Op amp problem ua741

[ArduinoGuy]

r3 r1 are 10k r5 Is 1kohm and c1 is 10uf cap to make input lp filter.
ua741 is op amp

it works but the problem im having is even with +in grounded there is always 3.6v on output.
if I put 5 volt I get 10v ect but at zero voltage input im getting 3.6v output. Can anyone shed some light.
im using a arduino to drive the 0-5v pwm and need to to give 0 to 10v output.

 

[ArduinoGuy]

 

[(*steve*)]

The 741 can't pull the output close to the supply rails.

If you use a +/-12V supply, you will be able to get the output to 0V. This is how it is normally done.

You may be able to get away with an op-amp which has rail to rail outputs, although the output may be less accurate as it approaches ground, especially if you are trying to sink current into it.

 

[Alec_t]

The 741 doesn't like input voltages within a couple of Volts of its +ve or -ve rail. Check the datasheet.

 

[Audioguru]

The design of the 741 opamp is 50 years old and it was designed to use only a plus and minus 15V supply. The input common-mode voltage range does not include its input voltages being within 3V from the positive or negative supply but your input is at its negative supply of 0V where it does nothing.
Its output voltage cannot go closer than 3V from your 0V or within 3V from your +12V, you will be lucky if you found a 741 opamp that has an output voltage as high as +10V when the positive supply is +12V. Its output cannot go anywhere near 0V without it having an additional negative supply.

 

[WHONOES]

There are opamps that will go very close to the plus and minus supply rails and include 0V (ground) input in common mode and also work on a single supply but, not the venerable 741 or its derivatives.

 

[AnalogKid]

The datasheet for the 741 lists its capabilities. In both tables and plots it shows the output voltage swing range. If you don't know how to interpret the datasheet, post a PDF of the one you have (there are many different versions from many manufacturers) and we can explain exactly where the information is.

As above, there are many newer opamps with output stages that swing much closer to the + and - rails. These are called "rail-to-rail" parts. Note that even for the newer parts, it is impossible for an output stage to swing down to 0.0 V (if that is what the negative rail is) while making any kind of useful output current, and the lower the load impedance is the farther away the output will be from the rail.

ak

 

[WHONOES]

Absolutely true AK. But with a little bit of lateral thinking it is possible to get within a few milli volts of the negative rail if that is the requirement.

 

[ArduinoGuy]

Absolutely true AK. But with a little bit of lateral thinking it is possible to get within a few milli volts of the negative rail if that is the requirement.

can you explain lateral thinking?

 

[(*steve*)]

can you explain lateral thinking?

I do it laying down...

You might have a pull-down resistor on the output, or minimise the amount of current the output has to sink, if you can, make the output source current (the pull-down resistor does this). You might also be able to redefine what zero means.

 

[WHONOES]

See the attachments.
You will need a 24V supply minimum to achieve 10V out because of limitations of the positive output of the 741 plus the gate source voltage of the FET. As well as that, there is the variation of the Vgs of the Fet to take into account and may take a little bit of fettling to get the required O/P.
But as depicted, you should be able to get the O/P to within a few millivolts of 0V depending on load.
Hope this helps.
You could of course use something like a CA3140 which will go very close to ground without the extra paraphernalia.

 

[poor mystic]

Hi
I just wanted to suggest the LM318, which is capable of replacing a 741 on pretty well any circuit board but has way better specs and can deal with rail-to-rail input voltages.

The data sheet: http://vakits.com/lm318-smt-high-speed-op-amp-ic

 

[Audioguru]

I just wanted to suggest the LM318, which is capable of replacing a 741 on pretty well any circuit board but has way better specs and can deal with rail-to-rail input voltages.

No, its inputs cannot deal with rail-to-rail input voltage swings since with a plus and minus 15V supply its minimum input voltage range is only plus and minus 11.5V. That is why its minimum supply voltage is plus and minus 5V. Its minimum output swing in to a 2k load is 3V less than each supply voltage which is also far from being rail-to-rail.

 

[poor mystic]

No, its inputs cannot deal with rail-to-rail input voltage swings since with a plus and minus 15V supply its minimum input voltage range is only plus and minus 11.5V. That is why its minimum supply voltage is plus and minus 5V. Its minimum output swing in to a 2k load is 3V less than each supply voltage which is also far from being rail-to-rail.

I have no axe to grind, but the data sheet I linked does guarantee rail-to-rail input voltages for many applications.

 

[Alec_t]

but the data sheet I linked does guarantee rail-to-rail input voltages for many applications.

The datasheet shows the minimum input range is ±11.5V when the supply is ±15V. That is not rail-to-rail input.

 

[WHONOES]

The inputs of the CA3140 I mentioned are guaranteed to 0.5V below ground if I remember correctly.

 

[OBW0549]

I have no axe to grind, but the data sheet I linked does guarantee rail-to-rail input voltages for many applications.

Where? Where in that data sheet do you see any such guarantee?

In fact, the Input Voltage Range and Output Voltage Swing specs listed in the Electrical Characteristics section of the data sheet make it crystal-clear that neither the LM318's inputs nor its output operate rail-to-rail:

 

[poor mystic]

Where? Where in that data sheet do you see any such guarantee?

In fact, the Input Voltage Range and Output Voltage Swing specs listed in the Electrical Characteristics section of the data sheet make it crystal-clear that neither the LM318's inputs nor its output operate rail-to-rail:

View attachment 41165

Let's not bicker, this is a nice place.

 

[Audioguru]

The design of a 741 opamp is 50 years old! Nobody dreamed of rail-to-rail 50 years ago.

 

[Harald Kapp]

Where in that data sheet do you see any such guarantee?

Note 4: For supply voltages less than ±15V, the absolute maximum input voltage is equal to the supply voltage

This does not, however, mean that the opamp will be functional when (ab)used that way, only that it will not be destroyed. That is not what rail-to-rail operation is about.
The functional input voltage range is +-11.5 V aas stated in the table in post #17.