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Can I use such simple reference???

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DZC

DZC

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In my circuit I need a reference which is equal to VDD/2 (varies according to VDD).
I simplely draw two transistors in series.To save the power comsumption I used long channel length,and for better matching I used large gate area.
Then I wander is it a practical circuit?If so,any other consideration should be taken?
And if not,any other better, practical circuits?
Since the VDD/2 is of great importance for my circuit, I come here to make sure about it.
Thanks for you comments.
 

In addition the output of the reference is connected to the gate of a OTA's input transistor.
 

Why u don't use resistors?
For ur circuit it's better to use same type MOSFETs (pmos or nmos) otherwise Vref <=> Vdd/2 for different corners. Term "matching" isn't suitable for different type MOSFETs.
Other for different corners current though ur ckt will vary at the least +/-100%.
If Vref is reference for comparator (or other switching ckts), small current though ur circuit can cause unexpected variation of Vref. In this case it's better to increase current (using more strong MOSFETs) or supply Vref though buffer ckt (OTA, AMP).
 

    DZC

    DZC

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I agree with DenisMark. You should use resistor.such as poly resistor.
Take care the resistor layout that can get a good vdd/2 voltage.
 

Hi,DenisMark
Thanks a lot for you wonderful comment.
By "matching" I mean more immune to process variation.
And as for the use of resistors, I just worried about the power and area trade-offs...
 

For two MOS transistors both of them should have source connected to well to prevent body effect.
So if your substrate is p-type use PMOS, if substrate n-type use NMOS.
 

Fom said:
For two MOS transistors both of them should have source connected to well to prevent body effect.
So if your substrate is p-type use PMOS, if substrate n-type use NMOS.
Click to expand...

Did you mean to use the same type of transistors in series?
 

That is two diode connection mos, P/N MOS, why you think you can get vdd/2 ? Vt of P/N mos is diff
 

large variations with process corners and temperatures, I think
 

DenisMark said:
Why u don't use resistors?
For ur circuit it's better to use same type MOSFETs (pmos or nmos) otherwise Vref <=> Vdd/2 for different corners. Term "matching" isn't suitable for different type MOSFETs.
Other for different corners current though ur ckt will vary at the least +/-100%.
If Vref is reference for comparator (or other switching ckts), small current though ur circuit can cause unexpected variation of Vref. In this case it's better to increase current (using more strong MOSFETs) or supply Vref though buffer ckt (OTA, AMP).
Click to expand...

I agree with denismark,poly resistors is a good choice.

another question is the Vref's stabilization and PSRR. LPF? shunt-series feed back?
 

You can use BG reference + LDO to realize the vdd/2 voltage, I think.
 

Poly resistor is your #1 choice
If u still like your original design, I think two npn will perform better (process variation and vt matching) than pnp and npn
 

If you still want to deign by MOS(tradeoff size). you can let the PMOS's gate connect GND, and add some option NMOS parallel with N current source control by digital signal.
1 the PMOS's Vgs is change only with Power supply.
2 You can adjust you NMOS diode.
But the corner and temperature variation cann't guarantee.
 

If the vref of circuit is very important .I think u can consider use BGR and voltage buffer to get the vref. This circuit has poor PSRR and temp coeffcient. If u have to use this circuit u can use 2 pmos or nmos in stead of cmos architecture. I think use a buffer behind it is a good choice. best wishes
 

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