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Old 12th January 2013, 07:11 AM   #1
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Default Question about input/output range for this opamp

I'm using an LM358N, datasheet here:

http://www.st.com/internet/com/TECHN...CD00000464.pdf

I have this opamp in a feedback loop for a series pass voltage regulator with a 2.5V reference, schematic attached.

Question.

At vz I am measuring 2.49V but at vout I get 20.8V, vfb at 2.93V, and vb at 25.16V.

I understand why vb is railing out at 25.16V (because vp>vn), but why isn't vb decreasing to compensate since it's in a feedback loop? Is 2.5V input too low to sense for the opamp? If so, where can I find this info in the datasheet?
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Old 12th January 2013, 07:16 AM   #2
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2,5V too low input?
This amp has a open loop amplification of 100dB.
You must be careful to set the amplification too high here.
10X is really too much with 2,5V in.
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Old 12th January 2013, 06:15 PM   #3
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Quote:
Originally Posted by TANDBERGEREN View Post
2,5V too low input?
This amp has a open loop amplification of 100dB.
You must be careful to set the amplification too high here.
10X is really too much with 2,5V in.
How do you determine that 2.5V is too low input?
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Old 12th January 2013, 11:18 PM   #4
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I've up the reference voltage to 6.2V, still the same issue. Is there something else going on here?
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Old 12th January 2013, 11:25 PM   #5
DUG is offline DUG  Canada
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A schematic of the circuit would be very helpful.
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Old 12th January 2013, 11:26 PM   #6
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Sorry I forgot to attach. Here it is.
Attached Images
File Type: png ss.png (19.0 KB, 31 views)
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Old 13th January 2013, 12:54 AM   #7
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Now I just tried with a 11V reference and a different PNP pass transistor. There is something I'm missing here as this works fine in simulation.

Non inverting input is always much more than inverting therefore, thus does not settle at a bias voltage to get the transistor output to the regulated voltage.
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Old 13th January 2013, 01:12 AM   #8
DUG is offline DUG  Canada
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In the line from OutA to Vb (labeled vb) place a 4-6V zener and a resistor from e to c of Q2.

The output of these devices does not go that close to the positive supply.

What is not in the simulator is maximum output swing to the positive rail of U1.
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Old 13th January 2013, 01:14 AM   #9
DUG is offline DUG  Canada
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Remove InvertingB from ground and connect to OutB.
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Old 13th January 2013, 02:11 AM   #10
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Quote:
Originally Posted by DUG View Post
In the line from OutA to Vb (labeled vb) place a 4-6V zener and a resistor from e to c of Q2.
What purpose does this serve?

Quote:
Originally Posted by DUG View Post
The output of these devices does not go that close to the positive supply.

What is not in the simulator is maximum output swing to the positive rail of U1.
Hmmm you may be right, because vfb > vz, vb needs to go higher to lower vfb, but vb doesnt go higher so it must be railed out.

Do I just need to use an opamp that will go rail to rail, or at least to the positive rail?
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