ECE3204 Lecture 7
Gain-bandwidth product
relationship
What’s the difference between
OPEN LOOP GAIN
CLOSED LOOP GAIN
?
Op-Amp as a classical
feedback system
Op-Amp Design Example
Handouts:
Design Example
Op-Amp Error Matrix
ECE3204 Lecture 7
7-1
Op-Amp Design Example
a) Design an inverting amplifier with an input resistance of 1kΩ and a gain of -100
b) If an LM741 (fT = 1MHz) is used, determine the closed loop transfer function, and express in the
form showing the DC gain and closed loop 3-dB frequency (bandwidth) f3dB
c) Determine vOUT(t) for an input step of 0 to 100mV. At what time will the output vOUT(t) reach within
100mV of its final value?
d) Determine vOUT(t) for an input sine wave of (100mV)sin(2π[100Hz]t)
!"!#$%&''()*+,-,./'0.1,23'!45)*+.6'
e) Determine vOUT(t) for an input sine wave at the upper edge of the audio frequency range
(100mV)sin(2π[20kHz]t)
57'
0.1,23'53',38./9,32'5)*+,-,./':,9;'53',3*<9'/.1,1953=.'>-'?%@A'53B'5'25,3'>-'C?%'
f) If we want D7'
a closed
loop 3-dB
frequency (bandwidth) f3dB = 20kHz for an audio application,
E-'53'FGH&?'IJK?GLM7',1'<1.BN'B.9./),3.'9;.'=+>1.B'+>>*'9/531-./'-<3=9,>3'
0.9./),3.'8
determine =7'
the op-amp
unity
gain frequency fT required
OPJ'->/'53',3*<9'1,3.':58.'>-'?%%)Q'1,3I$!'R?%%LMS'97'
B7'
0.9./),3.'8OPJ'->/'53',3*<9'19.*'>-'%'9>'?%%)Q'
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ECE3204 Lecture 7
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7-2
b) If an LM741 (fT = 1MHz) is used, determine the closed loop transfer function, and express in the
form showing the DC gain and closed loop 3-dB frequency (bandwidth) f3dB
!"!#$%&''()*+,-,./'0.1,23'!45)*+.6'
Gain-bandwidth product procedure:
1. Redraw circuit with all inputs suppressed57'(set =0.1,23'53',38./9,32'5)*+,-,./':,9;'53',3*<9'/.1,1953=.'>-'?%@A'53B'5'25,3'>0)
2. Find feedback factor β (fraction of output
to inverting
input)
D7'fed back
E-'53'FGH&?'IJK?GLM7',1'<1.BN'B.9./),3.'9;.'=+>1.B'+>>*'9/531-./'-<3=9,>3
'->/'53',3*<9'1,3.':58.'>-'?%%)Q'1,3I$!'R?%%LMS'97'
3. Closed loop bandwidth f3-dB will be unity=7'gain 0.9./),3.'8
frequency fOPJ
t multiplied by β
B7'
0.9./),3.'8OPJ'->/'53',3*<9'19.*'>-'%'9>'?%%)Q'
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ECE3204 Lecture 7
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7-3
ECE3204 Lecture 7
7-4
c)
Determine vOUT(t) for an input step of 0 to 100mV.
What is the 10%-to-90% rise time?
At what time will the output vOUT(t) reach within 100mV of its final value?
v
IN
v
OUT
ECE3204 Lecture 7
t
t
7-5
d) Determine vOUT(t) for an input sine wave
of (100mV)sin(2π[100Hz]t)
v
IN
v
IN
t
v
t
v
OUT
OUT
t
ECE3204 Lecture 7
e) Determine vOUT(t) for an input sine wave
at the upper edge of the audio frequency
range (100mV)sin(2π[20kHz]t)
t
7-6
f) If we want a closed loop 3-dB frequency (bandwidth) f3dB = 20kHz for an audio application,
determine the op-amp unity gain frequency fT required
ECE3204 Lecture 7
7-7
What about fT (unity gain bandwidth) variability?
If you need to depend on closed loop f3dB, get it from (better controlled) passive element values
ECE3204 Lecture 7
7-8