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Op Amp Saturation Definition

  • #1
JJBladester
Gold Member
286
2

Homework Statement



What exactly is op-amp saturation?

Homework Equations



[tex]V_{out}=A_{v}V_{in}[/tex]

The Attempt at a Solution



I am doing a lab where we use an LM339A quad voltage comparator and a voltage divider network to create a voltmeter. A single red LED is connected to each of the four output terminals. When the input voltage exceeds Vref of each comparator, the LED is forward biased, conducts current and lights up.

The lab asks us to find the saturation voltage of the comparator by "measuring any comparator output terminal when an LED is lit". I did this and found

[tex]V_{sat}=261mV[/tex]

The datasheet has a saturation voltage [itex]V_{OL}=250mV[/itex] (typical) and a voltage gain [itex]A_{V}=200V/mV[/itex] (typical).

So, my measured saturation voltage is about 4% off from the datasheet. Not bad... but I'm having a hard time describing exactly what is meant by saturation voltage. I read this PF post but I'm still slightly confused.

I believe that saturation occurs when the open-loop input differential voltage exceeds some small voltage and that since the op-amp has a high open-loop gain, the output is driven to ±Vout(max).

Here's what I wrote in my lab... Does it make sense?

The saturation voltage was measured to be 250 mV which is a 4.4% error from the datasheet value of 261 mV. The saturation voltage is the input voltage at which the output voltage [itex]\left ( A_{OL}\cdot V_{sat} \right )[/itex] will be at its maximum positive or negative value. So, if we have a voltage difference of 250 mV between the inverting and non-inverting inputs, the op-amp's high open-loop gain will force the output to ±Vout(max).
 

Answers and Replies

  • #2
UltrafastPED
Science Advisor
Gold Member
1,912
216
The "power rails" provide the maximum possible voltage outputs for any op amp. When you review the spec sheets for an op amp you may see that the op amp "saturates" prior to reaching the rail voltage; they will then provide some parameters.

The meaning is this: if your amplifier circuit voltage exceeds the saturation limit your op amp output will be pegged at that level. Thus you must know the input voltage range which corresponds to the "safe" non-saturated output range for your circuit.

You can test with a simple "op amp voltage follower", or with a definite amplification - open loop is not a good choice IMHO. And yes, your result may not match the spec sheet exactly.

What you should write is (a) exactly how you conducted the test, including the circuit used, and how the inputs were provided (and how you know that what they are!), and how the outputs were measured. By running the test in a series of small steps you can obtain a plot which shows what the actual amplification was, and where it saturated.
 

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