how to decide the overdrive voltage

[wjlzhx]

what is overdrive voltage?

if i want to design a circuit ,example a bandgap .how i detemine the overdrive voltage???
if it sensitive to current ,i get the overdrive low ,may be 150mv.
if it sensitice to voltage ,i get the overdrive hight ,may be 300mv.
am i right????

if you know ,please tell me .Thanks a lot

 

[wjlzhx]

does anyone know???
please help me

 

[rubink]

Isn't it the other way round - for a current mirror where you are trying to match current the overdrive should be larger eg 300mV. The other constraint is the voltage headroom for the mirror devices, the larger the overdrive the more volts drain-source you need to keep the devices saturated.

For the voltage case, for example the input pair of an amplifier, it is better to reduce the overdrive.

For a cmos ampifier if you make the channel length of the mirror devices longer by 3X or more than the input pair, then the input offset will be set by the mismatch of the input pair.