Ua 741 OpAmp how to connect it to get a gain of 2?

Hi maybe someone can help me out with this

I've been reading http://forum.arduino.cc/index.php/topic,19751.0.html

Basically I need to go from 0-5V to 0-10V.
So in that post Grumpy_Mike suggested to go with this approach

I also went to Online Circuit Simulator with SPICE to verify that it works.

When I built the circuit it behaves completely different.

What i have is a 5bit dac built by a R/2R ladder inspired by colins lab

this part works fine.
Now instead of just a voltage follower at the end I'm trying to increase the voltage 2 times.

Can't figure out how to connect the OpAmp.

//

I have a Pc psu with +12V(yellow) and gnd (black)
first I connected pin 7 to +12V and pin 4 to gnd.

Pin 6 goes to +12V with the resistors detached.
When I connect the voltage divider resistors(20K each) I get 4V on pin 6 and 2 V on pin 2.
Pin 3 is at 0V.

Then Was reading that OpAmps need a Dual powersupply.
So I tried making a +/-12V using 2 330K ohm resistors connected to a 24V power supply and using the center as ground.

result is the same 2V on pin 2(inverted input) and 4 on pin 6(output) 0V on pin 3
and some ripple idk where that came from.

So I kept reading and some suggest that that approach might not work unless I use a rail splitter?
something like this?

Anyone that can point me in the right direction?
How can I wire this to a 12 or 24 psu?

First off a 741 is not a good amplifier to use these days, when thy first came out just over 40 years ago they were quite state of the art but they have been superseded decades ago. They have several problems when used with today's circuits, the two main ones being they require a high supply voltage and the output can not go within a couple of volts of either supply rail.
If you use a decent amplifier ( Like an MCP 601 or MCP 602) and the first circuit in your post you should be able to attach the input to the output of your resistor ladder.

Note that we talk of an amplifier having a gain of 2 not a voltage multiplier.

A voltage multiplier is another circuit all together, used to change an AC signal into one twice the amplitude using passive components ( capacitors and diodes ).

Thank you for your reply.

I've changed the title.

So that's normal behavior for a 741? Well I'm surprised.
ryuujin seemingly got it to work back then.
The mcp 601/2 seems to take 7V max as Vdd.
I I need 10 V so that won't do.

Is there some other solution to get a gain of 2?

I I need 10 V so that won't do.

OK there are lots of op amps out there.
Ones that have a wider input range are:-
LM6142
LM358
TL062
TLV2370
There are many more, those are just the ones I have the data sheets for on my hard drive.

Thank you Mike

I've just bought 2 9V Batteries and hooked it up like a dual power supply and it works perfectly with the 741.

Now I need to figure out how to make a dual power supply.
Just a voltage divider doesn't work.

An lm317 adjustable voltage regulator?

So I kept reading and some suggest that that approach might not work unless I use a rail splitter?
something like this?

That diagram you posted has the output of the op amp connected to ground, you won't get much out with that.
Otherwise that is the way to implement a split rail only I would make those resistors 1K and put a capacitor across each one.
Try it with your two batteries in series.
I think the 741 needs a minimum of +/- 5V to work so with splitting just the one battery you don't have enough voltage.

The data sheet for the 741 says that the maximum output voltage for a +/- 15V supply voltage is typically +/- 12V so that is 3V off the rail. This gets worst as the voltage drops.

cjdelphi:
An lm317 adjustable voltage regulator?

I don't see how that will help produce a split rail.

It's a better alternative to a voltage divider... (just going from the resistor divider)

A split rail, check out this..

cjdelphi:
A split rail, check out this.

Can you explain why this is better than two 1K resistors?

Never claimed it was... I was a bit confused by the question now I understand....

cjdelphi:
It's a better alternative to a voltage divider... (just going from the resistor divider)

Grumpy_Mike:
Can you explain why this is better than two 1K resistors?

cjdelphi:
Never claimed it was...

Is repeating a post with no extra comments, that calls your verisimilitude into question adequate?

I've built a power supply and it works.

Here is the complete schematic.
Please note that as Mike suggested there might be better parts out there that do the job better.

Also The dropping resistor R15 should be checked if it has the appropriate value. Not very sure abt that.
the ua741 draws 150mW if I remember correctly ....R15 allows for 600mW.

Use at your own risk before verified.

Thank you everyone for your input.

Ps If anyone knows a better free image host... I'll re-upload.

I've built a power supply and it works.

Congratulations. :slight_smile:

Grumpy_Mike:

I've built a power supply and it works.

Congratulations. :slight_smile:

Thanks
I know I have a long way to go... :slight_smile:
Hope this helps someone someday.

3_35:
I've built a power supply and it works.

Here is the complete schematic.
Please note that as Mike suggested there might be better parts out there that do the job better.

Also The dropping resistor R15 should be checked if it has the appropriate value. Not very sure abt that.
the ua741 draws 150mW if I remember correctly ....R15 allows for 600mW.

Use at your own risk before verified.

Thank you everyone for your input.

Ps If anyone knows a better free image host... I'll re-upload.

sorry for the necro.

but please re-upload the image.

3_35:

this part works fine.
Now instead of just a voltage follower at the end I’m trying to increase the voltage 2 times.

I’m amazed that so many posts have been made and yet nobody has helped you yet.

Use the circuit you have, but instead of connecting the op-amp output directly to the inverting input, connect it as follows:

  • 10K resistor from output to inverting input.
  • 10K resistor from inverting input to ground.

That will set your op amp to have a gain of 2 (it currently has a gain of 1).

Also, your circuit will only produce about +7 or so volts max with a +/-9 volt supply (the output obviously cannot go higher than the supply voltage). If you need more than +7, then increase the supply voltage to +/-12 or +/-15 volts.

Usually, the supply voltages do not have to be equal. If you only need a positive output, a +5, -15 volt supply would be OK.

Lastly, although the 741 op-amp is an older design, there’s nothing wrong with it and there’s no reason to use something different.

Good luck.