2022-10-05 16:35 Status: # Common Drain Amplifier (aka [[Source Follower]]). Used if you want to lower the output impedance of a node. ## Layout Active biasing acts like a current source. ## Operating Principle  If the current in the device is constant it will have a constant gate-source voltage. Thus if the input voltage (at the gate) changes, the output voltage (at the source) changes by the same amount and we have a constant voltage drop. This means the small signal gain is close to 1. We can supply a decent amount of current in the transistor comared with what is driving the gate - e.g. a high impedance [Common Source Amplifier](Common%20Source%20Amps.md). ## Tee Model for Transistor  Purpose of the current source in the tee model ensures no current flows out of the gate (KCL at the node).  Thus the input resistance is large ($r_{in}=\infty$) Output impedances is parallel combination of resistors. $r_{out}=\frac{1}{g_{m1}+1/ r_{ds1}+1/ r_{ds2}}$ Gain can be found with a voltage divider: (since the current source only ensures no current flows into gate.) $A_v=\frac{r_{ds1}||r_{ds2}}{r_{ds1}||r_{ds2}\ +\ 1/g_{m1}}=\frac{g_{m1}}{g_{m1}+1/r_{ds1}+1/r_{ds2}}$ Gain is slightly smaller than 1. ## Bulk Connection  Including this in our gain and output impedance: $r_{out, bulk}=\frac{1}{g_{m1}+1/ r_{ds1}+1/ r_{ds2}+ g_{mb1}}$ $A_v=\frac{g_{m1}}{g_{m1}+1/r_{ds1}+1/r_{ds2}+g_{mb1}}$ Since $g_{mb1}\approxeq 0.2g_{m1}$ the gain becomes around 0.8 to 0.99. This is somewhere you can connect the body to the source (gnd, lowest potential). This is not possible in an nmos device using anm n-well but you could in a pmos! ## PMOS Design  Also referred to as Source Follower [^1] --- # References [^1]: [vr-4602-wk04-sc03-commondrain](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W4/vr-4602-wk04-sc03-commondrain.mp4)