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Current mirror image: why M1 has to be diode connected ?

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robeleon1

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current mirror

why is necessary that M1 be diode conected?
 

STOIKOV

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Re: curren mirror

It works as a resistor, so if a current flows through it, a voltage is generated. This voltage is applied to the other MOS gate and another current is generated.
You can obtain a desired current by changing the sizes of the transistors.
See Design of Analog CMOS ICs (Razavi).
 

robeleon1

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Re: current mirror

ok STOIKOV,

if i don't connect the gate with the drain of M1, a current that flows through M1 is not going to generate a gate voltage?
 

hysamir

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Re: current mirror

In my opinion, the requirement of current mirror is that.... both vgs has to be same

so to get the precise same gate voltge they always connected together... but on the other hand both FET have to be in saturation mode...

so the drain and gate voltage same is best configuration so far for the saturation
and it's also easy to deal with is

cause for the saturation
vds >= vgs -vth

so if vds and vgs are same then vds will always more than vgs-vth

and don't think FET connection can make diode. BJT transistor ... u can think of diode... not MOSFET
 

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