Thread: Differential amplifier active load

Hi,

While I was reading the basic architecture of differential amplifier, I noticed that the differential pair (say, a nmos pair) usually has a pmos current mirror as its active load. Why pmos current mirror in stead of 2 diode connected pmos transistors? What is the difference? Thanks.

pmos current mirror has higher output impedence, so dc gain is higher using current mirror, it's gmi*(rop||ron). the dc gain is gmi/gmd when using diode-connected active load, there 1/gmd is dc impedence of diode.

Originally Posted by jiangxb

pmos current mirror has higher output impedence, so dc gain is higher using current mirror, it's gmi*(rop||ron). the dc gain is gmi/gmd when using diode-connected active load, there 1/gmd is dc impedence of diode.

Thanks Jiangxb. For pmos current mirror, one pmos is actually diode-connected to sense the current, and the other pmos is not. Will this cause the unbalanced loads for the 2 nmos transistors?

Yes this will cause unbalaced loads for diff pair... but... It gives you a differential to single ended conversion as well...

If you want to have balanced load then u shud go for pmos current sources as load ... in this case you will haave to take care of output common mode range also.

Thanks Jiangxb. For pmos current mirror, one pmos is actually diode-connected to sense the current, and the other pmos is not. Will this cause the unbalanced loads for the 2 nmos transistors?

Yes, this is the cause of systematic offset in the differential pair design. We try to minimize by keeping the Vds of both PMOS almost the same.

But you pay in terms of offset for higher gain.