2022-10-05 15:51 Status: # Common Source Amps Most common gain block in ICs. ## Basic  ### Small Signal Diagram  Infinite input resistance $r_{out}=r_{ds}||R_L$ $A_v=\frac{v_{out}}{v_{in}}=-g_mr_{out}$ Intrinsic (max) gain: $|A_v|_{max}=-g_mr_{ds}$ Need a decent intrinsic gain, thus $g_mr_{ds} >> 1$ or the transistor is useless! This small intrinsic gain causes issues at nm scales, where intrinsic gain can be less than 10. Also input resistance is no longer assumed as infinite. ## Load Analysis ### Passive Load  Operating point is the intersection of the two equations. To increase the gain, need to increase the load resistance, meaning the slope on the load line is shallower. This would move the intersection point down the transistor graph and the output impedance of the circuit (VDS/ID or 1/slope(transistor)) would be very low. Thus higher gain requires us to increase the supply voltage, which cannot be done when we operate at the max voltage we can. ### Active Load   The slope of the n- and p- are much shallower - meaning a much higher gain can be established. $A_v=-g_m(r_{ds1}||r_{ds2})\approxeq \frac{1}{2}g_mr_{ds1}$ for equal drain source resistances. ### Biasing of Active Load Don't use a voltage source - current is very dependent on this. Use a [current mirror](Current%20mirrors.md).  ## Transfer Characteristic In the bias condition, we operate where the two graphs intersect. The n-mos characteristic depends on the input voltage.  If we reduce the input voltage, the output characteristic for the n-device would drop in current level and the output voltage would take on a higher value. Vice versa, a higher input voltage would result in a higher current in the n-mos so a lower operating output voltage.  ### DC Transfer Characteristic Need to turn on the n-os device before we get anything but VDD on the output. As we turn on the n device we get lower and lower voltage at the output.  ### Choosing the operating point The operating point should be chosen such that it sits on the steep part of the characteristic because that's where we have the largest gain: ## P-Type  [^1] --- # References [^1]: [vr-4602-wk04-sc02-commonsource](../../Spaces/University/ELEC4602%20–%20Microelectronics%20Design%20and%20Technology/Lectures/W4/vr-4602-wk04-sc02-commonsource.mp4)