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Design >> Analog Design >> gm boosting
https://designers-guide.org/forum/YaBB.pl?num=1151481288

Message started by grosser on Jun 28^th, 2006, 12:54am

Title: gm boosting
Post by grosser on Jun 28^th, 2006, 12:54am

hello

can you suggest me how to increase transconductance gmx of Mx transistor without increasing bias current?

what circuit solution i should use?

regards

Title: Re: gm boosting
Post by aamar on Jun 29^th, 2006, 6:20am

Hello,

I think you should try to plot the gm vs. vds (at your (vgs-vth) bias value) for your transistor for different channel lengths while keeping the W/L ratio constant (so that the current will not be changed).
i.e. W=10*L and use the L as a parameteric sweep variable

I tried it and I think the larger the channel length (while W/L is constant) you can increase your gm.

Best regards,

aamar

Title: Re: gm boosting
Post by Croaker on Jun 29^th, 2006, 8:23am

Aamar, what is the reason for that?

These don't predict a higher gm under the conditions W/L=constant and I=constant.

gm=k*W/L*(Vgs-Vth)
gm=sqrt(2*I*k*W/L)
gm=2*I/(Vgs-Vth)

Title: Re: gm boosting
Post by grosser on Jun 29^th, 2006, 10:31am

Croaker is right

It's not so easy, but i really need a good solution, so if You have any ideas please public it here

regards

Title: Re: gm boosting
Post by Croaker on Jun 29^th, 2006, 11:35am

grosser wrote on Jun 29^th, 2006, 10:31am:

Croaker is right

It's not so easy, but i really need a good solution, so if You have any ideas please public it here

regards

I wasn't saying he wasn't right, only that I didn't get how that worked based on the simple square-law eqns.

Title: Re: gm boosting
Post by vivkr on Jun 29^th, 2006, 11:24pm

grosser wrote on Jun 28^th, 2006, 12:54am:

hello

can you suggest me how to increase transconductance gmx of Mx transistor without increasing bias current?

what circuit solution i should use?

regards

Hi,

You can certainly increase the Gm of Mx by using larger W/L ratio. Your bias current is set by the lower transistors and so this can be kept constant. In a differential pair, one can easily apply some small amount of +ve feedback to boost the Gm. You can look up the paper by Castello et al. (JSSC, June '90) and see if you can adapt his ideas somehow. However, be aware that circuits with +ve feedback need to be designed very carefully or else there will be instability. If nothing else works and you have access to good NPN BJTs, use them. MOS devices can never beat BJTs in terms of Gm/I. You will obviously need some way to compensate for the base current there.

The change in Gm vs. L is not apparent from the square-law model, and you cannot derive it thus. Short-channel devices have reduced Gm owing to velocity saturation which tends to shift the I-V curve from square-law towards a linear one. Look up any good device physics book (Tsividis for example).

Regards
Vivek

Title: Re: gm boosting
Post by grosser on Jun 30^th, 2006, 1:14am

vivkr wrote on Jun 29^th, 2006, 11:24pm:

You can certainly increase the Gm of Mx by using larger W/L ratio. Your bias current is set by the lower transistors and so this can be kept constant.

The change in Gm vs. L is not apparent from the square-law model, and you cannot derive it thus. Short-channel devices have reduced Gm owing to velocity saturation which tends to shift the I-V curve from square-law towards a linear one.

Regards
Vivek

Certainly larger W/L gives better transconductance, but i've already used the largerst possible W/L. Using longer channel and constant W/L increases gmx, but the increase is very small. I need about 5 times gmx

regards

Title: Re: gm boosting
Post by mikki33 on Jun 30^th, 2006, 1:20am

You can make a replica of M_x and drive with that Nmos transistors at the output (25 pF), instead of cascoded current source. In this case you will have class A/B output stage with higher gain (and probably with lower current consumption)...

Title: Re: gm boosting
Post by aamar on Jun 30^th, 2006, 1:25am

Hello Croaker,

As Vivkr mentioned it cannot be derived from the square law model, or may be we cannot at the moment and it needs a look in a physics book, but from the practical point of view, I use the L to optimize the gm/gds of a transistor and in the same way I tried it only for the gm and the results were true, this means that gm can be improved with increasing the L till a limit where the change will not be noticable.

So I am sorry for not giving a clear answer now, but I will try to find it.

Best regards,

aamar

Title: Re: gm boosting
Post by mikki33 on Jun 30^th, 2006, 2:04am

see the pic

Title: Re: gm boosting
Post by grosser on Jun 30^th, 2006, 2:26am

thank you for the pict

you mean it's a fully diferential opamp to the left?

i have found this soulution in attached picture but it doesn't work. it happens cause vdd=1.3V and vout to the left is 0.8V. this implies very low voltages at the opamps inputs which are cut-off

Title: Re: gm boosting
Post by mikki33 on Jun 30^th, 2006, 3:00am

So, don't use sourse follower at the input. You may make OTA to be differential to differential instaed of differential to single ended and convert to singlr ended only at the output.

Title: Re: gm boosting
Post by grosser on Jun 30^th, 2006, 3:37am

mikki33 wrote on Jun 30^th, 2006, 3:00am:

So, don't use sourse follower at the input. You may make OTA to be differential to differential instaed of differential to single ended and convert to singlr ended only at the output.

if i don't use source follower, i have 0.8V at the non inverting input and about 160mV at the inverting one. One input transistor is cutt-off and introduces low frequency parasitic poles.