Hi guys,

I've read in a few places that op-amps are voltage devices and so when you require the output of an op amp to have a relatively high current delivery to the load, you may have to use something like a push-pull transistor pair to switch on higher current.

Well my question is why?! I mean if there is say 10V at the output of the op amp.. and you connect a load resistor to that then to ground... shouldnt there be current through the load from (10V - 0)/R?

Thanks for your time!
Regards

 

Originally posted by Kazagant@Apr 4 2006, 06:36 AM
Hi guys,

I've read in a few places that op-amps are voltage devices and so when you require the output of an op amp to have a relatively high current delivery to the load, you may have to use something like a push-pull transistor pair to switch on higher current.

Well my question is why?! I mean if there is say 10V at the output of the op amp.. and you connect a load resistor to that then to ground... shouldnt there be current through the load from (10V - 0)/R?

Thanks for your time!
Regards

[post=15799]Quoted post[/post]​

Yes, for some but not all values of R. The output stage of a typical opamp is current limited by the transistor geometry and other components. As the load demands more and more current the output voltage can no longer deliver that amount of current so the voltage starts to drop. There are opamps with short circuit protection and they generally drive a short with a very low voltage.

The external transistors that can deliver power to an external load like a 4 ohm speaker have HUGE silicon geometries and are packaged to disipate large amounts of power. An opamp in a SOT23-5 package just can't handle the heat.

Hope this helps.