Virtual Ground Common Drain Gain: (Drain Resistance)/(1/gm+Source Resistance) Low Res into the drain of Q3 High Res into the drain of Q4 Not a virtual ground (Strictly Speaking)
Analysis Technique Short circuit transconductance Output resistance
Short Circuit Transcondutance Virtual Ground Justification: 1.The drain of Q4 is grounded! 2.The resistance into Q3 is low! (e.g. 1/g m3 ) 3.Therefore, we have an approximate virtual ground. Approximations: 1.r o1 >>1/g m3 2.r o3 >>1/g m3 Implications of the short at the output: 1.r o4 is shorted out. 2.r o2 is shorted out.