The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> reference bias generation
https://designers-guide.org/forum/YaBB.pl?num=1222279281

Message started by smarty on Sep 24^th, 2008, 11:01am

Title: reference bias generation
Post by smarty on Sep 24^th, 2008, 11:01am

Hi,
I am trying to do a simple reference which should follow Vt. I just did a pmos diode connected and a resistor. I expected as I change the temperature, the threshold decreases and the voltage generated should be vgs-vth and it should change as per vth change, but I do not see that change. Is that I am missing something, this is in DSM in 65nm.

Is there any way to generate the voltage following the Vth to compensate for the temperature.

Thanks,
SBR

Title: Re: reference bias generation
Post by ywguo on Sep 26^th, 2008, 1:44am

Hi SBR,

I am not clear how you connect the resistor and the PMOS. Would you please depict it with a figure?

If you want to observe/measure the Vth vs temperature, you'd better give a constant or relative constant drain current for that PMOS.

Yawei

Title: Re: reference bias generation
Post by smarty on Oct 4^th, 2008, 12:38pm

Hi,
I am sending the fig. as per your request. I am working in 65nm and I see the mobility variation ~50% across temperature. I need to generate the bias which controls to generate the current which should vary to compensate for the mobility reduction across temperature to provide a constant delay cell.
The bias is generated for the constant delay cell.

Thanks and Regards,
SBR

Title: Re: reference bias generation
Post by smarty on Oct 4^th, 2008, 12:41pm

Hi,
Forgot to attach in my previous reply

Title: Re: reference bias generation
Post by Tlaloc on Oct 18^th, 2008, 10:28am

It really sounds like you are trying to follow g_m, not V_t since you are trying to compensate for mobility. There is a very simple constant g_m circuit that is found in a number of text books. One that I think of off the top of my head is the Johns and Martin book, "Analog Integrated Circuit Design".

I ran a quick search on google, and found a paper that describes the technique. www.eecg.toronto.edu/~kphang/papers/iscas2004_nicolson_opamp.pdf. I have not used the additional techniques presented by the paper, but it does give a nice overview of the traditional way to bias amplifiers. One difference is that I always implement the resistor on chip, so I don't ever see the stability problems that they were talking about.

I realize that everything that I have talked about is generated currents that will produce a constant g_m, and you asked for a voltage. Do you really need a voltage? If so, and if you have a low tempco resistor available, you can just drive the current into a resistor.

Title: Re: reference bias generation
Post by smarty on Oct 18^th, 2008, 12:30pm

Hi Tlaloc,
Thanks for the inputs. Actually I was working on a current starved inverter, and I wanted to compensate for the mobility across temperature to get a constant delay. So I tried the above approach and bias the current starved using this bias generation.

I thought of constant gm bias, but I was worried about the stability and compensating that. I wanted to make things simple, so I thought of this one.

I will have a look at the attached pdf.

Best Regards,
SBR