Question

5). In this problem, you are asked to consider the ac hybrid-x model for an NMOS transistor and to relate the capacitors to the physical device structure. Recall the oxide capacitance per unit area ox) appears in the DC Ip-Vs relationship for triode and saturation regions. The NMOS transistor has kn-0.2 mA/V2, W 10 μm, L-I μm and μ,-1000 cm2/Vsec. a) Find the total gate-to-channel capacitance for small VDs, CG-cho. Hint: this is the parallel-plate capacitance between the gate and the full channel area. b) Next, consider a bias point in the saturation regime. Since the channel charge density at the drain end of the channel decreases with increasing Vos ("pinch-off"), CG-cho does not divide equally between G-S and G-D components. Assuming that Cgs is 80% of CG-cho and Cgd s 10% of CG-cho, find Cgs and Cgd. c) Suppose that this transistor is used in a CS amplifier with the topology of the 2nd stage in Prob l (i.e. this transistor is used as M6). The DC hybrid-x parameters at the DC bias point are gm 2 ms and r,-100 k Find the Miller capacitance (CM) for this CS stage. Note that for an inverting amplifier, CM Cd (1-Av) d) Find the total effective capacitance between G and S (Cgs +CM) +V MS -O 匕 l M 1 V. M6 M4 M3

Answer 1

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