Electronics > Beginners
LM741 as comparator 1.9V problem
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Lee Leduc:
Wikipedia has a very nice write up of 741 op amp. It walks you through the device explaining how each stage of the circuit works. Very educational.

https://en.wikipedia.org/w/index.php?title=Operational_amplifier&direction=next&oldid=436142906
PerranOak:
Beaut, thanks, I'll have a look.
Zero999:

--- Quote from: PerranOak on May 04, 2018, 01:24:23 pm ---Thanks all.

I was just experimenting with comparators and found a circuit online. I had an LM741 so I gave it a go. When I used an LM393 all was well but I just wanted to understand what was happening.

I looked on the datasheet for "input common voltage range" but can't find it nor anything that mentions the 2V headroom. I do have difficulty with datasheets though: it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values.

Oh well, new stuff learned and thanks all.

--- End quote ---
Page 5, section 6.5, "input voltage range ±13V". It's specified with +/-15V supplies, which implies it's 2V of headroom from either supply rail. Note it also says ±13V typical, ±12V minimum, you you'll need to allow 3V of headroom to ensure it works over the entire temperature range.
http://www.ti.com/lit/ds/symlink/lm741.pdf
viperidae:

--- Quote from: PerranOak on May 04, 2018, 01:24:23 pm ---it says that for my LM741C it says that the supply voltage can be +/-18V but doesn't mention that it only works properly at the higher values.

--- End quote ---
It says the recommended minimum supply voltage is +/- 10V. All the values in the tables are only valid when running at +/- 15V
David Hess:

--- Quote from: PerranOak on May 05, 2018, 01:07:38 pm ---
--- Quote from: David Hess on May 04, 2018, 05:33:48 pm ---It also includes an equivalent schematic from which it is possible to derive the input and output voltage ranges without too much trouble.
--- End quote ---

Really. How is this done?
--- End quote ---

Just add up the transistor Vce (about 0.2 volts) and Vbe (about 0.6 volts) saturation values.
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