input & output limitations 2
TRANSCRIPT
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Input & Output Limitations – 2 TIPL 1131 TI Precision Labs – Op Amps
Presented by Ian Williams
Prepared by Art Kay and Ian Williams
Prerequisite: Input & Output Limitations
(TIPL 1130)
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Real World VCM Range
2
V+ V+
V+V+
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Simple MOSFET Input – VCM to Negative Rail
3
+
-V+
V-
5V
OPA336
-0.2V < VIN- < 4V
-0.2V < VIN+ < 4V
+
-V+
V-
+2.5V
OPA336
-2.7V < VIN- < 1.5V
-2.5V
-2.7V < VIN+ < 1.5V
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Input Pair Biasing
4
Q4Q3
Q1Q2
+VS
VIN+
VIN-
Vgs=0.9V
-VS
Vgs=0.9V
100µA 100µAVgs = -0.8V
Vgs = -0.9V
Vgs = -1.0V
Vgs = -1.1V
100µA
200µA
ID
Vds0.1V
Q5
Vsat
0.1V
Vsat
0.1V
Vsat
0.1V
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OPA336 Input Stage – Maximum VCM
5
Q4Q3
IS1
Q1Q2
+VS
VIN+
VIN-
To second stage
Vgs=0.9V
Vsat=0.1V
VCM swing to positive rail:
Vsat + Vgs = 0.1V + 0.9V = 1.0V
Therefore,
Vcm_max < +VS – 1.0V
-VS
For Vcm > Vcm_max,
Q4 will cutoff and
IS1 will saturate.
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OPA336 Input Stage – Minimum VCM
6
Q4Q3
IS1
Q1Q2
-VS
+VS
VIN+
VIN-
Vbe=0.6V
Vsat=0.1V
Vgs=0.9V
VCM swing to negative rail:
Vbe + Vsat – Vgs
= .6V + 0.1V – 0.9V = -0.2V
Therefore,
Vcm_min > 0.6V + 0.1V – 0.9V
= -VS – 0.2VTo second stage
For Vcm < Vcm_min,
Q4 will saturate.
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Typical Bipolar or JFET Input – Not Rail-to-Rail
7
+
-V+
V-
+12V
-5V
OPA827
-2V < VIN- < 9V
-2V < VIN+ < 9V
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OPA827 Input Stage – Minimum VCM
8
IS1
IN-VIN+
R1 R2IS2
Cc
D1
D2
R3
to output stage
+VS
-VS
VR2Vbe
Vbe
Vsat
Vgs
VCM swing to negative rail:
VCM > -VS + VR2 + 2Vbe + Vsat – Vgs
Therefore,
-VS + 3V < VCM < +VS - 3V
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MOSFET Complementary N-P-FET – Rail-to-Rail
9
+
-V+
V-
5V
OPA703
-0.3V < VIN- < 5.3V
-0.3V < VIN+ < 5.3V
+
-V+
V-
+2.5V
OPA703
-2.8V < VIN- < 2.8V
-2.5V
-2.8V < VIN+ < 2.8V
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OPA703 Input Stage – Simplified Schematic
10
NCH3
PCH2PCH1
VCM_Iswitch
IS2
IS1
T8
NCH4
T2
PCH input pair active for:
-VS – 0.3V < VCM < +VS – 2V
-VS
+VS
VIN+
VIN-
2V
NCH input pair active for:
+VS – 2V < VCM < +VS + 0.3V
Turns on when
+VS – 2V < VCM < +VS + 0.3V
Steals current from PCH
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OPA703 Complementary CMOS – Rail-to-Rail
11
VIN-
VIN+
-VS
Q1 Q2
Q3 Q4
+VS
Common Mode Voltage (V)
200
100
0
-100
-200
-3000.0 1.0 3.0 4.0 5.02.0
Inp
ut O
ffse
t V
olta
ge
(µ
V)
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Crossover Distortion
12
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 0.2 0.4 0.6 0.8 1 1.2
Vo
ut
(Vo
lts)
time (ms)
Vout vs. Time (Crossover Distortion)
3.8
3.85
3.9
3.95
4
4.05
4.1
4.15
4.2
0.11 0.12 0.13 0.14 0.15 0.16
Vo
ut
(Vo
lts)
time (ms)
Zoom in on Crossover Distortion
Vout Ideal
Vout Crossover
+5V
-
+
+
Vin
Vout
RL
1k
Common Mode Voltage (V)
5
2.5
0
-2.5
-5
-7.50.0 1.0 3.0 4.0 5.02.0
Inp
ut O
ffse
t V
olta
ge
(m
V)
Vout vs. Time
Vout vs. Time (Zoomed In)
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MOSFET Charge Pump – Rail-to-Rail
13
+
-V+
V-
5V
OPA365
-0.1V < VIN- < 5.1V
-0.1V < VIN+ < 5.1V
+
-V+
V-
+2.5V
OPA365
-2.6V < VIN- < 2.6V
-2.5V
-2.6V < VIN+ < 2.6V
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Remember from Earlier in the Presentation…
14
Q4Q3
IS1
Q1Q2
+VS
VIN+
VIN-
To second stage
Vgs=0.9V
Vsat=0.1V
VCM swing to positive rail:
Vsat + Vgs = 0.1V + 0.9V = 1.0V
Therefore,
Vcm_max < +VS – 1.0V
-VS
For Vcm > Vcm_max,
Q4 will cutoff and
IS1 will saturate.
