about bandgap voltage reference

[wonbef]

hi, all,

the following circuits are bandgap voltage referenc circuits. but Fig.1 can operate correctly, and Fig.2 will oscillate if the rise time of the power-on is very short (ex: 1us, 10us). but the open loop is stable. i don't know the reason why Fig.2 oscillate, does it result from start circuit? need your help, thanks in advance.
very sorry, i don't know how to paste the figure in here, so i upload the circuits file.

kind regards,
wonbef

 

[flushrat]

yes, it seems start up fails.

 

[rfsystem]

Lets see....

The startup generate initial bias current ~(VDD-Vth,p-Vth,n)^2

seems ok! But the reg opamp of the kT-generator see the impedance of the inital startup bias generator. So use ideal buffer with some real output impedance to isolate the effect. I suggest that oscillation if happen because of the impedance vanish for zero ohm and inifinite ohm. An alternate would be a startup which create parallel current source injection and have a switchoff by active bandgap.

By the way the circuit show a kT-generator instead of a full bandgap. The kT-multiplier is missing.

 

[DenisMark]

1. Circuit Fig.1 requre startup in any way. Bacause at some offset voltage of diff. amp. circuit can be in state when Vref=0.
2. Startup of circuit Fig.2 isn't situable. U control and monitoring the same node of circuit, its wrong (and oscilate). Simple solution: provide pulldown nmos to this node, the gate of nmos connect to inverter (with weak pmos), the input of inverter connect to Vref.
3. Too many stages (4). It can cause stability problems or if property compansated the circuit can be low frequency. Move stage with diode connected load. And connect output of second stage to node where startup connected.
4. Move C1, it cause phase delay in negative feedback.

 

[wonbef]

many thanks to all above! i will try again.

by the way, how to paste figure/picture here.

 

[stoned]

add an attachement is ok, and you file must be the pitrue format
press "Img" button and insert the url

stoned

 

[wonbef]

thank you very much!

 

[nickoo]

Nod to DenisMark