Operational Amplifiers

1
Operational Amplifiers
2
Important Parameters
Gain Small-Signal Bandwidth Large-Signal
Bandwidth Output Swing Linearity Noise and
Offset Supply Rejection
3
One-Stage Op Amps
4
One-Stage Op Amp in Unity Gain Configuration
5
Cascode Op Amps
Cannot operate at Vcm0
6
Unity Gain One Stage Cascode-difficult to bias
7
OCMR, ICMR (telescopic)

• VDD3V, VT0.7V, VOV0.3V

Vout max. 3-0.3-0.32.4V (constraint from load)
Vout max. Vb-(Vgs-Vt)1.6-0.31.3V
Only 0.4 V range
Vout MIN. Vb-Vt1.6-0.70.9V
8
OCMR, ICMR(telescopic) in feedback mode

• VDD3V, VT0.7V, VOV0.3V

Vout max. 3-0.3-0.32.4V
Vout max. Vb-(Vgs-Vt)1.6-0.31.3V
Vout MIN. Vb-Vt1.6-0.70.9V
9
Single-Ended Output Cascode Op Amps
10
Triple Cascode
Av app. (gmro)3/2 Limited Output Swing Complex
biasing
11
Folded Cascode Op Amps
Can operate at Vcm0
12
Folded Cascode Stages (cont.)
2v
1.3v
3v
0.7v
1v
Can operate at VcmVdd
13
Folded Cascode (cont.)

• VDD3V, VT0.7V, VOV0.3V

2.3v
1.7v
2v
Can operate at Vcm0
ICMR--- 0-1.4V
14
OCMR, ICMR (folded cascode) (BETTER) in feedback
mode

• VDD3V, VT0.7V, VOV0.3V

Vout max.Vin lt 1.7V
Vout MIN. Vb-Vt1.7-0.71V
Vout min 0
15
Folded Cascode (cont.)
16
GAIN

• SLIGHTLY LESS THAN TELESCOPIC
• POWER DISSIPATION HIGHER

17
Telescopic vs. Folded Cascode Pole
18
POLE AT FOLDING POINT

• POLE FREQUENCY LOWER THAN TELESCOPIC (possibly)

19
Example Folded-Cascode Op Amp
See Example 9.6
20
Two-Stage Op Amps
Design Approach for Two-Stage Op Amps
21
(No Transcript)
22
Single-Ended Output Two-Stage Op Amp
23
(No Transcript)
24
Output Impedance Enhancement With Feedback
25
Gain Boosting in Cascode Stage
26
Differential Gain Boosting
27
(No Transcript)
28
Differential Gain Boosting (cont.)
min voltage required increases
Vy
Vx
29
OCMR, ICMR

• Voutmax. Vdd-Vgs-2Vov. 3-1.3-0.3 2.4V
• Voutmin. Vgs52Vov. 1.6V
• Vx min Vgs5Vov.1.3V
• Vinmin Vx min.Vt2.3V
• Vinmin Vgs1Vov 1.3
• Vinmax Vgs1VovVt VxmaxVt 2.8v

30
CAN BE ZERO
31
Differential Gain Boosting (cont.)
32
OCMR, ICMR

• Voutmax. Vdd-Vgs-2Vov. 3-1.3-0.3 1.4V
• Voutmin. Vgs52Vov. 1.6V
• Vx min can be 0
• Vinmax Vt0.7V
• Vinmin Vgs1Vov 1.3

33
Fully differential circuitsDrawbackcommon
mode level can not be a stable desired value due
to process variations
34
Well defined common mode level
35
Common-Mode Feedback
36
Common-Mode Feedback (cont.)
37
(No Transcript)
38
High Gain Amp Model
39
(No Transcript)
40
How to sense?
41
How to bring a desired Vout cm value?
42
Common-Mode Feedback (cont.)
43
(No Transcript)
44
(No Transcript)
45
Resistive Sensing
Problem-----Gain decreases due to resistive
loading So, make R1 very very large Si area
problem
46
Remedy---Source-Follower Buffering
47
CMFB Example
48
Alternative CMFB for Folded Cascode
49
Deep Triode FET CM Sensing
50
Operation
Rtot is a function of Vout1 Vout2 only.
Hence this parameter can be used to sense Vcm
51
Returning CMFB with Triode Devices
Causes Voltage swing problem
See example 9.9
52
This value gets set automatically depending on
circuit parameters
53
Drawbacks
Remedy---
54
CMFB Triode circuit
fast
Auto correction
voltage change
Current change
55
Forcing desired Reference voltage
So, Currents should track each other Vgs15 Vgs9
or Vgs16 Vgs7
Vref desired
Shd. track
56
CMFB Triode Example with Reference (cont.)
TRANS. TRACK EACH OTHER
EXACT DUPLICATE CIRCUIT
57
Differential Pair with CMFB
58
Stability of amplifier with CMFB circuit

