Mos sooch cascode current mirror

Hi,

Can anyone explain me the use of the current mirror attached with this question thread?

this is not a current mirror. you are welcome.

frankrose said:

this is not a current mirror. you are welcome.

Click to expand...

Ok. May be I need to rephrase the question. What is the use of the above mos structure? Any references?

Thanks,
Rama

the 4 mosfets are equal with one diode connected device. we use diode connected devices in current mirrors, level shifters, loads, etc.
if you connect in series 4 devices as attached it is equal with the increasing of the length for only 1 device with 4x.
but from matching standpoint, especially when we want to implement differential loads or current mirrors, it is better if we use 4 same devices instead of 1 device with 4 times longer length.

Hi frankrose,

Thanks for the reply. I understand that the diode connected MOS with another MOS connected to its source as shown in the diagram will drive the MOS connected at the source in to linear region. So, for better matching we have used four such MOS connected in series to the source of the diode connected MOS.

My question is, what is the purpose of this kind of structure?

Thanks,
Rama

As I mentioned, to reach better matching. You can place the devices on the layout in common-centroid arrangement, and the error of the mirrored current will be less.

Hi frankrose,

Thanks for the reply.

But, your first reply to this thread mentioned that this structure is not a current mirror. But, now your latest reply says, this structure is for better current mirror with less error. These two replies are contradicting.

Thanks,
Rama

Nope. It is still not a current mirror, and it can be a part of a current mirror. There is a difference. Current mirror's have output current, your above circuit doesn't have any output, but diode connected devices are the basic parts of current mirrors.
https://en.wikipedia.org/wiki/Current_mirror#Basic_MOSFET_current_mirror

4 x little diodes are easier to fabricate accurately to get 4 x Vf than a single device with equivalent voltage drop.

It certainly isn't a mirror, how could it be with only two connections. As Frankrose states, it can be used as part of a current mirror circuit just as any other component can be but it isn't one in it's own right.

Brian.

betwixt said:

4 x little diodes are easier to fabricate accurately to get 4 x Vf than a single device with equivalent voltage drop.

It certainly isn't a mirror, how could it be with only two connections. As Frankrose states, it can be used as part of a current mirror circuit just as any other component can be but it isn't one in it's own right.

Brian.

Click to expand...

I completely agree that its not a current mirror. But I have only one question on this structure. What is the exact use of this kind of MOS structures? Is it used for generating particular voltage level to bias other circuits?

Direct me to any books/websites which explain the same. I referred Grey/Mayer book where its explained that this kind of structures are used in sooch current mirror. Pleas help me to understand.

Thanks,
Rama

About Sooch's current mirror you can read more here from his patent:
http://www.freepatentsonline.com/4550284.pdf
And a simplified version for it, I am not sure this is also Sooch's idea, but I didn't hear about it too much (page 3, bottom of the table):
**broken link removed**

The point is to reach higher output voltage swing for the cascoded current mirror. For this somehow we have to produce a bias gate voltage for the output cascode device, which is not coming from 2 stacked diodes (2*Vgs is too much simply). Thus on the input side of the current mirror the upper diode is halved to generate a lower voltage than 2*Vgs, but it is still enough to keep the bottom device on the output side in saturation. The minimum voltage which is necessary for this is Vgs+Vdsat, which is reachable with the halved upper diode on the input side, because the lower device of it is in triode region, and represents a resistor. But instead of resistor better to use MOSFET because the area is smaller for it and it follows better the process variation of the current mirror.
Sorry, I hope you can follow me and I answered your question.