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Input from students
(Week of Oct.8’12)
-LW response: wL=1/RC1 + 1/RC2 + ... Each time constant is calculated by
short-circuiting the voltage source and the other capacitances (coupling and
bypass), and opening the current sources.
-HW response: wH=1/(RC1+1/RC2+...) Each time constant is calculated by making
the other stray capacitances an open circuit, short-circuiting the
independent voltage sources and opening the current sources.
-hfe: ac current gain. Used under the B-cutoff frequency because is constant.
hfe(0)=B.
-Miller's theorem: divides Cu between 2 floating points into 2 grounded
capacitances.
(Prof. input)
Pretty good. No further additions.
(Week of Sep.24’12)
Important parameters of the differential amplifier:
1) Ad: Differential Gain
2) CMRR: Ad/Acm
3) Rin: Input resistance, usually comes from a current source output resistance.
(Prof. input)
Good. I can add
1. For balanced differential signals, the DA can be modeled as a half circuit (a CE or CS
amplifier)
2. The output resistance RI of the bias current source influences the CMRR of the DA.
Higher in the CMRR, better is the DA.
(Week of Sep.17’12)
No input(s) from any student.
(Week prior to Sep.17, 2012)
 Here are my points:
1. You can implement a basic current source using two matched BJT's or MOSFET's in the form
of a current mirror.
2. You control the current of the current source by either varying the resistance at the reference
current, or by varying the amount/configuration of the mirrored transistor.
 My understanding of ELEC312 in the past week is as follows:
1) The DC&AC calculation of BJT and MOS (review class);
2) The function and configuration of BJT current mirror and MOS current mirror;
3) DC bias design for BJT current mirror and MOS current mirror;
4)The fundamental theory of three Non ideal effects:
a. Base-width/channel modulation effect;
b. Effect of channel modulation in a MOS current mirror & the concept of tracking error;
c. Effect of finite β in a BJT mirror.
Input from the professor
All the above are right. I can add
1. Current source is required to provide a constant DC bias current to an electronic amplifier.
2. Current mirror is used to provide a DC bias current which is proportional to the current in the
reference current source.
3. The size of the mirror transistor can be (or need be) adjusted to provide different values of bias
current to different amplifiers.
4. An array of current mirrors is used to supply different bias currents to different amplifiers in a
multistage amplifier system.
5. The accuracy of tracking of the current value in the reference current source is affected by the
beta (in BJT) of the transistors as well as the slope of the output characteristics (arising out of
Early effect) of the transistors (BJT and MOSFET)