Design with Operational Amplifiers, also known as Op Amps

1. The World Leader in High Performance Signal Processing Solutions
THE FUNDAMENTALS OF DESIGNING WITH
SEMICONDUCTORS FOR SIGNAL
PROCESSING APPLICATIONS
Class 2 - THE OP AMP
Presented by David Kress

2. Analog to Electronic signal processing
Sensor Amp Converter Digital Processor
(INPUT)
Actuator Amp Converter
(OUTPUT)

3. Analog to Electronic signal processing
Sensor Amp Converter Digital Processor
(INPUT)
Actuator Amp Converter
(OUTPUT)

4. Amplifiers and Operational Amplifiers
 Amplifiers
 Make a low-level, high-source impedance signal into a high-
level, low-source impedance signal
 Op amps, power amps, RF amps, instrumentation amps, etc.
 Most complex amplifiers built up from combinations of op
amps
 Operational amplifiers
 Three-terminal device (plus power supplies)
 Amplify a small signal at the input terminals to a very, very
large one at the output terminal

5. Operational Amplifiers
 Operational
 Op amps can be configured with feedback networks in multiple
ways to perform ‘operations’ on input signals
 ‘Operations’ include positive or negative gain, filtering, nonlinear
transfer functions, comparison, summation, subtraction, reference
buffering, differential amplification, integration, differentiation, etc.
 Applications
 Fundamental building block for analog design
 Sensor input amplifier
 Simple and complex filters – anti-aliasing
 ADC driver

6. Original vacuum-tube op-amp from Philbrick Research in 1953 – it
used +/- 300V supplies

7. THE RELATIVE SCALE OF SOME MODERN
IC OP AMP PACKAGES
SC-70 SOT-23 MSOP8 mSOIC 8-SOIC 14-SOIC
0.1 in
(ALL PACKAGES ABOVE TO SAME SCALE)
SC-70 SOT-23

8. AD823 JFET Input Op Amp Simplified
Schematic
+VS
VBE + 0.3V Q43
Q44 Q57
Q72 Q61 Q58 Q49 A=1 A=19
J1 J6
(+)
R44 R28
INPUTS
(-)
Q62 Q60 OUTPUT
S1P S1N
VB
Q48 Q59 Q17
A=1 A=19
Q53 Q35
I1 Q56
-VS
BIAS CURRENT = 25pA MAX @ +25 C
INPUT OFFSET VOLTAGE = 0.8mV MAX @ +25 C
INPUT VOLTAGE NOISE = 15nV/Hz
INPUT CURRENT NOISE = 1fA/Hz

9. Standard op amp symbol
(+)
INPUTS
(-)
1-9

10. The Ideal Op Amp and its Attributes
POSITIVE SUPPLY IDEAL OP AMP ATTRIBUTES:
 Infinite Differential Gain
 Zero Common Mode Gain
 Zero Offset Voltage
 Zero Bias Current
(+)
OP AMP INPUTS:
INPUTS OP AMP OUTPUT  High Input Impedance
 Low Bias Current
(-)  Respond to Differential Mode Voltages
 Ignore Common Mode Voltages
OP AMP OUTPUT:
 Low Source Impedance
NEGATIVE SUPPLY

11. Operational Amplifier Circuit Design
 Use of negative feedback
 The output signal, or a controlled portion of it, is fed back to the
negative (-) input terminal
 The op amp will adjust the output signal until the input difference
goes to zero
 Example of high-gain
 Assume op amp gain of 106 (one million)
 Apply signal of one volt to positive input
 Feedback directly from output to negative input
 Output will go to one volt (minus one microvolt)
 Difference at input will be on microvolt

12. Key Op Amp Performance Features
 Bandwidth and slew rate
 The speed of the op amp
 Bandwidth is the highest operating frequency of the op amp
 Slew rate is the maximum rate of change of the output
 Determined by the frequency of the signal and the gain needed
 Offset voltage and current
 The errors of the op amp
 Determines measurement accuracy
 Noise
 Op amp noise limits how small a signal can be amplified with good
fidelity