• 4 gain calculations

59
1.Common mode gain voc/vcmc without CMFB
voc
vcmc
60
2.Common mode gain voc/vic without CMFB
small
61
3.Common mode gain voc/vic with CMFB present
voc
vic
vcms
ß
vref
ß
1
large
62
Common mode gain voc/vic with CMFB present
Example ---grey, meyer
ß1
63
4. Condition on loop gain for Voc Vref --With
Feed Back
Vref
CMFB Loop gain A ß Af ß--(Vref to Voc)
vcms/ voc----feedback amplifier
Af--Closed loop voc/ vcms ----forward amplifier
Vref
vref
Open loop voc/ vcms----forward amplifier
Feedback amplifier gain--ß
Vref
64
Dominant pole at output
SHOULD HAVE NON DOINANT POLES
65
Common mode gain voc/vcmc with CMFB
ASSUMING ONLY ONE DOMINANT POLE
shd. be non dominant pole
66
Stability of amplifier with CMFB circuit
For ß1 (max feedback condition)
Assuming single pole response
Maximum unity gain frequency
Shd. have sufficient phase margin at this freq.
Else reduce this freq. how? Decrease gm,
(desirable) or Increase CL-----causes
reduction of -3db freq of forward amplifier in
differential mode operation
67
To stabilize CMFBreduce gm by splitting
Disadvantage----gain voc /vcms reduces
Provides stable Ibias , shd. be gt 80
68
Increasing ICMR
69
Rail-rail ICMR
70
Constant Gm Circuits
Techniques are available to stabilize Gm
71
Slew Rate

• Slew Rate (SR) limit Real OpAmp has a maximum
  rate of change of the output voltage magnitude
• limit
• SR can cause the output of real OpAmp very
  different from an ideal one if input signal
  magnitude is too high
• Affects settling time of OPAMP

72
Normal Settling

• Linear RC Step Response the slope of the step
  response is
• proportional to the final value of the output,
  that is, if we apply a larger input step, the
  output rises more rapidly.
• If Vin doubles, the output signal doubles at
  every point, therefore a twofold increase in the
  slope.
• Completely decided by -3dB frequency

73
R-C charging
Rate of change of output node
1/f-3dB
74
Linear Feedback Systems
75
Step ResponseLinear Settling

• Again find dvout/dt
• It is still proportional to Vout magnitude

76
Realistic Opamp

• But the problem in real OpAmp is that this slope
  can not exceed a certain limit.

77
Why? Origin of slewing
?v small, always linear settling
78
?V largeOPAMP slew
Low To High Transition
Constant current charging ramp
behaviour
Slew rate dVo/dT Iss/CL Any further increase
in vin will not make charging of output node fast
79
High To Low Transition
Off
80
Slew Rate
81
Slewing

• Undesirable because Limits the speed of OPAMP
• Can not be eliminated
• Remedy ----
• Estimate max. speed that can be obtained
• Then make slew rate large How? provide
  additional current boosting,

82
Estimation of Full Power Bandwidth

• Full Power bandwidth the range of frequencies
  for which the OpAmp can produce an undistorted
  sinusoidal output with peak amplitude equal to
  the maximum allowed voltage output

How to know it?
fFP shd, be gt f-3dB
83
Slewing in Telescopic Op Amp
Slew rate Iss/(2CL)
84
Differential Slew Rate

• Positive slew rate---large positive step at input
• Negative slew rate----large negative step at input

85
Folded-Cascode Slewing
Slew rate Iss/(CL), indep. of Ip
86
Folded-Cascode (cont.)
87
Constraint on Ip

• Ip gt Iss

88
Slewing Recovery if Ip lt Iss
Large settling time
89
Slewing Recovery (cont.)
90
Techniques for improving SR

• provide additional current by
• adaptive bias for tail current boosting
• local common-mode feedback (LCMFB)
• Use clamping circuit

91
Performance parameters
92
Normal OTAclass AB Operation
work for large pos. input
work for large neg. input
Both work for small . input
93
Charging output
0.33, B1 in slew mode
0.25 in normal mode
94
Parameters
If B increases, SR as well as power consumption
both increases
Small signal behaviour
Poles at x and y node
95
Adaptive biasing with LCMFB
LCMFB
drawback
96
(No Transcript)
97
Operation
Icm in each arm
Under normal condition
for large differential input
Large swing at X
98
Operation
Can be large
I8 0
So,
Similarly for negative swing. Thus general
expression for vid ?0
99
CE for charging output in slew mode
Current efficiency Iout/ 2Iout2Icm ?0.5
(charge) for Iout gtgt Icm, B1
Iout/ Iout2Icm ?1 (discharge)
Current consumption 2Iout 2Icm
Here also, If B increases, SR as well as power
consumption both increases Aim is to achieve
high SR at low power consumption
100
Small signal behaviour
k1 ----transconductance factor
101
Adaptive biasing
102

• VB can be low to obtain low power consumption
  under low level inputs.

