diffirential amplifier

[rudie]

operation of ic ua741

hello,

I was working on an differential amplifier for DC voltages. After some simulating with Pspice i found out that for small voltage differences it won't work.

In the attachment of this mail you will find some pictures of the simulation results. In fig. 1 you will see the first simulation. Voltage source V1 has a voltage 4V and V2 a voltage of 3V at the output the voltage is 1V, so this is perfect. In figure 2 I narrowd the difference to only 0.1V. The output of the amp is now 0.4V(388mV). This is not good. As you can see the supply voltage of the opamp is on the positive pin 15V and the negative pin 0V. If i now make the negative supply pin -1V (fig.3) it works fine again, with the same difference of 0.1V between V1 and V2. How is this possible? is this the result of the voltage offset?. I only have a power supply of +15V does anyone know a simple ciruict with transistors or someting to make -1V out of +15V?

I hope that someone can help me

greets Rudie

 

[FvM]

diffirential

Hello,

you can either use an amplifier with rail-to-rail capability on output or have a sufficiant negative supply. 1V isn't enough for LM741 by specification.

Regards,
Frank

 

[rudie]

thank you for your reaction.

But why isn't -1V enough. I don't need the range between -15V and 15V. The lowest output i will get is 0V so I don't get why i need a negative voltage supply. I am also wondering why is my output 0.4V for low differences? In the datasheet of the Ua741 the voltage offset is 7mV so i don't get this 0.4V.

greets Rudie

 

[FvM]

can ua741 work on single voltage supply

The relevant specification is output voltage swing. Guranteed value is 3V difference to supply with no output load, 1V is typical. However, operation for unsymmetrical supply isn't specified excactly. May be 1 V is sufficiant in most cases.

 

[rudie]

I still don't get what the 0.4V is. the output voltage swing is the maximum peak voltage that the output can produce before it starts clipping. In the datasheet i looked the Ua741 has a voltage output swing of 12V. But how can this be may problem?

rudie

 

[FvM]

Maximum negative output is -12 V for -15 V supply

 

[rudie]

what do you mean. In the datasheet of the Ua741 the voltage is 12V. But i can't see what this has to do with my problem please explain...

greets rudie

 

[Eugen_E]

As FvM said,

Maximum negative output is -12 V for -15 V supply

So, for 100mV output, you would need at least V7=-3.1V.
But, again, as FvM said,

However, operation for unsymmetrical supply isn't specified excactly.

So you may need V7=-4V, you need to try.

Replace LM741 with LM358, wich is a rail to rail opamp. From what I simulated long ago, LM358 can go as low as Vout=30mV, with a single 5V supply.

 

[rudie]

I am sorry but i still don't get it.

In my application i don't use negative voltages so i dont need the negative voltage supply (correct me if i am wrong). When i narrow the voltage difference between the pin the opamp doesn't go lower than 0.4V. But if i put my negative power supply to -1V it can go lower. But why do i need this negative voltage supply if i don't use a negative voltage on the input. How do you determine how "low" an opamp can go i can't find it anywere on the datasheet.

please explain

Greets ruud

 

[steadymind]

As far as i know IC uA741 needs a differential supply voltage for proper operation.
It may work for some input ranges without differential supply. Try it out.

Check the 4 page of the this pdf.

https://www.ortodoxism.ro/datasheets/texasinstruments/ua741.pdf

 

[rudie]

On page 4 you only see the absolute maximum ratings i don't see any minimum?

But Eugen_E said someting about how 'low' an opamp can go. How can i determine that voltage? Can somebody explain that?

greets rudie

Added after 2 hours 22 minutes:

forget what i said in my previous post. After some simulating i found out what you mean. And i think i have enough information to solve the problem.

Thank you for you time and advice

Greets rudie

 

[linaweku]

i have an idea..
do u had tried MULTISIM?

in multisim it have a wizard for designing diff op amp...

it very easy...
try first...

 

[hock]

the output voltage can not go to a voltage lower then the Vce Sat value, which for a transistor is 0.4 v. if your -ve rail is at 0 v the minimum output voltage will be +0.4V. if you have a lower voltage say -1V then your rail voltage will go to -0.6 V.
i think you shoul use an opamp which has rail to rail output specifications. do not use a normal opamp.
hock

 

[manish_kumar]

which cmos op amps ic avilable in market and are their specifications