# abt op-amp

#### fari

Joined Nov 14, 2005
2
hi
i have a very basic question about op-amp.as we know that the input current is
ideally zero due to infinite input impedence.then how all the circuitry works if input
current is zero.

#### pebe

Joined Oct 11, 2004
626
Originally posted by fari@Nov 14 2005, 08:50 PM
hi
i have a very basic question about op-amp.as we know that the input current is
ideally zero due to infinite input impedence.then how all the circuitry works if input
current is zero.
[post=11646]Quoted post[/post]​
Those op-amps that have a very high input impedance use FETs. They are controlled by a voltage - not a current.

Have a look at the tutorials on this site where op-amps and FETs are explained very well.

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by fari@Nov 14 2005, 02:50 PM
hi
i have a very basic question about op-amp.as we know that the input current is
ideally zero due to infinite input impedence.then how all the circuitry works if input
current is zero.
[post=11646]Quoted post[/post]​
Your thought process is sound. "Zero current" is certainly a condition that would appear to be somewhat non-productive.

It is a good idea for you to carefully review the material you will find at the link that Pebe has directed you to. There you will discover that "zero input current" and "infinite impedance" are assumptions made for the purpose of simplifying the calculations associated with opamps such as gain and frequency response.

Modern opamp technology including the fet-input opamps to which pebe referred has progressed significantly from the early days. The new devices come very close to achieving these two conditions. For that reason, the new opamps behave more predicably than the earlier generation devices.

If you plan to do any analog design you will do well to become familiar with opamps for they play an important role in analog circuits of all types.

hgmjr