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I'm using this circuit to measure the RPM of a DISC rotating at 24000 rpm. This circuit is working fine. But the problem is with amplitude. It should be of 4.2V, but actually is of 2V as shown in picture below. The question is how to increase the amplitude upto 4.2V.

The op. amp. is an LM324A

enter image description here

enter image description here

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    \$\begingroup\$ You have omitted the part number for U2A and a link to its datasheet. Please edit the question and add it in. You might also explain why you're using an op-amp rather than a comparator. \$\endgroup\$
    – Transistor
    Mar 28, 2021 at 15:02
  • \$\begingroup\$ Yes it's LM324A \$\endgroup\$
    – mastermind
    Mar 28, 2021 at 15:06
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    \$\begingroup\$ That doesn't explain why you aren't using a comparator. Anyway, the LM324A is not suited for this, not when running it off of 5 volts. \$\endgroup\$
    – Hearth
    Mar 28, 2021 at 15:11
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    \$\begingroup\$ A single supply op-amp with rail to rail outputs might work, but for this application you should use a comparator. It is a classic comparator application. \$\endgroup\$
    – mkeith
    Mar 28, 2021 at 16:01
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    \$\begingroup\$ You can add a pull up to the output (pin 1 to Vcc) to get a bit more voltage. \$\endgroup\$ Mar 28, 2021 at 16:25

3 Answers 3

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From the comments:

... in the case of comparator the issue is of level adjustment. As this circuitry can be repeated upto 100000 times in the industry.

No. Many op-amps latch up when driven to the supply rails. Comparators are designed to be driven to the supply rails. Repeating a mistake a million times won't fix it.

enter image description here

Figure 1. From the datasheet we can see that with a 30 V supply driving a 2 kΩ load the max output voltage will be 26 V. They don't specify what it will be with a 5 V supply.

Most comparators feature an open collector or open-drain output and with the addition of a pull-up resistor full supply voltage can be achieved on the output.


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    \$\begingroup\$ A different data sheet does specify Vout for LM324A with +5v DC supply (and 2k load)....**typically 3.5V**. A fast rail-to-rail opamp might do better. \$\endgroup\$
    – glen_geek
    Mar 28, 2021 at 16:11
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To add to @Transistor's answer:

The On Semiconductor LM324 datasheet (On Semiconductor) has the following specification with a 5V supply. As you can see the typical maximum output is only 3.5V.

This is because the output stage of the LM324 consists of a double emitter follower giving two VBE drops.

There are many newer opamps that can get within a couple of hundred millivolts of the supply rail, these are referred to as rail-to-rail output.

There is however another issue that is preventing the high output going above 2V: The negative feedback from the opamp through D2 and R33 will force an equilibrium with no light present that looks to be in that 2V region, the opamp never gets to saturation. You don't explain what that feedback is supposed to do. If it is intended to be an adaptive bias level compensation then the resistors values, eg R28, need to be changed. Or that circuit removed and bias the photodetector directly from 5V through a resistor. Or you can just follow the opamp by a comparator to create a proper logic level. enter image description here

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  • \$\begingroup\$ Interesting that they were seeing only 2 volts, then. I wonder if there's more to the problem than I thought; I didn't bother looking up a datasheet, since I already knew that the 324 isn't rail-to-rail. \$\endgroup\$
    – Hearth
    Mar 28, 2021 at 16:20
  • \$\begingroup\$ @Hearth - Good point I'll expand my answer. \$\endgroup\$ Mar 28, 2021 at 16:29
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By shifting the +Ve reference level from 0.88V to 1.68V solved my problem. This is achieved by changing the resistance R29 from 10k5 to 25k. Thanks everyone for helping.

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