how do I calculate the current in a cascode current mirror

[iou]

how do I calculate (or set) the current in the following cascode current mirror circuit?
Can someone explain the circuit and how to design and analyse it.

Secondly, I'd like to set i to 2.5mA, I know VDD is 1.8V but how to I size the transistors to achieve this using .18um technology?

Opps, I think there is a resistor between the PMOS and NMOS.

 

[sergeyr77]

Re: how do I calculate the current in a cascode current mirr

Usually current is calculated with very well known formula of Id.... Spice programs
calculate it very fast.

 

[alchen77]

1. I don't think this circuit can work in 1.8V. You have 3 diodes but only 1.8V that mean all the MOS threshold voltage needs to less 0.55V in slow corner.
2. If the Vth is not the problem, the PMOS should be connected like CS not diode. because in your case, the current will dependent on the process.
3. If you want to calculate the current you just use the equation I=1/2*uC(W/L)(Vgs-Vt)^2.
Vgsp+2*Vgsn=1.8
4. you can setup a basic number for Vgsp or Vgsn then you will get the I (also W/L needs a number in the beginning)
5. also if you have already know the current is 2.5mA then you can give the Vgsp & Vgsn then calculate the W and L.

 

[sergeyr77]

Re: how do I calculate the current in a cascode current mirr

This circuit will work at any voltage at any technology. Vth is parameter that marks two regions- subthreshold (weak inversion) and strong inversion.

 

[alchen77]

not having currrent means work
Maybe you take some example that 1V technology use this kind of circuit?

 

[ambreesh]

Re: how do I calculate the current in a cascode current mirr

Is their a resistance between PMOS and NMOS.
If not it looks like a inverter with input and output shorted.

 

[iou]

Re: how do I calculate the current in a cascode current mirr

Yes there should be a resistor between the PMOS and NMOS.

Is their a resistance between PMOS and NMOS.
If not it looks like a inverter with input and output shorted.

 

[flushrat]

Re: how do I calculate the current in a cascode current mirr

I have ever set up such a circuit under tsmc .18 process. For a moderate W (such as several um), the current is around 1uA, If u want to get more than 2mA current, W maybe quite large. Furthermore, if we consisder the worst case, such as 1.6V ss corner 120 centidegrade, the task is even more difficult. By the way, I have also try to increase L to get lower Vth and bigger Vdsat, but it will cost more area.....

 

[ambreesh]

Re: how do I calculate the current in a cascode current mirr

dear iou,
I am not sure that one can fix the current exactly
The resistance between pmos and nmos would have (Vdd-Vtp-Vtn-2Von) drop across it, this would decide the current.
We have Vt' varying, R varying both with process and temperature.
Vt' variartion may not have as drastic an effect as R variation ±20%

hope it helps.

 

[mady79]

Re: how do I calculate the current in a cascode current mirr

This is self biased current mirror configuration .
R=1.8-(3Vov+3Vt)/I ;
Vov=overdrive =50mV R approx is 60 ohms ,calculate W/l for pmos and nmos .
The bias current has resistor variations that needs to be accomodated in design .

 

[ambreesh]

Re: how do I calculate the current in a cascode current mirr

daer mady,
the bias current would also see variation due to threshold voltage cariation across slow, fast and combinational corners.
So it is just not resistor variation

 

[mady79]

Re: how do I calculate the current in a cascode current mirr

Amby,

I think process corner's & tolerances are inherent .