Berkeley ELENG 105 - Lab 8 SPICE example (4 pages)

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Lab 8 SPICE example



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Lab 8 SPICE example

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Pages:
4
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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EE105 Fall 2006 Nathan Pletcher Lab 8 SPICE example In this lab you were to simulate the cascode given in the Lab 8 manual A simplified version of the circuit is shown in Figure 1 with a current source supply instead of the PMOS cascode given in Figure 3 This example will show how to simulate the amplifier in Figure 1 The main difficulty is to figure out how to bias the gate of M1 the input device As shown in the figure the gate is a floating node and SPICE will complain about it The reason is that there is nothing connected to that node to set the voltage On one side is the 10uF cap which has infinite DC resistance On the other side is the gate of M1 which is a capacitor and also has infinite DC resistance Somehow we need to get an appropriate bias voltage on the gate of M1 In this case appropriate means that we set up the gate voltage so that it will be able to have a drain current of Isup which is set by the top current source Remember the simple saturation equation Id W L unCox Vgs Vt 2 We need to give M1 the correct Vgs so that it can have the Id dictated by the current source In the SPICE simulation I will do this by connecting a voltage source to the gate with a resistor I use a huge 1x1015 ohm resistor to make sure that there is no voltage division between the 1Mohm source resistance R1 Alternatively you could use 90kohm because this is the approximate input resistance of the bias network shown in Figure 3 which is what we are trying to emulate But then you will get a voltage division right at the input just like in the lab measurement and that must be taken into account in your calculations For now let s just stick with a 1x1015 ohm resistor and try to simulate the gain of the cascode alone without any input voltage division Next I sweep the bias voltage vvin in my SPICE deck over a narrow range to see when the output voltage is about Vdd 2 That s where I set my DC voltage for vvin I found the approximate solution using the Id equation above and then simply



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