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[SOLVED] Bandgap voltage reference Vref =+0.45 Vdd = +1.2

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chandlerbing65nm

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Hi all!

The ramp function from my ramp generator is an input to a comparator(vin-) and another input is vref = 0.45V(vin+), the resulting wave is a square wave with 50% duty cycle.

Now when I made a schematic of bandgap voltage reference(attached image) and simulate it I have difficulty having an output of Vref = 0.45V, I can only get 0.8V-1V vref no matter how much I change the resistors.

My source voltage is 1.2Vdd.

I try using the formula(attached image), but I don't really know what is Vt and n. I assume it is my source voltage and bjt ratio of areas. Please enlighten me.

Also if you can recommend some better bandgap voltage reference I would really appreciate it.

Thank you very much for taking time reading this:thumbsup:
 

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  • bandgap.PNG
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  • bandgapformula.PNG
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Bandgaps have a fix 1.2V output voltage, so your circuit won't operate with 1.2V Vdd. Simply not possible.
Bandgap requires OPAmp not a comparator. Comparator is instable in a feedback loop.
Vt is the thermal voltage (k*T/q), n is the multiplier of the BJT device, which is actually yes, the ratio of the areas.
 
Last edited:

Thanks for the info. Can anyone recommend a voltage reference circuit that can output 0.45V at 1.2vdd? I need a temperature independent voltage reference. What are my options other than BGP?

Can I use a simple voltage Divider(resistors and supply voltage)?
 

yes, if the resistors have the same TC, and it won't bother you if the supply is changing your reference voltage will change too.
 

Hi,
You can google this paper:

Curvature-Compensated BiCMOS Bandgap with 1-V Supply Voltage - Piero Malcovati.

The bandgap in this paper has 0.54V, maybe it helps.
 

Dear ZoOneR,
It is really interesting, I have never heard about similar. My only concern about it is how does it work when the resistors or BJTs have small mismatch?
I am not sure but the shifted characteristics of the BJT devices can introduce maybe higher curvature differencies over temperature which come from asymmetrics.
Maybe you have got more experiences about this, I am realy interested, I couldn't figure it out from this article.
 

If you are not really pressed for area and current a simple
resistor divider off the Vref point can give you any lesser
output you want (provided that the pass transistor MP17
is up to the task, in its min/max gate drive and sizing).

The source voltage is the problem.

It seems to me (though I have never personally put it
to the test) that the bandgap could be "folded" so the
TCR resistors are not stacked on top of the PTAT pair
but PTAT match-point current could be put onto (say)
a divided-down Vf and (say) a half-value TCR resistor
to get a half-bandgap voltage. The Vf/2 would need an
amplifier or a properly tuned resistance (wonder if the
divider resistor, and the TCR resistor, could have a
"magic point" that allows them to be one and the same?).

You might chase through "sub-bandgap reference" art
and see what's been going on; it has been somewhat
popular ever since Vcore dropped below 1.8V.
 


Hi frankrose,
Sorry for the very late reply. I haven't used this circuit yet, since a simple bandgap satisfied my needs until now. If you try it share with us the results :thumbsup:
 

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