When I try to simulate a circuit I designed in Proteus, I'm getting an unexpected output from the op-amp.

As you can see from the image, for op-amp U1:A, the inverting terminal voltage is about 3.25V while the non-inverting terminal voltage is set to 2.5V reference.

Hence as the inverting terminal voltage is greater than the non-inverting terminal voltage, the output should be -VSat i.e. 0V.

However, I'm getting 3.72V as output.

Can you guys please tell me where I've made my mistake? The op-amp is an LM324 as a single rail op-amp. ( +Vsat = 5V, -Vsat = 0V.)

The expected inputs for JK F/F are 0 1. Which means output of U1:A should be 0V and U1:C should be 5V (Logic 1.)

Reference Image:

  • \$\begingroup\$ LS TTL requires a couple of milliamps to pull the input low. R16 won’t let that happen. \$\endgroup\$
    – Kartman
    Commented Feb 10, 2021 at 7:47
  • \$\begingroup\$ Can you suggest any modification in the current circuit. What value of resistor should be used ? \$\endgroup\$
    – Mihir
    Commented Feb 12, 2021 at 6:15
  • \$\begingroup\$ For your own education, I suggest you read the datasheet for the 74ls08 and find the spec for input low current. You can then apply Ohm's law to solve for resistance. Or chose a 74HC08 and avoid the issue. \$\endgroup\$
    – Kartman
    Commented Feb 12, 2021 at 9:45

2 Answers 2


Can you guys please tell me where I've made my mistake? The op-amp is an LM324 as a single rail op-amp.

You are measuring AC volts not DC volts: -

enter image description here

This means that your analysis is invalid.


In addition to not setting up the measurements correctly, you're using an op-amp as a comparator. Not the best idea, since most op-amps suck as comparators, and LM324 is no exception. Use an actual comparator, like LM139/239/339, LM2901, etc.

The op-amp is an LM324 as a single rail op-amp. ( +Vsat = 5V, -Vsat = 0V.)

That's not even remotely close to being true. There is no single "saturation voltage". The input and output voltage ranges are separate specifications, and the output voltage range varies with load impedance as well.

The input voltage range of an LM324 is 0..(Vcc-1.5V). The output voltage range with 2-10kOhm of load is 0.02..(Vcc-2V) across temperature. That's not good enough to drive a TTL load. You'd need to power that op-amp with 6.5-7V to drive a TTL load reliably.

I highly recommend that beginners do not use op-amps as comparators. There are gotchas to such use that can bite even experienced engineers. No free lunch. Use comparators when you need comparators. LM339 is dirt cheap. It can drive TTL no problem. The datasheet and separate application note shows how to drive various kinds of loads.


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