Re: Transistor as Resistors
I though you were referring to a different thing ! Actually, if you put your transistor like that you have a diode and not a resistor (relation between voltage and current is quadratic and not linear).... but i now understood you...
First of all, the pmos transistor that it is connected to the point A (on the top left) must be connected as a diode, in order to mirror the bias current to the rest of the amplifier....
Regarding your question, you can put whatever you want (current source, resistor, diode) connected between node A and GND to set the bias current. The problem is the variations that you will have in the bias current, with the variations of the supply voltage, temperature and process.
If you use a resistor, then the bias current will be determined by the supply voltage and by the series of a diode connected transistor (the PMOS I mentioned before) and that resistor. For example, when the supply voltage or resistor value varies, your current will also vary. A similar situation is found if you use a NMOS diode instead of the resistor.
Note that it is not possible to create a large resistor with a MOS connected like this -> 1/gm is small for reasonable currents.
The only way to have a stable current is to put there a current source - the most simple is a transistor biased in the saturation region. Of course that, in this case, you must have a separated circuit which generates the bias current, that is then mirrored to your amplifier by this transistor (industrial designs use one circuit that generate a bias current, which is then mirrored to all the blocks of the circuit)
If your purpose is to make simple tests in the amplifier and you can add a pin just for this, the solution of controlling the bias current externally is probably the best one for you.
Hope this helps