2022-10-05 11:05 Status: # Current mirrors Fundamental IC building block. ## Layout "diode-connected" transistor  ## Basic Working  If same $V_{GS}$, same current! ## Current Scaling Can also scale by changing the width ratio (keep length the same for matching purposes):  ## Output Response  When $V_{DS}$ is equal to the effective voltage, saturation is reached. At Vin, for matched width, $I_{out}=I_{in}$ Linear section in saturation can be modelled using a small signal model. ## Small-signal Model  The [diode connected transistor](diode%20connected%20transistor.md) current source acts like a resistor - where the current flow through it is proportional to the voltage across it.  ### Small-signal Input Resistance $r_{in}=\frac{1}{{g_{m1}+1/r_{ds1}}}\approxeq\frac{1}{g_{m1}}$ as $g_{m1}>>\frac{1}{r_{ds1}}$ ### Small-signal Output Resistance $r_{out}=r_{ds2}$ ### Small-signal Gain $v_{in}=i_{in}\frac{1}{g_{m1}}$ At zero-load (the output is shorted to ground since it is a current output) - $i_{out}=g_{m2}V_{in}=\frac{g_{m2}}{g_{m1}}V_{in}$ If the only thing that differs is the transistor width, the transconductance ratio is the same as the width ratio (same as for large signal). Small signal gain: $A_I=\frac{i_{out}}{i_{in}}\frac{g_{m2}}{g_{m1}}$ ## Uses of current mirror ### Single Stage Amplifier Can act as a single stage amplifier, with key LF small signal parameters of input and output resistance and small signal gain. ### Biasing Distributing bias currents can be used by a multiple output current mirror.  All have same gate-source voltage. $I_{out,k}=I_{in}\frac{W_k}{W_{in}}$ ## PMOS  Entirely symmetrical to [N-type](n-type.md). [^1] --- # References [^1]: [vr-4602-wk04-sc01-currentmirrors](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W3/vr-4602-wk04-sc01-currentmirrors.mp4)