DC current in normal condition
103
Ac behaviour
K2 here
104
ICMR
105
PSRR
106
(No Transcript)
107
(No Transcript)
108
(No Transcript)
109
(No Transcript)
110
PSRR OF A CIRCUIT
111
(No Transcript)
112
(No Transcript)
113
(No Transcript)
114
PSRR Calculations
115
(No Transcript)
116
(No Transcript)
117
(No Transcript)
118
(No Transcript)
119
2 stage CMOS OP AMP
120
(No Transcript)
121
Without Cc
Ap Ap2 A2 Ap1
A - A1 A2
122
(No Transcript)
123
With Cc
124
Only second stage
125
(No Transcript)
126
Noise
127
Noise

• Small current and voltage fluctuations that are
  generated with in the device

128
Origin of noise

• Existence of noise is due to the fact the charge
  is not continuous but is carried in discrete
  amounts equal to electron charge
• Thus noise is associated with fundamental
  processes in integrated circuit devices
• so it can not be removed

129
Why should we study noise

• Because noise represents a lower limit to the
  size of electrical signal (min. detectable
  signal) that can be amplified by a circuit
  without significant deterioration in signal
  quality
• Noise results in upper limit to the useful gain
  of an amplifier because if gain is increased
  without limit, then due to noise fluctuations at
  output node, transistors may go to linear region

130
Noise-Random signal

• Value of noise signal cannot be predicted at any
  time even if past values are known

131

• If microphone drives a resistive load, More heat
  will be generated in case b.
• ?Average value of ac signals

132
How to estimate Noise?

• Observe noise for a long time
• Using measured results, prepare a statistical
  model
• Extract useful properties (here, noise power)
  from this model that can be predicted
• Use noise power for doing noise analysis

133
Average Power

• Average power delivered by a periodic

134
Noise power
135
How to find Average power

• Square the signal
• Area under the waveform is calculated
• Normalize the area to T
• Pav expressed in V2

Vn2
136
Noise content

• Noise content varies with frequency
• Noise power spectral density is obtained i.e to
  find the magnitude of low and high noise
  components

137
How to obtain Noise spectrum
138
Noise spectrum
139
Types of noiseThermal noise
140
Representation of thermal noise
141
MOSFET noise---thermal noiseNoise generated in
the channel
142
MOS---flicker noise
143
(No Transcript)
144
Representation
145
MOS noise
146
Noise corner frequency
147
Computation of Noise in circuits
148
Uncorrelated noise sources

• Noise produced by resistor is independent of
  noise produced by transistor

149
Output noise voltage
150
output noise for comparison

• Drawbacks of using output noise for comparison
• Consider two amplifiers of gain A1, A2
• Amp1 has Vout 1V, Vn v30nV/ vHz.
• Amp2 has Vout 3V, Vn v60nV/ vHz.
• Which is better? Difficult to make comparison

151

• A2 generates more noise, but has higher gain
• A1 has low gain but generates less noise

152
COMPARISON PARAMETER

• Signal to noise ratio-how lage is signal in
  comparison to noise
• Should be large
• Input referred noise voltage(indep. Of gain)
  fictitious quantity as it can not be measured at
  the input
• This indicates how small an input the circuit can
  detect
• Should be small

153
Representation
154
(No Transcript)
155
How to reduce input referred noise voltage

• gm1 should be maximized

156
2nd circuit
157
How to reduce input referred noise voltage

• gm1 should be maximized, gm2 must be minimized

158
Frequency response
159
Total output noise
160
SNR
161
Csacode amplifier
162
Resistive load differential amp
163
(No Transcript)
164
(No Transcript)
165
(No Transcript)
166
(No Transcript)
167
Active load diff amp
168
(No Transcript)
169
(No Transcript)
170
(No Transcript)
171
Vx
172
Vy
173
(No Transcript)
174
(No Transcript)
175
(No Transcript)
176
(No Transcript)