# Is it possible to have active low window comparator using open collector output opamps?

My textbook uses opamps with active pullup and diodes:
$$\A1\$$ drives the load when $$\v_{in}\lt LTP\$$
$$\A2\$$ drives the load when $$\v_{in}\gt UTP\$$
None drives the load when $$\LTP\lt v_{in} \lt UTP\$$

There is a serious limitation with this as it cannot drive loads with different supply like TTL.
So I'm wondering if this can be done using open collector opamps. Any help?

Btw I know how to work the active high output window comparator using open collector opamps. It is the active low output as shown in the diagram I'm not able to get..

• You could use open collector comparators. – Peter Smith Apr 1 at 12:52
• I want to use basic open collector opamps. This is fixed. Logically it seems I need to make OR gate using open collector inputs. Looks it is impossible? Because open collector can only ground the node, so only AND gate is possible with pullup resistor? – across Apr 1 at 12:57
• There is little difference between an opamp and a comparator (a comparator is not internally compensated) and I am not aware of any open collector / drain opamps; why make your life more difficult than needed? – Peter Smith Apr 1 at 13:05
• @PeterSmith actually I don't want to make anything haha! I'm only wondering if it were impossible to get active low output – across Apr 1 at 13:08
• @PeterSmith something like this prnt.sc/rqujpc – across Apr 1 at 13:11

• Open collector opamps don't exist as an opamp needs to have an output that can be driven low or high.

• Open collector comparators do exist, the LM393 is a popular example.

As mentioned in the comments, with open collectors outputs and diodes you can only make an OR function. We can still use something similar as can be seen in the schematic that I made:

simulate this circuit – Schematic created using CircuitLab

Note that I have drawn the open collector comparator explicitly as a comparator with an NPN. That is similar to what is inside for example the LM393. So Comp1 and Q1 would be on the same chip.

Now the output signal will be high when Vin is within the window.

Just add an inverter and you will get a signal that is low when Vin is inside the window.

• Clever! But why do we need those diodes ? Can't we just short the bjts' outputs and hook directly to the pullup resistor? Your circuit is functionally equivalent to this prnt.sc/rqujpc right? – across Apr 1 at 13:33
• Can't we just short the bjts' outputs and hook directly to the pullup resistor? Indeed we can. The diodes aren't needed. I left them in to make the circuit look more similar to the one from your question. – Bimpelrekkie Apr 1 at 14:07
• I see thank you so much:) By inverter hopefully you mean a pulldown npn with a pullup resistor... – across Apr 1 at 14:11
• By inverter hopefully you mean a pulldown npn with a pullup resistor. Yes or an inverter that is inside an IC for example the 7404, it has 6 inverters. – Bimpelrekkie Apr 1 at 14:32