Current reference using MOS transistors and using VDD and a resistor

[sathi.ec]

Hi
What is the advantage of using current reference (using MOS transistors) over Current reference (using VDD and a Resistor) ? :|

Thanks
Sathi

 

[caosl]

Re: Current Reference

If you are going to look into the current value variations caused by VDD & Temperature,you will find the answer.

Best wishes!

 

[rajkumaru]

Re: Current Reference

Hi Sathi,
as we know resistor is highly affected by supply voltage. So, we will not get supply independent current/voltage reference. value of reference voltage will vary with supply and temperature.
So, we dont prefer resistor base reference circuit.
Generally we prefer BJT based band gap reference circuit because it is temperature independent
Regards,
Raj

 

[qutang]

Current Reference

but reference also depend on resistor.

 

[shaikhsarfraz]

Re: Current Reference

hi,
We prefer current referneces using MOS rather than those made of resistors, because the temperature variation of resistors is much more than that of MOS.
Also MOS gates take lesser area then those of resistors.

thanks
sarfraz

 

[surianova]

Re: Current Reference

Hi
What is the advantage of using current reference (using MOS transistors) over Current reference (using VDD and a Resistor) ? :|

Thanks
Sathi

Definition of current souuce- current independent of voltage across the transistor or rersistor.

MOS can be used as voltage control current source and control by the gate voltage and the current independent of voltage across the MOS but current through resistor depend on the voltage across the resistor.

 

[leelamadhav]

Re: Current Reference

Also the drop across the resistor is more limiting swings

 

[nagula suresh]

Re: Current Reference

resistor value varies by process around 20 to 30%.,
to avoid this we will use current references.

 

[wonbef]

Re: Current Reference

mainly related with power supply and temperature dependence.

 

[tandr]

Re: Current Reference

Transistors have high ac output resistor , so more like idea current source

 

[sengyee88]

Re: Current Reference

To generate a current reference, you still need some kind of reference voltage block, eg Vbg etc. The generated current ref could be PTAT, CTAT or independant of temp depending on your needs.

MOS transistors are mainly for mirroring purpose. It guarantee the current reference are mirrored well to other places assuming negligible channel length modulation (big Length) and sufficient head room for them to stay in saturation over PVT variations.

Unlike a passive resistor, any fluctuation across it will be totally translated into current error.