LM358 AC coupled non inverting amplifier

Sergio Restrepo

Other Parts Discussed in Thread: LM358

Hello.

I just want to know what the 1meg resistor (labeled as R3 in the LM358 datasheet) is used for in that circuit. I´m a little lost here.

Somewhere in the datasheet it reads: "In most cases, input biasing is not required". The value for input bias current compensation resistance is the parallel between the feedback resistors but R3 does not meet that condition.

Thanks in advance for the help.

over 9 years ago

0 Ron Michallick over 9 years ago

TI__Guru**** 165146 points

Sergio,

R3 puts DC bias on non-inverting input. C2 can't connect directly (R3=0) because it would short AC input. It can't be open (R3=1/0) because input would float up to about VCC -1.3 V. Output will be greatly distorted (clipping).

0 Sergio Restrepo over 9 years ago in reply to Ron Michallick

Prodigy 50 points

Ronald, thanks a lot for your answer.

Ok, now I understand its importance to ac inputs but I found an Analog Devices aplication note (AN 581) in which is writen: "provides a dc return path for the VS/2 reference voltage" So my question is ¿Why is that return path needed? In the simulation I can see that the Vs/2 reference voltage changes a little (about 20mV for Vs=9V) when R3 is cero but I guess that it's due the input bias current altough it's too small.

0 Ron Michallick over 9 years ago in reply to Sergio Restrepo

TI__Guru**** 165146 points

Sergio,

Every circuit needs a DC solution; sometimes called an operating point.

Amplifiers

Amplifiers forum

LM358 AC coupled non inverting amplifier