EENG 2610: Circuits Analysis Class 6: Operational Amplifiers (Op-Amp), 1/2 Oluwayomi Adamo...
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Transcript of EENG 2610: Circuits Analysis Class 6: Operational Amplifiers (Op-Amp), 1/2 Oluwayomi Adamo...
EENG 2610: Circuits AnalysisClass 6: Operational Amplifiers (Op-Amp), 1/2
Oluwayomi AdamoDepartment of Electrical EngineeringCollege of Engineering, University of North Texas
Operational Amplifier (Op-Amp) Op-amp is the single most important integrated circuit (IC) for analog circuit
design; it has been extensively used in circuit design at all levels. Op-amp is consisted of individual transistors and resistors interconnected
on a printed circuit board (PCB) Op-amp was originally designed to perform mathematical operations such
as addition, subtraction, differentiation, and integration. We have learned tools to analyze practical circuits using op-amps !
Op-Amp Models Op-amp is just a really good voltage amplifier! Example: LM324 from National Semiconductor
General purpose quad (four in a pack) op-amp.
unit: inch
Dual Inline Pack (DIP) style packageFour identical op-amps in the package IN +: noninverting input IN -: inverting inputOUT : output VCC: positive voltage VEE: negative voltage or ground
)(00 ININAV
Typically, A0 is between 10,000 and 1,000,000 !
In-Out Voltage Relation:
Op-amp is modeled using a dependent voltage source and resistors
Power Supply and Ground
A simple model of op-amp
gain amp-op :
resistoroutput :
resistorinput :
A
R
R
o
i
)( vvAvo
In-Out Voltage Relation:
voltage source
Effects of Power Supply Each op-amp has minimum and maximum supply ranges over
which the op-amp is guaranteed to function For proper operation, the input and output voltages are limited to no
more than the supply voltages (VCC, VEE). Inputs and output are called rail-to-rail, if the inputs and output can
reach within a few dozen millivolts of the supplies. An op-amp is said to be in saturation,
If an increase in the input voltage may not yield a corresponding increase in the output voltage
Saturation and Rail-to-Rail
)( vvAvo
In-Out Voltage Relation:
Rail-to-rail output voltage
PA03
Unity Gain Buffer Circuit
iin
inOOout
inOOis
IRv
vAIRv
vAIRIRV
0
0
iOO
is
out
RAR
RV
v
1
1
1S
outO V
VA
That’s why it’s called Unity Gain Buffer,or Voltage Follower.PERFORMANCE OF REAL OP-AMPS
Op-Amp BUFFER GAINLM324 0.99999LMC6492 0.9998MAX4240 0.99995
VoltageGain:
?,, ooi ARRWhat should be the values for
Lo
Lo
Thi
i
S
o
RR
RA
RR
R
V
v
1
Voltage Gain:
To achieve large overall gain independent of ideally 0,, oio RRA
LTh RR ,1
(Commercial op-amps do have this tendency !)
Equivalent resistance of voltage source Equivalent load
resistance
Op-amp Model
Ideal Op-Amp Model
ARR iO ,,0 : Model Ideal
vv
ii 0
i
i
iR
)(
vvAv
A
o
0 vv
Analyze unity gain buffer using ideal model
ARR iO ,,0 : Model Ideal
vv
ii 0
v
v
0i
0i
1i
LR
CCV Where does the current come from?1i
v
v
so vvvv
Why use unity gain buffer? Unity gain buffer is buffer amplifier
Unity gain buffer isolates driving circuits from load circuits, which is called buffering
The load current (or energy) comes from op-amp power supply, which have plenty of current (or energy) output capacity, rather than the driving circuit.
CONNECTION WITHOUT BUFFER CONNECTION WITH BUFFER
Sv
consume source energySR does not consume source energySR
driving circuitdriving circuit
load load0 0 CCV
iRvv sso so vv
Op-Amp Circuit Analysis General rule for op-amp circuit analysis
Use the ideal op-amp model conditions Write nodal equations at the op-amp input terminals
ARR iO ,,0 : Model Ideal
vv
ii 0
Example 4.2: Basic inverting op-amp configurationDetermine gain using both non-ideal model and ideal model
Note: the ground can all be connected to a single node.
Equivalent
1. Identify op-amp nodes
2. Redraw the circuit cutting out the op-amp
v
v
ov
4. Redraw as needed
2R
v
v
v
v
ov
3.Draw components of linear op-amp
v
v
oviR
OR
( )A v v
Using non-ideal op-amp model:
)1
()1
()11
)(111
(1
)1
)(1
()11
)(111
(
))(1
(
22221
1
2
22221
12
ooi
S
o
ooi
S
oo
RA
RRRRRRR
RR
v
v
RA
RRRRRRR
R
v
RA
Rv
51
2lim
R
R
v
v
S
o
A
Typical values:
kRkR
RRA i
5,1
,10,10,10
21
085
59996994.4 S
o
v
v2v
3v
v
v
ov
Using ideal op-amp model:
vv
ii 0Ideal op-amp model:
000
0
21
R
v
R
v
vv
oS1
2 R
R
v
v
s
o
From now on, unless otherwise stated, we will use the ideal op-amp model to analyze circuits containing op-amp.
General rule for op-amp circuit analysis Use the ideal op-amp model conditions Write nodal equations at the op-amp input terminals