start up in current source

[mostafah67]

there is a current source here which is independent to supply voltage and it have a start up circuit (M5) that prevents zero current in the loop:



(figure from:analog cmos integrated circuit by: behzad razavi)

i simulate the circuit but M5 is cutoff, i think i choose the wrong value for Rs. can you help me that what is the start up condition in this circuit:shock:

 

[sarge]

I think that in work mode, M5 must be in cutoff mode and no current flows through it, and you must choose value of Rs only for wanted current value (equation 11.4)

 

[mostafah67]

thanks, before turning of M5 it is on for a specific time, do am right? is it specific condition to work the start up transistor correctly? and is it possible this transistor don't work correctly so that the circuit current was other than thats equation 11.4 was predicted?

 

[sarge]

thanks, before turning of M5 it is on for a specific time, do am right?

Yes, you are right.

is it specific condition to work the start up transistor correctly?

Yes, drain-source of diode must be larger Vth

and is it possible this transistor don't work correctly so that the circuit current was other than thats equation 11.4 was predicted?

Yes, value of resistor may be very big or vdd very small and transistors works not in saturation. Here my example circuit:

There is zero volt on drain of M1 for 10 us.
And current waveforms for startup wit diode and without diode:


Without diode start time more than twice.

 

[mostafah67]

thanks a lot, can you give me the value of R and Vdd, and the ratio of Mos??
another question which i have, what is the switch in the right side of circuit? i didn't see it before in the similar circuits, is it necessary to circuit work correctly?

 

[sarge]

thanks a lot, can you give me the value of R and Vdd, and the ratio of Mos??
another question which i have, what is the switch in the right side of circuit? i didn't see it before in the similar circuits, is it necessary to circuit work correctly?

If i remember correct W/Ln = 36, W/Lp = 56, R = 10 Ohm, Vdd = 2V.

I just make zero current as initial with that switch. You can delete it without any troubles.