problems while using op-amp 741 as buffer.

[vijayjadhav.595]

Hi...
i've been trying to use 741 as a buffer. pins 4 and 7 are connected to -5V and +5V respectively. Inverting teminal is shorted to the o/p pin for unity gain. input is being applied to to the non-inverting terminal. Ideally the voltage on the o/p side should follow the voltage applied to the input.In my case when the input is 5v the o/p goes upto 4.5v which is fine but when the input is connected to ground(-5v) the o/p is stuck at 1.84v. I have tried connecting the fixed ends of a 10k pot to the offset pins and the slider to the ground but was of no use.
can any one help me out with this.

cheers...
vijay

 

[monsoon]

I believe that the input range will not be rail to rail for 741. If your circuit works satisfactorily between (say -3.5V to 3.5V), then there is nothing wrong with the circuit.

 

[vijayjadhav.595]

hi...
so is there some other opamp IC which you think i could use...?

 

[monsoon]

You might use Linear Technology's LT1677. You might find others by searching for- opamps with rail to rail common mode.

 

[Junus2012]

Hello
741 is rail to rail op-amp no need to change for other one
first you have large offset voltage (5-4.5=0.5V) but this not the problem
do one thing plz
sweep the input from 5V to -5V by 0.5V and record the output to see at which point the problem start

 

[vijayjadhav.595]

Hello
741 is rail to rail op-amp no need to change for other one
first you have large offset voltage (5-4.5=0.5V) but this not the problem
do one thing plz
sweep the input from 5V to -5V by 0.5V and record the output to see at which point the problem start

hi...
the buffer works the way its supposed to from 5v to 1.9v. as the input goes lower than 1.84 the o/p is stuck at 1.84...

 

[Junus2012]

hi
with bipolar op-amp like 741 has a feature of low offset voltage, in 741 data sheet it give a value of maximally 6mv while you are getting an offset voltage of 0.5 V and here I think is the problem. so I will suggest you three things to do. first try to compensate the offset voltage you are getting by putting potentiometerresistor between NC pins and the negative supply and adjust it to have zero offset voltage after that repeat your test for the buffer. if still does not work then try to work with +-15V supply voltage .if non of them work then I suggest you to replace your op-amp with other 741 op-amp.

 

[monsoon]

Datasheet says that for +-15V supply swing is limited to only +-12V supply. Moreover with 4 to 5 transistors cascoded rail to rail swing is not possible. However, this does not explain what you are observing.

 

[crutschow]

..................
741 is rail to rail op-amp no need to change for other one

A 741 is definitely not a rail-to-rail op amp. Neither the input or output can operate to the power rails. Just look at the data sheet common-mode input range and the maximum output voltages.