How to improve PSR of the bandgap?

[pugongying]

PSR of bandgap

What is the reasonable power supply rejection of this bandgap structure?

how can I improve PSR of the bandgap ?Thanks

 

[manissri]

Re: PSR of bandgap

making cascode of upper transistor u can increase the psrr in opamp

 

[leo_o2]

PSR of bandgap

Use common-source PMOS in the last stage of opamp to drive the two PMOSs.

 

[she_long]

Re: PSR of bandgap

And increase the gain of op can improve the PSRR too.

 

[pfd001]

PSR of bandgap

Output capapcitor can help you to improve the PSRR

 

[jluhzw]

PSR of bandgap

I don't think that higher op amp gain can improve the psr. As I see, the following idea is can improve the psr.
1. Take advantage a pre-regulator inner power for the bandgap.
2. use the cascode current mirror.
3.refine the op amp's design. Set proper DC gain and make sure the compensation is ok. Generally, 60dB gain is ok.If the gain is too high, it is difficult to compense the OP AMP for enough PM.
4.connect a output capacitor can do it.
Depend on your bandgap's structure ,complex and the SPEC parameters, you can choose ideas to improve it PSR.

 

[leo_o2]

PSR of bandgap

For low frequency PSRR (DC PSRR), it is about A*R1/R2. Here A is DC gain of the opamp. Usually, R2 is less than R1 for temperature compensation. So the structure is not for high PSRR. However, increasing A will still help to increase PSRR(DC). In the other hand, high DC gain for the opamp is difficult to be stable. For high frequency PSRR, it is more complicate that depends on your compensation network. The compensation will decrease high frequency PSRR because it will degrade the gain.

 

[wwwww12345]

PSR of bandgap

Dear Leo, How did you know the low frequency PSRR is about A*R1/R2,thanks~

 

[leo_o2]

PSR of bandgap

You can calculate with small-signal analysis

 

[chenmy]

Re: PSR of bandgap

I think another important point to take care is :
If the OPAMP and the bandgap share the same power supply,Make sure the OPAMP's PSR do not degrade the bandgap's PSR.

Best regards!
chenmy

 

[PaloAlto]

PSR of bandgap

In my experience, the opamp BW does not degrade PSR over freq, it is actually the PMOS current mirrors and ultimately the bipolar core itself. So, in the end, to have good PSR over freq I had to add big decoupling caps :-(