13. Non-inverting Mode Op Amp Stage
OP AMP G = VOUT/VIN
= 1 + (RF/RG)
RF
VIN VOUT
RG
1-13

14. Inverting Mode Op Amp Stage
SUMMING POINT
RG RF
G = VOUT/VIN
= - RF/RG
VIN OP AMP
VOUT
1-14

15. Non-Inverting Amplifier gain
V OUT
RFB G=
V IN
I VOUT
RFB
VIN
= 1+
R IN
RIN
1-15

16. Single Pole Op Amp Active Filters
-
-
+
+
(A) (B)
LOWPASS HIGHPASS
5-38

17. Open Loop Gain (Bode Plot)
OPEN OPEN
LOOP LOOP
GAIN 6dB/OCTAVE GAIN 6dB/OCTAVE
dB dB
12dB/
OCTAVE
1-17

18. Gain-Bandwidth Product
GAIN
dB OPEN LOOP GAIN, A(s)
IF GAIN BANDWIDTH PRODUCT = X
THEN Y · fCL = X
X
fCL =
NOISE GAIN = Y Y
WHERE fCL = CLOSED-LOOP
R2 BANDWIDTH
Y=1+
R1
fCL LOG f
1-18

19. Noise Gain
IN +
+
+
+ +
+
+
+ +
+
+
+
A B C
R1 -
-
-
- R1 -
-
-
- R1 -
IN IN R2
R2 R2
Signal Gain = 1 + R2/R1 Signal Gain =- R2/R1 Signal Gain =- R2/R1
R2
Noise Gain = 1 + R2/R1 Noise Gain = 1 + R2/R1 Noise Gain = 1 +
R1R3
 Voltage Noise and Offset Voltage of the op amp are reflected to the
output by the Noise Gain.
 Noise Gain, not Signal Gain, is relevant in assessing stability.
 Circuit C has unchanged Signal Gain, but higher Noise Gain, thus
better stability, worse noise, and higher output offset voltage.
1-19

20. AD847 Open Loop Gain
1-20

21. AD8051 Phase Margin
1-21

22. VFB and CFB Amplifiers
VIN + +
VIN
–T(s) i
×1
v ~–A(s) v VOUT i
T(s)
×1 VOUT
i RO
–
–
R2 R2
R1
R1
T(s) = TRANSIMPEDANCE OPEN LOOP GAIN
A(s) = OPEN LOOP GAIN
VOUT = -T(s)*i
VOUT = A(s)*v
1-22

23. Current Feedback Amplifier Frequency
Response
GAIN
dB G1
G1 · f1 G2 · f2
G2
f1 f2 LOG f
 Feedback resistor fixed for optimum performance. Larger values
reduce bandwidth, smaller values may cause instability.
 For fixed feedback resistor, changing gain has little effect on
bandwidth.
 Current feedback op amps do not have a fixed gain-bandwidth
product.
1-23

24. Standard Input Stage (Differential Pair)
VIN
1-24

25. PNP Input Stage
+VS
-VS
1-25

26. Compound Input Stage
+VS
-VS
1-26

27. Output Stages. Emitter Follower for Standard
Configuration And Common Emitter for “Rail-
to-Rail” Configuration
EMITTER FOLLOWER COMMON EMITTER
+V S +V S
OUTPUT OUTPUT
-V S -V S
1-27

28. INPUT OFFSET VOLTAGE
-
VOS
+
 Offset Voltage: The differential voltage which must be applied
to the input of an op amp to produce zero output.
 Ranges:
 Chopper Stabilized Op Amps: <1µV
 General Purpose Precision Op Amps: 50-500µV
 Best Bipolar Op Amps: 10-25µV
 Best FET Op Amps: 100-1,000µV
 High Speed Op Amps: 100-2,000µV
 Untrimmed CMOS Op Amps: 5,000-50,000µV
 DigiTrim™ CMOS Op Amps: <1,000µV

