collector and comparator

In common comparator circuits , Usually used OC Output .

Typical comparator ：LM239 Single power comparator
max rating 2V-36V Wide power supply

The comparator is used to compare the voltage at the positive and negative input terminals .
Vin+ > Vin-, High output ;
Vin+＜Vin- , Low output ;
Avoid equal ends , Otherwise, it will oscillate .

Reference article

1. How to understand drain open drain output and push-pull output
2. Open drain output and push-pull output