analog integrated circuits lecture 8: two-stage...
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Analog Integrated Circuits
Lecture 8: Two-Stage Opamp Design
Analog Integrated Circuits
Lecture 8: Two-Stage Opamp Design
ELC 601 – Fall 2013
Dr. Ahmed Nader
Dr. Mohamed M. Aboudina
Department of Electronics and Communications Engineering
Faculty of Engineering – Cairo University
Introduction to two-stage opamps
© Mohamed M. Aboudina, 2011
2
Two-Stage Opamp
• 1st stage DC-gain ~ ����
• 2nd stage DC-gain ~ ����
• Total DC-gain ~ ����� (Find
Exact !!)
• Do we need a high-swing first stage? NO
• Why don’t we replace it with a high-gain stage and lower swing ?
• Total DC-gain ~ ����� (Find Exact !!)
• High Output Swing
Closed-loop opamp stability
© Mohamed M. Aboudina, 2011
3
Introduction
• What happens if βH(s) = -1 ? OSCILLATIONS or INSTABILITY
– Phase = -180 and Magnitude ≥ 1
Closed-loop opamp stability
© Mohamed M. Aboudina, 2011
4
Single vs Multiple Poles
• Remember:
– The slope of the magnitude changes by +20dB/dec at every zero frequency and by -20 dB/dec at every pole frequency.
– The phase begins to change at one-tenth of the pole (zero) frequency, changes by -45 degrees (+45 degrees) at the pole (zero), and approaches a -90-degree (+90-degree) change at 10 times the pole (zero) frequency.
Closed-loop opamp stability
© Mohamed M. Aboudina, 2011
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Effect of β
• Changing the feedback factor β
does not change the phase plot.
• From a PM point of view: worst
case comes at β = 1.
Closed-loop opamp stability
© Mohamed M. Aboudina, 2011
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Phase Margin
• If Gain = 1 when phase = 180 � Oscillations,
• Phase Margin (PM) presents how far we are away from the cross over point ≡ phase @ (0 dB gain) + 180
Closed-loop opamp stability
© Mohamed M. Aboudina, 2011
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Phase Margin – How much is adequate?
• Frequency Compensation:
– Minimize number of poles.
– Keep one dominant pole.
For a step input, x(t)
Telescopic Opamp Stability
© Mohamed M. Aboudina, 2011
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Example
• What if the PM is not good enough ?
– Add more CL .
Telescopic Opamp Stability
© Mohamed M. Aboudina, 2011
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Example
• Design for a 450 phase margin.
• Keep increasing CL to move down the dominant pole � increase PM to reach 450 or more.
Original dominant
pole
New dominant
pole
Two-stage opamp stability
© Ahmed Nader, 2013
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Pole Locations
• 3 poles:
– �� =�
�������
– �� =�
�������
– �� =���
��
• How to compensate
it?
Two-stage opamp stability
© Ahmed Nader, 2013
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Compensation – Pole Splitting
• ����� = ����!" + �$ 1 + &'� ≅ �$&'�
• �� =�
������)*+�
• �� = ?
– At high frequencies, �$ can be considered a short circuit � Output
resistance ≅�
���(prove !! )
• �� =���
��
Two-stage opamp stability
© Ahmed Nader, 2013
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Compensation – Pole Splitting
• �� =�
������)*+�
• �� =���
��
Two-stage opamp stability
© Ahmed Nader, 2013
13
Two-stage opamp compensation – Pole Splitting
• �� =�
������)*+�
• �� =���
��
��
��
&-�
./ ≡ unity-gain freq.
Two-stage opamp stability
© Ahmed Nader, 2013
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Two-stage opamp compensation – Pole Splitting
• �� =�
������)*+�
• �� =���
��
• ./ = 012 =���
�)
��
��
&-�
./ ≡ unity-gain freq.
012 = &-� × ��
= &'�&'� ×1
4�/5��$&'�
=���4�/5�
4�/5���
=���
�$
Two-stage opamp stability
© Ahmed Nader, 2013
15
Transfer Function Zeros
• 6�/5 =�
�78
9:�
+�
�78
9:�
=�7;(
�
9:�7
�
9:�)
(�78
9:�)(�7
8
9:�)
=�7
8
9>
(�78
9:�)(�7
8
9:�)
• Multipath creates zeros.
1
1 +?
��
1
1 +?
��
1
1 +?
��
1
1 +?
��
@A�
Σ @�/5
-@A�
_
+
Two-stage opamp stability
© Ahmed Nader, 2013
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Transfer Function Zero
• B�$ = B�� + B��/5 ⇒ @D − @�/5 ?�$ = @D��� +'���
����
• ? = ?F@ @�/5 = 0 ⇒ ?F =���
�)(Right or Left half plane zero?)
• IJKLJ M�KN?.O� .PNQRSTN =*U) ��
8
9>
(�78
9:�).(�7
8
9:�)
• Does this zero increase or reduce the PM? – This zero reduces PM and can cause instability
Cc
@D
RoutB�$
= (@D−@�/5)?�$
B�� = @D���
Two-stage opamp stability
© Ahmed Nader, 2013
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Transfer Function Zero - Solution
• @ @�/5 = 0
– B�$ ='W
�>7�
8X)
– B�$ = B�� ⇒
• Options:
– Option 1: Set F = ∞ ⇒ 4F =�
���
– Option 2: Use F to cancel one of the already existing non-dominant poles to improve PM even further.
Cc
@D
Rout
B�� = @D���
Rz
?F =1
��1
���− 4F
?F =1
��1
���− 4F
Two-stage opamp
© Ahmed Nader, 2013
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Recap
• Max. Output Swing = 6-- − 6f-ghfi − 6f-jhfi
• &-� = &'�&'�
• 012 = ./ =���
�)(Assuming Dominant first pole)
• �� =���
��
• F =�
�X�
k����>
Two-stage opamp
© Ahmed Nader, 2013
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Noise
• The noise of the second stage can be neglected since it is divided by the gain of the 1st stage when refereed to the input
• The noise performance of 2-stage amplifier is similar to a 1-stage amplifier
Two-stage opamp
© Ahmed Nader, 2013
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Slew Rate
SR+ = ISS/CC
SR- = ID3/CC
If I1 < ISS
© Ahmed Nader, 2013
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Multi-Stage Opamps
• More stages can be cascaded to get more gain.
• Each stage contributes a pole.
• That requires multi-level compensation (for example Miller) to
result in only 1 dominant pole.
• A buffer stage (for example common-gate) can be added at the end
to reduce the output impedance if needed. (Note that if the
Opmap is used in feedback the output impedance will be already
reduced by the loop gain).
• An Opmp will no output buffer stage is referred as OTA.