2022-10-12 10:00 Status: # Differential Pair  Performance is better with a constant current source (current mirror, possibly a [cascode](cascode.md) current mirror). The output of a differential pair is the difference between the two drain currents (transconductive device). Input is the difference between bias voltages.  ## Uses of Differential Pair Usually used for a small differential input voltage.  Device is transconductive. ## Input Resistance The input resistance of the device is large since no current flows into the gate. ($r_{in}=\infty$) ## Gain $G_m=\frac{i_{out}}{v_{in}}=g_m$ for a matched set of transistors ## Output Resistance Depends on load connected. Commonly use a current mirror for an active load:  $r_{out} = r_{ds2}||r_{ds4}$ -> looks like it's M4 and M2 in parallel. Requires a full small-signal diagram to derive. Current source used to bias is normally connected to negative power supply (almost always in practice). ## PMOS  [^1] --- # References [^1]: [vr-4602-wk04-sc05-differentialpair](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W4/vr-4602-wk04-sc05-differentialpair.mp4)