EE 140 HW 8
Due Tuesday 3/20/01 at 5 pm in 497 Cory
1) Consider a PMOS-input 2-stage opamp similar to the one shown in figure
5.2 in J&M.
For this exercise:
ignore the compensation network formed by Cc and Q16,
don't use an output stage (Q8 and Q9).
assume that all transistors are 100/1 except Q7 which is 200/1 and Q13
which is 25/1 (quick quiz: why change Q7 and Q13?)
assume that the input pair has a desired Vdsat of 0.2 V
assume that the supply is +-5V
2) Use the op-amp above with a 5V supply to simulate the effect of digital
circuits switching nearby your analog circuits. For the digital circuits,
we will assume that there are 100,000 minimum sized transistors (say W/L
of 1/0.5) in the digital circuit. At any given transition, half of
them may be switching. We'll simulate this with two giant digital
inverters, each made with 50,000/0.5 NMOS and PMOS devices.
One will switch, the other won't (we include the non-switching transistors
to represent parasitic capacitance on the power supply).
Calculate the desired bias currents flowing in Q1 through Q7
Calculate the gm, ro, and gain of the first and second stage.
Choose Rb such that the supply-independent bias network (Q10-15) provides
the appropriate bias voltage to Q5 and Q6.
Design a much simpler bias network to replace Q10-15 - use only a resistor
and a diode-connected PMOS transistor.
For both the supply-independent bias network, and your simpler bias network,
calculate the change in currents, Vdsats, gm, ro, and gain that would result
from the supply changing from +-5 to +-2.5 as the batteries die.
Use SPICE to verify your hand calculations for bias currents, Vdsats, gm,
ro, and gain calculations. Make a table with your calcs, and SPICE's
calcs. The table should have 9 columns:
Hand calculations for the Q10-15 network with +-5V and +-2.5V
Hand calculations for the simplified bias network with +-2.5 V (hopefully
+-5V would be the same as for the Q10-15 network!)
SPICE calculations next to each of the hand-calculated columns.
% difference between hand calculations and SPICE calculations
Using a 5V supply with a 10 ohm series resistance, use transient analysis
in spice to determine the maximum current spike during switching.
Use a 1ns rise/fall time and a 5ns pulse. What is the magnitude of
the voltage spike on the digital supply?
Hook your op-amp from problem 1 up to the same voltage supply as the digital
circuit, and measure the output voltage spike due to digital supply noise.
Do this for both the supply-independent bias circuit, and the simplified