Why do I need to run ac simulation on bangap refernce?

[sharas]

Please excuse this questio, which I'm sure most of you will regard as stupid: bangap reference circuit is intended to produce a stable DC voltage. Why is there any need to check the ac behaviour? What are the pitfalls?


Thanks,


Sharas

 

[flatulent]

You can learn two important things.

The first is the transfer function between the power supply and the reference output. This will tell you the effects of noise on the power supply.

The second is the output impedance at different frequencies. This will tell you the effects of a load which draws a varying, periodic current or the effects of a step in load current.

 

[Teddy]

cause there is a) feedback b) amplifier combined it could start oscillating

 

[jinxingsun]

oscillating is a important fator in designing a bandgap.so you must check AC analysis.

 

[sharas]

Thanks for your answers, guys.

flatulent, do you mean that my reference voltage will change with the frequency due to the change of the output impedance? and regarding the noise in the power supply - isn't it enough to perform a DC sweep of +-10% of my the power supply and check that the output voltage is stable?

 

[flatulent]

Yes, any circuit with a finite output impedance will change voltage with load current. Usually the impedance increases with frquency.

 

[aryajur]

and regarding the noise in the power supply - isn't it enough to perform a DC sweep of +-10% of my the power supply and check that the output voltage is stable?

DC sweep always considers all capacitors to be open. So you do not know how a time varying noise signal on the power line gets transferred to the output. So you need to do an AC analysis or a Noise analysis.

 

[huojinsi]

In the bandgap reference, it has a closed-loop negative feedback. When the disturbance signal is injected in the loop,the circuit probably will oscillator.So u must do ac analysis.

 

[steve_guo]

if you use opamp in your bandgap
u should simulate its phase margin and GBW
your bandgap should have enough PM to prevent oscillation and large GBW to have good transient respone