How can a transistor be used as resistor and capacitor? I read somewhere that for the resistor, we can use a constant current source made up of transistors?
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Using a transistor in place of a resistor or a capacitor is sometimes done in integrated circuit design, because some processes don't offer suitable resistors or capacitors or they simply would take up too much area.
An example is shown below:
The transistor MC is a capacitor. The gate acts as one plate, the channel as the other. Source, drain and bulk are commonly tied together.
Transistor M1 acts as a resistor. It's simply a MOSFET operating in the linear region where it behaves as a resistor.
Another possibility to mimic a resistor would be a switched-capacitor circuit.