operational amplifier shematic problem 741

[yassin.kraouch]

i designed a circuit using uA741 from ST https://www.st.com/stonline/books/pdf/docs/5304.pdf in order to amplify voltage using a gain of 22 ( R2 = 22k and R1 = 1k), the shematic is attached, but without résult the circuit is unfonctionnel, the pin 7 i used a 5V alimentation and in pin 4 i connect the ground,

 

[alexan_e]

Actually the non inverting amplifier like this has a gain of 1+(22k/1K)=23 , it is always the resistor ratio +1
other than that the schematic is fine, if you apply +0.1v to the + input you will get +2.3v to the output

Alex

 

[IanP]

I think you have to change the horse – the UA741 was not really meant to be used in low voltage applications ..
Google for an opamp rated for single 5V supply, perhaps rail-to-rail output, to give you full 0-to-5V output voltage swing ..
:wink:
IanP

 

[zorro]

Hi yassin,

With this connection, you should connect a negative supply voltage at pin 4.
Regards

Z

 

[yassin.kraouch]

3 answer but i don't know which is the true answer,
i tried simulation and it works fine, i don't know why my circuit does not work

 

[alexan_e]

I thought you had problems with the simulation and it works fine there, also the schematic can work in reality too but with a different opamp like for example LM358.
The 741 need a symmetrical power supply.

Alex

 

[yassin.kraouch]

and i can't obtain -5V from the +5V ? have you an idea about this ?

 

[alexan_e]

What kind of signal do you intend to amplify?
If it is a DC signal that it would be much easier to use a different opamp

Alex

 

[yassin.kraouch]

Yes it is a DC signal, i have to amlify 80 mV in order to put it in the ADC of an MCU, and just i have a supply voltage of 5V

 

[alexan_e]

I would suggest to change the opamp, do you have any other available opamp?

Alex

 

[yassin.kraouch]

no it is late now, i will try to change it tomorrow, what kind of O.A you will suggest to use ?

 

[alexan_e]

You probably need a rail to rail opamp , I have only used OP279 but I'm sure you can find many more, I don't have any specific in mind.
The LM358 is not rail to rail and you can't get an output close to 5v with a supply of 5v.

Alex

 

[yassin.kraouch]

i did not need an output of 5V just i have a supply voltage of 5V and i don't want to add a boost or regulator,

---------- Post added at 17:42 ---------- Previous post was at 17:38 ----------

why did you talk about rail to rail, i did not need a rail to rail opam, the output needed is about 1 V

---------- Post added at 17:45 ---------- Previous post was at 17:42 ----------

look at the LM324 alex i think he can do the job **broken link removed**

 

[alexan_e]

Then LM358 will work fine (it's a dual opamp but it doesn't matter, it is cheap anyway).
I thought you wanted to use the full range of the ADC (assuming 5v reference)

Alex

 

[yassin.kraouch]

no my mcu work with 3.3V and i will the refernce of 1.5V

 

[FvM]

LM358 is basically able to do what you want. Your designed gain is however 23 rather than 22.

 

[alexan_e]

look at the LM324 alex i think he can do the job **broken link removed**

The LM324 is the quad version of LM358, you can use it if you have more channels to amplify or if you like to have a quad opamp for future projects

Alex

 

[LvW]

---------- Post added at 19:18 ---------- Previous post was at 19:17 ----------

At the moment, I am not sure if the 741 can operate with a supply as low as +- 2.5 volts (or +5V single supply).
However, one thing I know: Your circuit cannot work without a suitable bias circuitry for single supply operation (very, very often discussed in this forum). More than that, the feedback resitor to ground must be in series with a capacitor in order to give the circuit a dc gain of unity. This is always necessary for single supply operation.
If this is fulfilled, of course you can use the 741 for this task - no doubt.

 

[alexan_e]

---------- Post added at 19:18 ---------- Previous post was at 19:17 ----------

At the moment, I am not sure if the 741 can operate with a supply as low as +- 2.5 volts (or +5V single supply).
However, one thing I know: Your circuit cannot work without a suitable bias circuitry for single supply operation (very, very often discussed in this forum). More than that, the feedback resistor to ground must be in series with a capacitor in order to give the circuit a dc gain of unity. This is always necessary for single supply operation.
If this is fulfilled, of course you can use the 741 for this task - no doubt.

He wants to amplify a DC signal, how is this going to work with an amplifier that has a unity gain?

Alex

 

[yassin.kraouch]

---------- Post added at 19:18 ---------- Previous post was at 19:17 ----------

At the moment, I am not sure if the 741 can operate with a supply as low as +- 2.5 volts (or +5V single supply).
However, one thing I know: Your circuit cannot work without a suitable bias circuitry for single supply operation (very, very often discussed in this forum). More than that, the feedback resitor to ground must be in series with a capacitor in order to give the circuit a dc gain of unity. This is always necessary for single supply operation.
If this is fulfilled, of course you can use the 741 for this task - no doubt.

can you explain more ? as i understand i can supply my opam with VCC-=GND and VCC+=5V ?? the supply that i whould output is 80 mV and the gain is 23