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OPA365 MOSFET Charge Pump – Rail-to-Rail
15
VOUT = +VS + 1.8V
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Zero Drift MOSFET – Rail-to-Rail
16
+
-V+
V-
5V
OPA333
-0.1V < VIN- < 5.1V
-0.1V < VIN+ < 5.1V
+
-V+
V-
+2.5V
OPA333
-2.6V < VIN- < 2.6V
-2.5V
-2.6V < VIN+ < 2.6V
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0.0 1.0 2.0 3.0 4.0 5.0
Common Mode Voltage (V)
Inp
ut O
ffse
t V
olta
ge
(u
V)
200
100
0
-100
-200
-300
Common Mode Voltage (V)
200
100
0
-100
-200
-3000.0 1.0 3.0 4.0 5.02.0
Inp
ut O
ffse
t V
olta
ge
(µ
V)
VIN-
VIN+
-VSUPPLY
Q1 Q2
Q3 Q4
Zero Drift MOSFET – Rail-to-Rail
17
With Offset
Correction
No Offset
Correction
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18
Thanks for your time! Please try the quiz.
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Input & Output Limitations – 2 Multiple Choice Quiz TI Precision Labs – Op Amps
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2
Quiz: Input & Output Limitations – 2 1. Single supply amplifiers have _____________.
a. Common mode range that extends to the negative supply.
b. Common mode range that extends to the positive supply.
2. (T/F) Single supply amplifiers can be used in a dual supply configuration.
a. True
b. False
3. (T/F) The common mode range of an amplifier is limited by the saturation and cutoff voltages of transistors in the input stage.
a. True
b. False
4. (T/F) Most rail-to-rail amplifiers use bipolar transistors.
a. True
b. False
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3
Quiz: Input & Output Limitations – 2 5. What is a potential issue associated with using a charge pump to power the input stage on an op amp?
a. The charge pump can create crossover distortion.
b. The charge pump will cause a significant increase in the device quiescent current.
c. The charge pump requires external components.
d. Charge pump switching noise can introduce errors.
6. Consider the CMRR specification below. Does this device have crossover distortion?
a. Yes.
b. No.
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4
Quiz: Input & Output Limitations – 2 7. Which of the following will NOT minimize crossover distortion on rail to rail amplifiers?
a. Use an internal charge pump to boost the supply on the input stage
b. Use external TVS diodes to minimize the distortion.
c. Use a zero drift amplifier to minimize the overall offset so that the offset shift is small.
8. Consider the CMRR specification below. Does this device have crossover distortion?
a. Yes.
b. No.
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Input & Output Limitations – 2 Multiple Choice Quiz: Solutions TI Precision Labs – Op Amps
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6
Solutions: Input & Output Limitations – 2 1. Single supply amplifiers have _____________.
a. Common mode range that extends to the negative supply.
b. Common mode range that extends to the positive supply.
2. (T/F) Single supply amplifiers can be used in a dual supply configuration.
a. True
b. False
3. (T/F) The common mode range of an amplifier is limited by the saturation and cutoff voltages of transistors in the input stage.
a. True
b. False
4. (T/F) Most rail-to-rail amplifiers use bipolar transistors.
a. True
b. False
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7
Solutions: Input & Output Limitations – 2 5. What is a potential issue associated with using a charge pump to power the input stage on an op amp?
a. The charge pump can create crossover distortion.
b. The charge pump will cause a significant increase in the device quiescent current.
c. The charge pump requires external components.
d. Charge pump switching noise can introduce errors.
6. Consider the CMRR specification below. Does this device have crossover distortion?
a. Yes.
b. No.
![Page 26: Input & Output Limitations 2](https://reader034.vdocuments.net/reader034/viewer/2022042101/62560ff59a822a79473e2435/html5/thumbnails/26.jpg)
8
Solutions: Input & Output Limitations – 2 7. Which of the following will NOT minimize crossover distortion on rail to rail amplifiers?
a. Use an internal charge pump to boost the supply on the input stage
b. Use external TVS diodes to minimize the distortion.
c. Use a zero drift amplifier to minimize the overall offset so that the offset shift is small.
8. Consider the CMRR specification below. Does this device have crossover distortion?
a. Yes.
b. No.