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

ELEC 350
Electronics I
Fall 2011
Review Topics for Exam #1
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, this list has been made as comprehensive as possible.
General amplifier concepts
- “analog” vs. “digital” electronics
- linear vs. nonlinear regions of operation
- Thévenin and Norton equivalent circuit representations of input and output ports
o determination of open-circuit voltage, short-circuit current, and equivalent
resistance
o test-source method for finding Thévenin (Norton) resistance
o treatment of dependent vs. independent sources
- concept of a signal; voltage and current signals
- concept of a circuit “port” (pair of terminals)
- input and output resistances of amplifiers, sources, and loads
- distinguishing characteristics of “good” voltage amplifiers (very high input resistance
and very low output resistance)
Operational amplifiers
- ideal op-amp characteristics
o infinite open-loop gain
o infinite input resistance between input terminals
o zero output resistance
o zero current flow into the inverting and noninverting inputs
- closed-loop voltage gain vs. open-loop voltage gain
- virtual short if neg. feedback is present and op-amp operates in linear region
- non-ideal characteristics
o finite open-loop gain, non-zero voltage across op-amp’s input terminals
o finite input resistance between input terminals
o non-zero output resistance
- op-amp equivalent circuit model (for both ideal and non-ideal cases)
- output voltage limited by power supply voltages (clipping)
- output voltage limited if maximum output current is exceeded
- lack of virtual short when clipping occurs or output current limit is exceeded
- inverting amplifier
- noninverting amplifier; voltage follower
- analysis of non-ideal op-amp circuits
o nodal analysis is usually preferred
o identification of dominant parts of expressions for gain, Rin, and Rout
o assumption of ideal behavior is sufficient for good accuracy
o determination of Rin and Rout (not to be confused with rin and rout of op-amp)
o negative feedback causes Rout to be far less than rout
o negative feedback causes Rin to be far greater than rin (for noninverting amp)
o effect of RL (load resistance) on input resistance is minimized, but not zero
o effect of Rg (source resistance) on output resistance is minimized, but not zero
Difference amplifier
- has “floating” inputs
- differential-mode vs. common-mode signals
- purpose/advantage of differential amplifier
- problems encountered with physically separated local grounds
- differential gain vs. common-mode gain
- differential (balanced) vs. single-ended (unbalanced) input/output ports
- input resistance of individual ports w.r.t. ground
- input resistance between ports (differential mode)
- common mode rejection ratio (CMRR)
- performance problems due to resistor tolerance; worst-case CMRR
Instrumentation amplifier
- gain can be controlled by varying a single resistor value
- common-mode signals are not amplified by first stage (input amplifiers)
- amplifier circuits in 1st stage and diff amp in 2nd stage do not have to be carefully
matched for circuit to operate effectively
Non-ideal properties of op-amps
- input offset voltage (and output offset voltage)
o model using voltage source VOS in series with non-inverting input or inverting
input
o polarity of VOS varies from op-amp to op-amp (unpredictable)
o determination of component of output voltage due to VOS (use superposition)
o use offset null potentiometer to mitigate effects
- input bias current
o model using current sources IB1 and IB2 at inverting and non-inverting terminals,
respectively
o currents IB1 and IB2 always flow into op-amp terminals
o IB = 0.5(IB1 + IB2), that is, it is the average of IB1 and IB2
o input offset current defined as IOS = |IB1 – IB2|
o unequal bias currents can be expressed as IB1 = IB + 0.5IOS and IB2 = IB − 0.5IOS
(addition and subtraction could be switched; either IB1 or IB2 could be greater)
o mitigate effects of average bias current (IB) using resistor in series with one of the
inputs (R3 in Fig. 2.34 of textbook)
o mitigate effects of offset current using offset null potentiometer, as with VOS (also,
IOS and VOS could partially counteract each other)
o determination of component of output voltage due to IB1 and IB2 (can use
superposition)
- slew rate limit (max. slope of output voltage vs. time)
o slope of output voltage = dvo/dt
o slope of some parts of waveform might be less than slew rate, but output might
need to “catch up” from earlier time interval when slew rate was exceeded
o virtual short disappears when slew rate is exceeded
- output current limit
o due to internal protection circuitry (a good thing!)
o can cause virtual short at op-amp’s inputs to disappear, as with clipping
- output voltage saturation levels
o output voltage can’t actually reach VPOS and VNEG in a real op-amp
o virtual short disappears when saturation occurs
Relevant course material:
HW:
Labs:
Textbook:
Lecture notes:
Web Links:
Mathcad:
Matlab:
#1-#3
#1-#4
Sections 2.1-2.8 (light on Secs. 2.5 and 2.7 and Subsec. 2.8.4)
“Ideal voltage amplifiers
“Voltage Gain of Non-Ideal Inverting Amplifier”
“Input/ and Output Resistances of Non-Ideal Inverting Amplifier”
Burr-Brown INA 105 Differential Amplifier Module
(none)
(none)