# Non Inverting Amplifier

Thread Starter

#### donjacob

Joined Sep 20, 2005
1
Can some one please tell me :

Why do we connect a resistor to the negetive input of the op-amp and its other end earthed ? What is the roll of this resistor in the circuit, other than in mathematical calculation for gain ?

< Would be greatful to recieve a response at the earliest >
Thanks in advance

#### Brandon

Joined Dec 14, 2004
306
Originally posted by donjacob@Sep 20 2005, 11:31 AM
Can some one please tell me :

Why do we connect a resistor to the negetive input of the op-amp and its other end earthed ? What is the roll of this resistor in the circuit, other than in mathematical calculation for gain ?

< Would be greatful to recieve a response at the earliest >
Thanks in advance
[post=10477]Quoted post[/post]​
It forms a voltage divider which connects to the negative input. The negative input is 180 degrees out of phase with the positive input so that the overall gain of the omamp is clamped down and you get a larger useable frequency range. Its just a simple feedback system.

For instance you head an OPAmp having a 100,000 gain w/ 20khz bandwidth open loop. By adding negative feedback, you can get more useable bandwidth with sacrificing gain. This also gelps stabilize the amp.

The Gain Bandwidth Produt then is 20,000*100,000 = 2e9

Now say you needed a bandwidth of 2MHz and whats the maximium gain you can get with this bandwidth, you take 2e9/2e6 and you get 1e3 = 1K of useful gain.