The use of biasing MOS transister

[hysamir]

I wanted to know why we use biasing transister?

I am attaching a circuit diagram which has photodiode circuit connected with difference amplifier. Now the question is why there is need to have biasing transister. (at this moment i don't know what will be the voltage accross each transister. the biasing transister's gate voltage will be desided but for sure it's not vdd.

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From my understanding the biasing is needed so mosfet can operate in saturation region. so why we need EXTRA MOS transister? can we just lower the voltage or higher the voltage where we need to biasing?

i hope u understanding ...
pls help

 

[hysamir]

or in this circuit... why v_CMbias required?

Here is the description of that circuit.

he output impedance of this simple current mirror is finite, as a result, the
output current is sensitive to the fluctuations of the drain-source voltage of M2 due
to channel-length modulation. Various methods of enhancing the current mirror out-put impedance are outlined in [45]. In this design we use a feedback circuit (M3, M4) to keep a stable drain-source voltage across M2. The output impedance is given by rout 0.5×(gM3gM4rds2rds3rds4)[45]. Transistor M7 is used to bias the feedback circuit. Alternatively, from an analog VLSI viewpoint, M2 and M3 can be thought of as a current conveyor. Current output from M7 controls the gate voltage of M3, or the drain voltage of M2. M4 then acts as a current bu?er, and conveys current from the high conductance source to the low conductance drain. When row select (M5) is on, pixel current is output to the column bus. Transistor M6 allows current discharge when the row is not selected


how does M7 will help.... and how come M7's current will controll gate voltage of M3