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Design >> Analog Design >> Generating common mode in a differential amplifier
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Message started by jockeymonto on Dec 24^th, 2013, 12:34am

Title: Generating common mode in a differential amplifier
Post by jockeymonto on Dec 24^th, 2013, 12:34am

Hi all,
In CMOS diff amplifiers we have to generate the common mode voltage which serves as gate bias for the input diff transistor pair. I know there are several bias schemes based on resistors given in text books. But the total voltage at the gates of input diff transistor pair for diff amp is given as:

VG1=Vcm + (delta/2)
VG2=Vcm - (delta/2)

where delta= Vin1-Vin2.

I need to know how in practical design we create Vin1-Vin2/2. Do we use op amps to subtract them? How do we divide them by tow? Do we just ride the Vin1 and Vin2 on Vcm? How is the above equations implemented?

Title: Re: Generating common mode in a differential amplifier
Post by carlgrace on Dec 25^th, 2013, 8:47pm

There are different ways to set the input common mode of a differential amplifier. It really depends on the application. If you are AC coupled (meaning the DC level at the input of the amplifier isn't set by the output of the driving circuit) then either you use resistors or if you are using a switched-cap system you set the common mode explicitly with switches.

If it is DC coupled the input common-mode is set by the output common mode of the circuit driving it. This is where the "common-mode range" specification for differential amplifiers comes in. A differential amplifier can handle any common-mode as long as the input devices (and the tail current source) stay in saturation.

Typically the common mode can vary some significant percentage of your power supply and you're still ok.