input impedance of invertiong amplifier.

[BABESH]

hello
I am new to analog design.
I need small clarification.
can i use 10giga ohm resistor as input Ri in basic inverting amplifier configuration?

--babesh
[/img]

 

[IanP]

You can use 10GΩ resistor with some special opamps; in most cases, however, this resistance is just to large ..
Read this article on "Practical considerations: bias current" ..
https://www.allaboutcircuits.com/vol_3/chpt_8/15.html
and you will learn why ..

Regards,
IanP

 

[hamdard]

See the specification of partcular OPAMP.
Opmap needs certain current at its input to get driven. If the current supplied is less than required one, OPamp will get work.

 

[Kral]

BABESH,
There are practical problems with using large value input resistors.
Let's assume that the resistance path to ground equals the input resistor, in an attempt to cancel the effects of the input bias current. You still have a problem with common mode input voltage. The bias current (Ib) of the op-amp will create a common mode voltage equal to Ib X Rin. Unless the input bias current is extremely small, this common mode voltage will exceed the common mode voltage rating of the op-amp. Also, with large value resitors, the sensitivity to capacitively coupled noise will be very high.
Regards,
Kral