[SOLVED] opAmp Amplifier - strange results ?

Hello everyone !
First,thanks a lot.Replies to my previous thread helped me so much !!

Now, I've designed an amplifier using LM324 and 741 and the results are not as I have expected. The circuit is :


[Both IC powererd by 4.3 V (accidently attached the diode AFTER 7805) and Offset not nulled on 741 (plan to cancel it in software).Also both are properly grounded and there appear to be no short circuits on PCB]

> I'm trying to buffer the input signal by using 741 as a voltage follower
> LM324 is acting as non-inv amplifier with gain = 2

The problem is , the outputs are crazy (or I am )
(1) When input is grounded,I expect 741's o/p to be 0v. But it shows up as 1.88v
(2) When I ground the LM324's non-inv i/p , the expect the o/p to be 0v. But it's 2.66 V
(3) When the signal is 0.5 V :
> 741's o/p is 2.06 v
> 324's o/p is 3.01 v

The 100k & 0.1uF are supposedly (not my idea) for prevention of oscillations.The 110E is supposed to compensate for the bias currents.

The other 3 opamps of 324 are acting as buffers and they are working just fine ! I've also tried 2 741 ICs but both give the same results.This leads me to suspect faulty design. I've just begun in analog design (was doing digital till now) and my stupidity might be at play here.
So would you guys please give me any hints so as to where am I going wrong ?

Thanks a ton !!:smile:

You showed only one power supply connection of 741 and none of 324. Assuming, this is single supply, the 741 can't work as expected. Refer to the datasheet's common mode voltage specification to know why.

In addition, the 220 ohm resistors around 324 are too low for reasomable operation. And you can't operate an inverting amplifier with single supply and grounded +ve input.

Your calculations are valid only for Opamps supplied with a simetrical power supply (ex: -5V to +5V)

(1) When input is grounded,I expect 741's o/p to be 0v. But it shows up as 1.88v

Click to expand...

Wrong for 2 reasons. First, like I said, the result is only valid when your opamp is connected to a simetrical power supply. Second your input voltages cannot be so close to the supply rails, you always have to give some margin.

The 100k & 0.1uF are supposedly (not my idea) for prevention of oscillations.

Click to expand...

The 741 is compensated internally, there is no need for external compensation to prevent opamp oscillations. The resistor and capacitor are probably used as a low pass filter, to remove input noise, but it's not implemented correctly. The resistor should be to the left of the capacitor

The 110E is supposed to compensate for the bias currents.

Click to expand...

I don't think you need that, the input impedance is very high. I doesn't harm though.

Thanks for the replies!

@FvM : yes,741 was powered with single rail supply. I'll search for an alternative op amp now.
Just curious-why do the LM324 resistors need to be high value ? Low value res will just pull more current,right? or do they cause other problems too ?

@fcFusion : Thanks for the suggestions :-D

For standard OP circuits, you can assume a reasonable resistance level of kohm up to 10 k ohm as a first guess. 220 ohms risks to run into OP output current limit and will at least reduce the voltage swing by causing unwanted saturation voltages.

LM324 would be better suited for single supply operation with input voltages downto to 0.