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A strange problem with LM324

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aibelectronics

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I connected one of the 4 opamp sections of the LM 324 in the buffer/unity follower configuration.

I noticed that for relatively low voltage levels the output of my buffer was in sympathy with the input. But as I started increasing the input ( I once connected the input directly to the 5v Vcc line) the output was stubbornly clamped at 3.81 volts, and increase in input wasn't reflected in output.

I tried the other opamp sections as well.

What could be the reason for this?
 
aibelectronics said:
I connected one of the 4 opamp sections of the LM 324 in the buffer/unity follower configuration.

I noticed that for relatively low voltage levels the output of my buffer was in sympathy with the input. But as I started increasing the input ( I once connected the input directly to the 5v Vcc line) the output was stubbornly clamped at 3.81 volts, and increase in input wasn't reflected in output.

I tried the other opamp sections as well.

What could be the reason for this?

I suspect you're only powering the chip with +5V supply. Checkout the datasheet. The specification for the "Input Common-Mode Voltage Range" is 0 to Vcc-1.5V. That means if the supply voltage is +5V, the maximum voltage at the inputs is 3.5V for it to operate correctly.

The reason it is limited to this voltage limit is obvious if you look at the internal schematic diagram of the chip. The each input use two PNP transistors. The transistor bases have to be forward bias to operate. That requires approximately 0.6 volts each for a total of 1.2V drop from Vcc.
 
Yes, that should help. There will still be the 1.2v drop, but outof 15v it is a lot less significant. Alternatively, you could use an 074/084 quad op-amp, with its FET inputs I assume it has virtually no drop :?:
 
One other little known behavior is that if the input of your follower goes more than 0.3 Volts below the negative rail, the output will swing to the positive rail minus the 1.2 to 1.5 volts previously mentioned. If all the signals are confined to your board this should not be a problem. If your inputs are coming from an external source, an input clamp diode might come in handy.
 
Dr.EM said:
Alternatively, you could use an 074/084 quad op-amp, with its FET inputs I assume it has virtually no drop :?:
The inputs can work at the positive supply voltage (good for measuring supply current through a resistor) but the negative common-mode voltage rating is 3V above the negative supply. If the input voltage goes lower than the negative common-mode voltage limit then the output suddenly inverts. Try overdriving one as a follower for a very odd sound effect.
 
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