about weak inversion CMOS bandgap

[marlboro_x]

Here is a weak inversion bandgap reference.I've simulated.And the output voltage is about 1.8V when temp-co is very low.I want to know ,whether i can get a 1.2V voltage with low temp-co,and how?Another question is does the two pMOS both work in weak inversion?
Thx in advance.

 

[Vamsi Mocherla]

Of course you can try and get 1.2 V. Try and control ID6 and the value of VR1 by tweaking the resistor on the output branch. Low tempco will affect your Bandgap voltage variation and not the output voltage. Also, you have to take care of your output current so that your output PMOS is in weak inversion.

 

[JPR]

Are you sure that M2 and M4 really are in weak inversion? If the current densities are high, they will not be in weak inversion, and your PTAT current will not intercept at 0 kelvin.

It seems that this is likely, given your results of zero tempco at 1.8V rather than near 1.2V.

Try making M2 an M4 significantly wider (or add large multiplier) to reduce current density and push the devices into weak inversion, and see if this reduces your zero tempco voltage.

 

[Pipeline]

If u can get very low TC when the Vref=1.8V, I do not think u can get low TC when Vref=1.2V. Ideally, the Vref is fixed for this configuration when TC=0.

 

[marlboro_x]

Thx for ur reply.
I got M2=5/5 M4=40/5 R1=1M ohms,my PMOS VT0 is about 0.83V.However,output voltage is still about 1.7V with zero tempco(and also R2 resistor is very large ,several M ohms.)
But in ricon mora 's paper,it says the output only depends on Vgo.Here is the paper(in page 12).

 

[Vamsi Mocherla]

Try and reduce R2 for desired values of Bandgap voltage. Remember the final output voltage is sum of voltage drop across the resistor and the VBE5. Since, you do not want to tweak the values emitter area of the bipolar device because you do not want to increase the current density,you can try and reduce the values of the resistor. Remember one thing, you can sacrifice a little bit of accuracy in the case of Weak inversion bandgap references.