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I would like to know why op-amps saturate below their supply voltage. For instance I have a circuit with an LM358-N amplifier IC that is powered at +5 [V] and connected to ground on the other rail. Theoretically the saturation should be 5 [V] but it saturates actually at 3.7 [V].

  1. Why is this happening?
  2. What is the term for this offset that I can find in the datasheet?
  3. Are there op-amps that can go saturated all the way up to the supply voltage?

Thanks a lot!

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    \$\begingroup\$ "rail to rail" might be a useful term to google for you \$\endgroup\$
    – PlasmaHH
    Apr 10, 2015 at 14:39
  • \$\begingroup\$ Look for high level ouptut voltage on the spec sheet. Vcc-1.5 V is fairly typical. \$\endgroup\$ Apr 10, 2015 at 14:51

2 Answers 2

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Why is this happening?

Because physics. Op amps have an output stage made with transistors, and no practical transistor saturates to a perfect short. That's the simple answer, if you really want to know the physical reason, that needs to be a different question about solid state physics.

What is the term for this offset that I can find in the datasheet?

It varies with manufacturer, usually it will be labelled output swing, and will be spec'd at some supply voltage and several loads, say \$ 1k\Omega \$ and \$ 10k\Omega \$.

Are there op-amps that can go saturated all the way up to the supply voltage?

Because of physics, no op amp will get all the way to the rail. The rail to rail op amps mentioned in comments can get within about 50-100mV. If you absolutely must have 0-5V performance, you need to have a supply voltage slightly over 5V, and a negative bias generator.

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    \$\begingroup\$ A few op amps have internal converters to provide internal rail voltages outside supply rails BUT apart from these, which are "cheating", I agree. \$\endgroup\$
    – Russell McMahon
    Apr 11, 2015 at 8:10
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The simplest type of regularly used opamp has a push-pull emitter follower output stage and as with any emitter follower amplifier, the theoretical output can swing to about 0.7 volts of either supply rail. More modern opamps have a different output configuration that is marketed as "rail to rail" but, under full output loading might get to within 0.1 volts of the rails and maybe 10mV on very light loads.

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