04_Lecutre - Lecture 4 EE114 Lecture 4 Operating Point...

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Lecture 4 EE114 R. Dutton, B. Murmann 1 EE114 Lecture 4 Operating Point Calculations A Look at the Inner Workings of Spice R. Dutton, B. Murmann Stanford University R. Dutton, B. Murmann 2 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 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
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Lecture 4 EE114 R. Dutton, B. Murmann 3 Example Given: V I =2V, W=20 μ m, L=1 μ m, R 1 =5k Ω , R 2 =25k Ω , V DD =5V Device parameters: μ C ox =50 μ A/V 2 , V t =0.5V Calculate: I D EE114 V DD R 2 V o R 1 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 small-signal transfer function in homework 2.
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