# Help with Circuit Analysis problem

#### pichu

Apr 21, 2013
1
power supplies: VDD = +5V, VSS = -5V
transistors: Vtp = Vtn = 0.3V and (k'W/L)p = (k'W/L)n = 1mA/V2

Find the lower and upper -3dB frequencies of the amplifier below. Leave your answer in terms of the circuit components shown and the transistor gm, ro, Cgs and Cgd. Assume Cin >> Cgs, Cgd.
Do not neglect channel length modulation.

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#### davenn

Moderator
Sep 5, 2009
14,081
Hi there Pichu
welcome to the forums

Sounds like homework so have moved it to the appropriate place

what have you attempted so far to work out a solution ?

we dont give direct answers in this section, rather you will get help in working your way through the steps to finding a solution

Dave

#### john monks

Mar 9, 2012
693
There are two distinct questions and if one was to answer completely this would require differential equations. But short of that we will assume that the low frequency roll off and the high frequency roll off are very far apart and therefore has little effect on each other. So for the low frequency roll off we see three resistors essentially in parallel with each other and one capacitor, Cin. So can you see how to calculate the -3dB point from there?
Next you have Rs in parallel with the gate drain capacitance of the mosfet.
So from here we use complex numbers, and a TI-83 if you wish, or you can do it long hand.
Please work on these and tell me what you get. Where are you getting stuck?

There are only three things you can be confused about, the function of the resistors, the fucntion of the capacitances, and the mosfet. Where is the confusion?

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