Dual Comparator Circuit

Question

I've received a board with the following circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The INPUT is a digital signal coming from a MCU, with HIGH value being 3.3V. Based on received INPUT, the circuit should operate in two states:

• STATE 1 : When INPUT is HIGH, OUTPUT1 is HIGH and OUTPUT2 is LOW
• STATE 2 : When INPUT is LOW, OUTPUT1 is LOW and OUTPUT2 is HIGH

OUTPUT HIGH = -7V
OUTPUT LOW = 0V

The circuit was tested and controlled by a MCU, worked fine in both states. However, when we tested another MCU, we started with STATE 1, and then driven the INPUT LOW, but it didn't switch to STATE2. In fact, it kept holding in STATE 1. And it only goes to STATE 2 when we touched the input channel with Voltmeter. We tried to add a pulldown circuit but that didn't solve the case.

Q1 & Q2 are BC516-D27Z

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EDIT:

1. Updated with the correct circuit schematic.
2. Added OUTPUT HIGH/LOW values.
3. Both MCUs are the same.

Answer 1

AD820 is an op-amp, not a comparator. It's a rather lousy comparator in terms of speed, and for this application a simple LM393 powered from +/-7V would work much better.

If the circuit is as you have drawn, then you're using the PNPs with collector and emitter swapped. They will operate with a very low current gain. That's not what you want. If that's how the circuit was assembled, then that's your problem. Flip the transistors upside down and things will start to work.

Like this:

^{simulate this circuit – Schematic created using CircuitLab}

The output is indeed complementary:

But all this seems like a whole lot of work for little gain. This same circuit could be put together with a couple MOSFETs in place of the op-amp. To retain the spirit of the original circuit, I've decided to leave two BJTs there :)

^{simulate this circuit}

The cost is 5x lower than that of an AD820. Instead of using 6 discrete MOSFETs, this could also be a single chip - CD4007, for the same cost, but higher level of integration.

The output switches in about 100ns, and is driven push-pull, i.e. without pull-ups. The static current consumption of this circuit is about 0.2mA.

For an even lower cost, an LM393 circuit will do the job:

^{simulate this circuit}

The output switches reasonably quickly still:

Answer 2

I ran the simulation, and also added 1k resistors to the op-amps' outputs. Here is a plot of the DC sweep from 2V to 4V:

I see that I don't know how to do a DC sweep. I tried V(INPUT) but that gave me an error. And now CircuitLab has locked me out again.

And I see that I need to add a voltage source as @KubaHasntForgottenMonica has in their answer, but I can't use CircuitLab anymore.