lm324 voltage follower o/p behaviour

[sabu31]

Hi

I am using LM324 as a voltage follower. The Supply to opamp is 5V and -ve supply is ground 0. Two strange behaviours i am getting from the opamp.

When i directly connect the probes to o/p of opamp, there is amplification of input spikes. The voltage follower behaves normally, when a 100ohm load is connected, but it gets saturated at around 2.65V, ie.. o/p does not increase even if input is increasing. In datasheet its mention that o/p short circuit current of opamp is 50mA, but 2.65V at 100ohm is 26.5mA.
The saturation is avoided if I use 300ohms resistor . and if use a lesser value resistor like 20 ohms , it gets saturated at around 1V and there is amplification of spikes. I feel there is something related to loading, but not clear. Can somebody clarify this. Thanking you.

 

[IanP]

The sink/source currents are given at 15V supply, so don't expect them to be near those values at 5V ..
The output won't go higher than, say, Vcc-1.5V and lower than GND+0.9V ..
If you want an opamp to perform well within the supply voltage range you have to select one that is rated for rail-to-rail (output, input or both) operation ..

:wink:
IanP

 

[sabu31]

Hi

Thanks for the reply. Does that mean, that it will be linear upto 3.5 and then start saturating . Or the output will saturate as it tends to towards Vcc-1.5V, say from 3V onwards (in case of 5V Vcc).