29. Offset Adjustment Pins
+VS OR -VS +VS
1
2
-
8
7 6
4
3
+
-VS
1-29

30. External Offset Adjustment
R2 (B) R2
(A)
R1 R1
VIN
VIN
– –
VOUT
VOUT
+ +
NOISE GAIN =
NOISE GAIN =
RP R2
R2 R3 1+
R3 1+ R1
R1||(R3 + RA||RB)
RA RB RA RB
–VR +VR –VR +VR
RP
VOUT = – R2 VIN ± R2 VR VOUT = – R2 VIN ± 1 + R2 VR
R1 R1 RP + R3
R1 R3
MAX MAX
OFFSET OFFSET
RP = R1||R2 IF IB+  IB-
RP  50 IF IB+  IB-
1-30

31. Input Bias Current
+
IB+
IB-
1-31

32. Bias Current Compensation
R2
R1
–
IB–
VO
IB+
+
R3 = R1 || R2 VO = R2 (IB– – IB+)
= R2 IOS
= 0, IF IB+ = IB–
NEGLECTING VOS
1-32

33. Total Offset Voltage Calculations
B R1 IB– R2
 –
VOS
VOUT
A R3
IB+
+ GAIN FROM =
"A" TO OUTPUT
GAIN FROM R2 NOISE GAIN =
= –
"B" TO OUTPUT R1 R2
NG = 1 +
R1
 OFFSET (RTO) = VOS 1 + R2 R2
R1 + IB+• R3 1+
R1 – IB–• R2
R1•R2
 OFFSET (RTI ) = VOS + IB+• R3 – IB–
R1 + R2
FOR BIAS CURRENT CANCELLATION:
R1•R2
OFFSET (RTI) = VOS IF IB+ = IB– AND R3 =
R1 + R2
1-33

34. Input Impedance
IB+ IB–
Zdiff
+ INPUT – INPUT
Zcm+ Zcm–
1-34

35. Current Feedback Input Resistance
+INPUT X1 - INPUT
Z-
Z+
1-35

36. Voltage Noise
–
VN
+
1-36

37. Equivalent Noise Bandwidth
GAUSSIAN SINGLE POLE
NOISE LOWPASS
SOURCE FILTER, fC
SAME
IDENTICAL LEVELS RMS NOISE
LEVEL
GAUSSIAN BRICK WALL
NOISE LOWPASS
SOURCE FILTER, 1.57fC
EQUIVALENT NOISE BANDWIDTH = 1.57  fC
1-37

38. Current Noise
-
IN–
IN+
+
1-38

39. Total Noise Calculation
R2 V
R2J
V
R1J R1 V
n
V
ON
In-
V
RPJ Rp
+
In+
BW = 1.57 FCL
FCL = CLOSED LOOP BANDWIDTH
VON = BW [(In-2)R22] [NG] + [(In+2)RP2] [NG] + VN2 [NG] + 4kTR2 [NG-1] + 4kTR1 [NG-1] + 4kTRP [NG]
1-39

40. Dominant Noise Source Determined
by Input Impedance
EXAMPLE: OP27 VALUES OF R
CONTRIBUTION
Voltage Noise = 3nV / Hz
FROM
Current Noise = 1pA / Hz 0 3k 300k
T = 25°C
AMPLIFIER
VOLTAGE NOISE 3 3 3
+
AMPLIFIER
OP27 CURRENT NOISE 0 3 300
R FLOWING IN R
–
JOHNSON 0 7 70
NOISE OF R
R2
R1
RTI NOISE (nV /  Hz)
Neglect R1 and R2 Dominant Noise Source is Highlighted
Noise Contribution
1-40

