lab2 - ECE 3150 Lab 2 3/26/2010 MOS TRANSISTOR MATCHING AND...

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ECE 3150 Lab 2 3/26/2010 MOS T RANSISTOR M ATCHING AND S ERIES /P ARALLEL MOS N ETWORKS 1 Objectives In this lab, you will examine how closely the characteristics of nominally identical MOS transistors on your chips match one another. You will also examine the characteristics of closely matched MOS transistors connected in series and in parallel. 2 Prelab 1. Diode-Connected MOSFET . Recall that a semi-ideal diode has an exponentially increasing current when the voltage across it is positive (i.e., when it is forward biased ) with a slope of η × 60mV/decade of current ( η≥ 1 is the ideality factor), but it conducts only a tiny saturated current when the voltage across it is negative (i.e., when it is reverse biased ). It is common to use an MOSFET as a “diode” by connecting its drain and gate together. Explain why the MOSFET is always in saturation and why it behaves like a diode when connected in this manner. Draw a diode symbol and show how you would replace it with a diode-connected n MOSFET and with a diode- connected p MOSFET. 2. MOS Transistors in Series and Parallel . Consider the pair of matched n MOSFET, shown on the left in each part of Fig. 1. By calling these transistors matched, we mean that their characteristics are identical (e.g., they have the same W/L , threshold voltage, normalized threshold current, V Dsat , normalized I Dsat , etc. and they are operating at the same temperature), so that, if the same terminal voltages were applied to both devices, their channel currents would be identical. For this question, we shall assume that the Early effect is negligible for now. Recall that we can express the channel current of an MOSFET in a source/drain symmetric form as in the EKV model: ( 29 ( 29 ( 29 , ,V V f ,V V f I I DB GB SB GB s - = (1) where I S is approximately the channel current at V th and is a function that assumes an exponential form in weak inversion and a quadratic one in strong inversion. For an n MOSFET, the bulk is connected to ground, so the three potentials in the model would simply be V G , V D and V S . (a) Use the source/drain-symmetric model to show that two identical
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This note was uploaded on 03/26/2010 for the course ECE 3150 taught by Professor Spencer during the Spring '07 term at Cornell University (Engineering School).

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lab2 - ECE 3150 Lab 2 3/26/2010 MOS TRANSISTOR MATCHING AND...

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