Download review for elec 105 midterm exam #1 (fall 2001)

REVIEW FOR ELEC 351 EXAM #2 (SPRING 2002)
Posted March 23, 2002
 The following is a list of topics that could appear in one form or another on the exam.
Not all of these topics will be covered, and it is possible that an exam problem could
cover a detail not specifically listed here. However, I have attempted to make this list as
comprehensive as possible. You should be familiar with the topics on the review sheet
for the previous exam in addition to the topics below.
Small-signal modeling of amplifier circuits
- separation of bias considerations (quiescent levels; output voltage swing
range) from small-signal considerations (gain; input and output resistance)
- small-signal model of BJT
- small-signal model of JFET
- small-signal model of MOSFET, possibly including body effect (change in VTR
due to non-zero substrate-source voltage VBS)
- modeling body effect by multiplying gm of FET by the factor 1+, where

-
-
-

2 2 F  VBS
replacement of DC voltage sources with short circuits (because voltage can’t
change across a DC voltage source) and DC current sources with open
circuits (because current through DC current source can’t change)
why DC voltage sources are typically bypassed at AC (i.e., at signal
frequency) using capacitors
derivation of small-signal voltage gain for all kinds of BJT and FET amplifier
circuits
simplifications in gain and input/output resistance formulas when one term of
formula is much greater/smaller than another term
familiarity with typical values of important device and circuit parameters such
as o, ro, bias resistor values, and saturation voltage of BJTs
small signal analysis of BJT circuits using either o and ib, or gm and v
dependence of gm of BJT on quiescent collector current IC, and of gm of FET
on quiescent drain current ID
derivation of the input or output resistance “looking into” a particular pair of
terminals using a test source (vt and it)
use of MOSFETs as “pull-up” loads in MOSFET amplifiers; Thévenin
equivalent resistance of pull-up loads
relationship of transistor small-signal output port resistance to Early voltage
(VA); that is,
V
V
ro  rce  A for BJTs, and ro  rds  A for FETs
IC
ID
transistor small-signal input port resistance rx (real, physical resistance due to
ohmic contacts)
Two-port amplifier representation
- input of amplifier has an equivalent input resistance rin
- output of amplifier can be treated as a Thévenin equivalent circuit with voltage
av vin (where av is the small-signal voltage gain) and output (Thévenin
equivalent) resistance rout
Differential amplifiers
- concept of differential-mode and common-mode signal voltages (vidm, vodm,
vicm, and vocm)
- concept of a differential (or “floating”) voltage vs. a “single-ended” voltage
- use of a BJT to provide an emitter bypass function in a regular single-input
common-emitter amplifier circuit (avoids use of a capacitor; and technique
works at very low frequencies – in fact, all the way to DC)
- two-input BJT differential amplifier (superposition of two single-input commonemitter amplifiers with emitter bypass)
- calculation of differential-mode, single-ended output voltage gains Adm-se1 and
Adm-se2
- calculation of differential-mode, differential output voltage gain Adm-diff
- identification of appropriate KVL loops and KCL nodes to aid in the derivation
of voltage gain and input/output resistance
- calculation of common-mode, single-ended output voltage gains Acm-se1 and
Acm-se2
- calculation of common-mode, differential output voltage gain Acm-diff (usually
equal to zero for perfectly-matched transistors and loads)
- Admdiff  Adm se1  Adm se2 and Acm diff  Acm  se1  Acm  se2
-
-
common-mode rejection ratio (CMRR) for differential output and single-ended
outputs
expression of CMRR in dB
BJT diff amp input and output resistances (for both differential and singleended outputs)
BJT diff amp biasing
o current source based on common-emitter amplifier topology with
emitter degeneration (second half of Sec. 8.3.6)
o simple BJT current mirror (Sec. 8.3.7)
o BJT Widlar current source (Sec. 8.3.8)
biasing multiple diff amp circuits using a single “reference leg”
desirability of diff amp biasing circuit to act as close to an ideal current source
as possible (should have very large Norton equivalent resistance)
Relevant sections in the textbook:
Sections 7.4.2-7.4.6, 7.5, 8.1-8.3
You should also review the lab handouts. Any material covered in the lab experiments
could also appear on the exam.