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I have an issue with Op Amp LM324.

First, I supply with this, and everything is work like op amp. I have output twice than input.

schematic

simulate this circuit – Schematic created using CircuitLab

But, when I use different source, the output is equal with input.

schematic

simulate this circuit

Then I change the schematic. Output has twice than input.

schematic

simulate this circuit

Does Op Amp need supply + and -, and cannot be + and ground? Thanks for your answer.

EDIT QUESTION AFTER 4 HOURS

Sorry it was my mistake. I didnot draw correctly.

Oke, let me clear this. This wiring has output 6 Volt.

schematic

simulate this circuit

Why this one has output that same with input?

schematic

simulate this circuit

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  • \$\begingroup\$ Your circuits are quite wrong, and the behavior you're getting from the simulator is due to something specific in the op-amp model (like its estimate for input bias current). We have no assurance that the output will be 6V from the first circuit. An ideal op-amp model doesn't give us that, and neither do specific real op-amp assumptions. \$\endgroup\$
    – Kaz
    Commented Feb 20, 2016 at 3:22
  • \$\begingroup\$ Suppose that the inputs have a very high impedance. Current doesn't flow (much) through R1, and so the voltage on both ends of R1 is about the same. One end of R1 is tied to ground, and so the other end is also approximately at ground. Thus you're forcing the (-) input of the op-amp to ground. Yet, you're also forcing the (+) input to +3V. This is isn't how op-amps are normally used. Op-amps are used typically by forcing at most one input to a particular reference voltage, and allowing the feedback loop to settle the other input on an almost identical voltage. \$\endgroup\$
    – Kaz
    Commented Feb 20, 2016 at 3:24
  • \$\begingroup\$ According to the data sheet, the input bias current for a LM324 is low, only around 20 nA (nano Amperes). So, at most that much current can flow through R1. According to V = IR, if I is 0.000000020A, and R is 100 ohms, then V is near zero. Thus the (-) input in circuit 1 is held almost exactly to ground potential. Why 6V is being calculated as the output voltage is probably because you're asking the op-amp to drive a 100 ohm load to ground, requiring 120 mA of current at 12V. The LM324 can source only around 40 mA current (typical, 70 mA max) according to the data sheet. \$\endgroup\$
    – Kaz
    Commented Feb 20, 2016 at 3:36

1 Answer 1

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Does an op-amp need a bipolar supply? And why can't it operate from a single supply?

This is entirely dependent on the op-amp in question and the circuit you are using it in. There are op-amps that require a bipolar supply, and op-amps that are happy with a single supply. The LM324 is designed to swing within ~5mV of ground and within ~1.5V of the positive supply rail (when powered by reasonable voltages, say, 5V-30V).

All of your circuits are rather strange. While it initially appears that you have connected feedback paths, this is not true. The first two circuits simply have a 100Ω output load resistor, and no feedback. If yo draw the schematics in a more usual manner, this should become immediately apparent. Because of this, you are operating the op-amp as a comparator (the + input is 3V higher than the - input), and you should not trust the simulator. Intuitively, when the + input is significantly higher than the - input, the output will go to the rail. However, with a 100Ω load, the op-amp may not be able to source enough current. The point is, your output voltage is unpredictable, and the simulator may not accurately model this behaviour.

The third circuit is clearly marked incorrectly, you have marked a grounded node as "6V". Additionally, you are shorting the output of the op-amp to ground, which will make the op-amp sad.

Regarding your edited circuits: the first circuit is a non-inverting op-amp with an input of 3V and a gain of 2. Hence, the output is 6V.

The second circuit is still problematic; as I have already explained above, the op-amp is trying to drive the output to the + rail, but the load is large enough that the output is drawn down. Your label is misleading, it is very likely not 3V. It might be close to 3V, but the output is unpredictable and may depend on idiosyncrasies of the simulator model.

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  • \$\begingroup\$ hi @uint12, the wiring is not correct. I have edited my question, Would you like explain me why op amp need + and -? And when supply with + and ground is not correctly anymore as op amp. \$\endgroup\$
    – ucon89
    Commented Feb 20, 2016 at 5:11
  • \$\begingroup\$ I have edited my answer in response to your two new circuits. There are many online resources about why op-amps need bipolar supplies, and when they don't. Here's a nice application note, there are many more like it. \$\endgroup\$
    – uint128_t
    Commented Feb 20, 2016 at 5:27

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