Hi,
I have a question on the bandgap refence. I am refering to the the bandgap reference where the Opamp is used to match the two nodes X & Y and bias the current sources that set the current in both the legs , where v(X) = VBE1 and V(Y) = dVBE + VBE2.  

(Razavi, Pg:391, Fig:11.18)

The current is mirrored to generate the PTAT current and then the Vref is obtained by using another Resistor ( R3) and BJT #3, such that  Vref = Iptat* R3 + VBE3 .

My question is, why cant we just use the node voltage V(y) as Vref instead of mirroring the PTAT current and using another resitor,BJT ..?

If we can tap the voltage V(Y), then what about the voltage  V(X)..? SInce the Opamp forces, both X and Y to be the same, is V(X) also a bandgap voltage..?

Thanks.

Hi,

Interesting question. After some calculations, the I realized that it is not possible to get a bandgap voltage at Y. The voltage at Y is VY = VBE + VTln(A2/A1). To get 0 TC at 300k, assume a -2mV/K change in VBE, the ratio A2/A1 will be exp(23.2), which is a whopping 1.2e10. So i guess it is not possible anyway to have bandgap voltage at Y. Did I make any mistakess?


Regards,
Chase

Hi,

Just want to clarify that A2/A1 is not the ratio of a resistor but the ratio of BJT ratio where n=A2/A1 in Razavi book, A1 is always 1. As for Vx, I am sorry that I guess I do not have the answer. Maybe someone else will be able to help.

Chase

Chase is correct... Even though v(y) is Vbe + dVbe, the dVbe term cannot be made large enough for v(y) to become a "bandgap."  The current ratio required is something like 1e9:1.

Thank you all for actively participating in this discussion.

I am mainly interested in the  temp coefficient of V(X)  vs V(y) if they are forced to be equal by the opamp.  

I am not worried about the huge ratio of  A2/A1 required to make V(y) a bandgap voltage.  Lets  assume (only for this discussion)  that in some way, ( ex: adding R3 below the BJT, as mentioned by qltong) we can acheive V(y) as the bandgap voltage.

Now, since the nodes v(X) and V(y) are forced to be the same, will there temp coeff also be the same..?
The reply by posted by "river", suggesting that temp coeff of v(x) may be the same as V(y) , as Ic is PTAT, sounds reasonable, but I am still not sure..

Thanks

chase,RobG and Qltong: You were  right...

V(Y) cannot be a bandgap reference  because of the missing factor for dVBE to make it large enough to acheive the bandgap voltage. Hence, V(y) and so does v(x) are not badgap references though they are equal. There temp coeffs are the same, but not zero...

thanks