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COLORADO STATE UNIVERSITY
ECE 332: ELECTRONIC PRINCIPLES II
HOMEWORK 5
1. In a MOS cascode amplifier, the cascode transistor is required to raise the output resistance
by a factor of 40. If the transistor is operated at |𝑉𝑂𝑉 | = 0.2 𝑉, what must its 𝑉𝐴 (the early
voltage) be? If the process technology specifies 𝑉′𝐴 = 5𝑉/𝜇𝑚,(Early voltage per unit channel
length in m). what channel length must the transistor have?
2. For a cascode current source shown in the figure on the
right, show that if the two transistors are identical, the
current 𝐼 supplied by the current source and the output
resistance 𝑅𝑜 are related by 𝐼𝑅𝑜 = 2|𝑉𝐴 |2 /|𝑉𝑂𝑉 |. Now
consider the case of transistors that have |𝑉𝐴 | = 4 𝑉 and
are operated at |𝑉𝑂𝑉 | = 0.2𝑉. Also, let 𝜇𝑛 𝐶𝑜𝑥 = 100𝜇𝐴/𝑉 2 .
Find the 𝑊/𝐿 ratios required and the output resistance
realized for the two cases: (a) 𝐼 = 0.1𝑚𝐴 and (b) 𝐼 = 0.5𝑚𝐴.
Assume that 𝑉𝑆𝐷 for the two devices is the minimum
required (|𝑉𝑂𝑉 |).
3. Design the circuit in the figure from problem #2 to provide an output current of 100𝜇𝐴. Use
𝑉𝐷𝐷 = 3.3 𝑉, and assume the PMOS transistors to have 𝜇𝑝 𝐶𝑜𝑥 = 60𝜇𝐴/𝑉 2, 𝑉𝑡𝑝 = −0.8 𝑉, and
|𝑉𝐴 | = 5 𝑉. The current source is to have the widest possible signal swing at its output.
Design for 𝑉𝑂𝑉 = 0.2 𝑉, and specify the values of the transistor 𝑊/𝐿 ratios and of 𝑉𝐺3 and 𝑉𝐺4.
What is the highest allowable voltage at the output? What is the value of 𝑅𝑜 ?
4. The cascode transistor can be thought of as providing a
“shield” for the input transistor from the voltage variations
at the output. To quantify this “shielding” property of the
cascode transistor, consider the situation in the figure on
the right. Here we have grounded the input terminal (i.e,
reduced 𝑣𝑖 to zero), applied a small change 𝑣𝑥 to the output
node, and denoted the voltage change that results at the
drain of 𝑄1 by 𝑣𝑦 . By what factor is 𝑣𝑦 smaller than 𝑣𝑥 ?
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