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Unformatted text preview: 1 Lecture 4 Operating Point Calculations Operating Point Calculations A Look at the Inner Workings of Spice R. Dutton, B. Murmann R. Dutton, B. Murmann 1 EE114 (HO #7) Stanford University Operating Point Calculations Calculating the operating point in the amplifier example from last lecture was relatively straightforward Assume transistor operates in saturation region Compute drain current (via a simple closed form expression) Check if result is consistent with assumption (V DS >V GS-V t ) If not, repeat calculation with triode equation Computing the operating point in general, and in more complex circuits can be difficult Primarily due to the transistors nonlinear I-V characteristics and because their region of operation is not known a prior R. Dutton, B. Murmann 2 and because their region of operation is not known a priori To give you a better feel for this issue, we will work through a slightly more complex circuit And take a look at how Spice handles the calculation EE114 (HO #7) 2 Example Given: V I =2V, W=20 m, L=1 m, R 1 =5k , R 2 =25k , V DD =5V Device parameters: C ox =50 A/V 2 , V t =0.5V Calculate: I D V DD R 2 V o W/L V i Note: This circuit is called a common source amplifier with source degeneration. In this lecture we will only analyze the DC operating point of this circuit. You will take a closer look at its sma signal R. Dutton, B. Murmann 3 EE114 (HO #7) R 1 take a closer look at its small-signal transfer function in homework 2....
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