41. 1/f Noise Bandwidth
NOISE 3dB/Octave
1
nV / Hz en, in = k FC
f
or
pA / Hz 1
CORNER
en, in f
WHITE NOISE
k
FC LOG f
 1/f Corner Frequency is a figure of merit for op amp
noise performance (the lower the better)
 Typical Ranges: 2Hz to 2kHz
 Voltage Noise and Current Noise do not necessarily
have the same 1/f corner frequency
1-41

42. RMS to Peak to Peak Voltage Comparison
Chart
1-42

43. The Peak-to-Peak Noise
in the 0.1 Hz. to 10 Hz. Bandwidth
1-43

44. Resistor Noise
VNR
R
 ALL resistors have a voltage noise of V =   4kTBR)
NR
 T = Absolute Temperature = T ( ° C) + 273.15
 B = Bandwidth (Hz)
-23
 k = Boltzmann ’ s Constant (1.38 x 10 J/K)
 A 1000  resistor generates 4 nV /  Hz @ 25°C
1-44

45. THD & THD+N Definitions
 Vs = Signal Amplitude (RMS Volts)
 V2 = Second Harmonic Amplitude (RMS Volts)
 Vn = nth Harmonic Amplitude (RMS Volts)
 Vnoise = RMS value of noise over measurement bandwidth
V22 + V32 + V42 + . . . + Vn2 + Vnoise2
 THD + N =
Vs
V22 + V32 + V42 + . . . + Vn2
 THD =
Vs
1-45

46. Slew Rate
OVERSHOOT
FINAL VALUE
90%
VOLTAGE
V
SLEW RATE =
T
V
10%
T RINGING
TIME
1-46

47. Slew Rate and Full Power Bandwidth
Slew Rate = Maximum rate at which the output voltage of
an op amp can change
Ranges: A few volts / s to several thousand volts / s
For a sinewave, V out = Vp sin2  f t
dV/dt = 2π f Vp cos 2π f t
(dV/dt)max = 2fVp
If 2 V p = full output span of op amp, then
Slew Rate = (dV/dt) max = 2 * FPBW * Vp
FPBW = Slew Rate / 2 Vp
1-47

48. Settling Time
OUTPUT
ERROR
BAND
DEAD SLEW RECOVERY FINAL
TIME TIME TIME SETTLING
SETTLING TIME
Error band is usually defined to be a percentage of the step 0.1
%
0.05%, 0.01%, etc.
Settling time is non
-linear; it may take 30 times as long to settle to
0.01% as to 0.1%.
Manufacturers often choose an error band which makes the op
amp look good.
1-48

49. Common Mode Rejection Ratio
for the OP177
160
140
120
CMR 100 CMR =
dB 20 log10 CMRR
80
60
40
20
0
0.01 0.1 1 10 100 1k 10k 100k 1M
FREQUENCY - Hz
1-49

50. Power Supply Rejection Ratio
160
140
120
PSR 100
PSR =
dB 20 log10 PSRR
80
60
40
20
0
0.01 0.1 1 10 100 1k 10k 100k 1M
FREQUENCY - Hz
1-50

51. Maximum Power Chart (from the AD8001)
1-51

52. Input Protection
+V S
RS
+
-
-V S
1-52

53. Typical Absolute Maximum Ratings
1-53

54. Single-Supply Op Amps
 Single Supply Offers:
 Lower Power
 Battery Operated Portable Equipment
 Requires Only One Voltage
 Design Tradeoffs:
 Reduced Signal Swing Increases Sensitivity to Errors
Caused by Offset Voltage, Bias Current, Finite Open-
Loop Gain, Noise, etc.
 Must Usually Share Noisy Digital Supply
 Rail-to-Rail Input and Output Needed to Increase Signal
Swing
 Precision Less than the best Dual Supply Op Amps
but not Required for All Applications
 Many Op Amps Specified for Single Supply, but do not
have Rail-to-Rail Inputs or Outputs

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