Problem #3: A MOSFET Differential Amplifier Problem

A MOSFET based differential amplifier. Assume all nFETs and pFETs are matched to each other with the (W/L) ratios as shown. Assume Vref sets a bias current of 2mA to the nFET current source transistors when biased in saturation.

Further, K'n = 32 mA / V2, K'p = 10 mA / V2, and VT = 1V, Ith' = 0.25 mA, VA = 25V for both nFETs and pFETs.

(a) What is the bias currents flowing through the differential-pair transistors? Are any of the pFETs in the ohmic region?

(b) Draw the Small-signal Model for this differential amplifier assuming a common-mode signal applied to V1 and V2, and then the small-signal model for a differential signal applied to V1 and V2. Solve for the small-signal parameters.

(c) Solve for the differential, common-mode gain, and the common-mode rejection ratio for this amplifier.

(d) What is the common mode and differential mode gain if Vref sets a bias current of 0.25mA to the nFET current source transistors?

(e) What practical issues make this implementation difficult to use?

Solutions Here