Bandgap Voltage Reference Cascode

[cr4zypt]

Hello ppl,

I have a question here.



Can you guys please explain the purpose of the EXTRA top three PMOS (left image)!
Is it to increase estability of the circuit since ampop input terminals are switched on both images?
Shouldn't the extra 3 pmos decrease the amount of control de ampop has on the current mirror?

Because I simulated both circuits under an ideal opamp and the result is nearly the same! The difference is only a few mV at Vref.

Thanks.

 

[dick_freebird]

The cascoding is for PSRR particularly, although the higher
gain (from higher Rout) may make a small difference in the
setpoint voltage and tempco (not much, since Rout is
dominated by the diodes' operating point conductance).
Cascoding kills the lambda current-match errors you'd get
in a simple mirror.

 

[cr4zypt]

The thing is that those PMOS transistors are opeating in Triode region, and their Vds is nearly 0 all the time! So, do they serve their purpose of increasing PSRR at all?
NOTE: Every transistor as the same size.

 

[nitishn5]

The thing is that those PMOS transistors are opeating in Triode region, and their Vds is nearly 0 all the time! So, do they serve their purpose of increasing PSRR at all?
NOTE: Every transistor as the same size.

Those PMOS transistors need to be in active region. Else even the current mirroring won't be accurate at all and you can forget about PSR, etc.
You would need to correct the design and keep them in active.