common mode input voltage of amplifier

[jszair]

Where does this come from ? How do I get this in real design? suppose I have a small sine voltage, how do I add common mode voltage to that so my amplifier can actually operate.

Can I use a voltage divider to get a dc voltage down to like 0.6V and connect to both input of op amp as common mode input?

 

[ashwinvl]

Common mode voltage is nothing but the dc voltage.. Yes you can use a voltage divider or give a sine voltage with some DC value

 

[LvW]

.......... suppose I have a small sine voltage, how do I add common mode voltage to that so my amplifier can actually operate. Can I use a voltage divider to get a dc voltage down to like 0.6V and connect to both input of op amp as common mode input?

This sounds as if you intentionally want to create a common mode voltage. Is this correct? Why?

 

[ashwinvl]

You have to give a certain common mode input voltage(DC) on which the differential signal(ac) is superimposed.
This common mode input voltage acts as the bias voltage for the transistors inorder to work at the operating point. I hope you know the importance of bias voltage.
Hence it is important to give a common mode input voltage as a bias voltage.. although the differential amplifier amplifies only the differential(ac) signal and rejects the common mode signal.

 

[jszair]

This sounds as if you intentionally want to create a common mode voltage. Is this correct? Why?

Yes. The differential signal comes from a sensor and needs to be amplified. The common mode voltage coming out of the sensor is very low.

In single supply amplifier this would be a problem. In dual supply the common mode input range covers 0 V common mode.

So my question is how to give the signal a common mode voltage.

 

[ashwinvl]

I assume you are asking how do I bias using a single supply.. Correct me If I am wrong..
You have various methods for biasing.... voltage divider, self-bias etc.

 

[kender]

The differential signal comes from a sensor and needs to be amplified. The common mode voltage coming out of the sensor is very low.

What type of sensor do you have?