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THIẾT KẾ VI MẠCH TƯƠNG TỰƯƠCHƯƠNG 6: Frequency reponse of Amplifiers
TP.Hồ Chí Minh 04/2012
Hoàng Trang-bộ môn Kỹ Thuật Điện Tử[email protected]
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Frequency Response of Amplifiers
1. General Considerations1. General Considerations2. Common-Source Stage3 Source Followers3. Source Followers4. Common-Gate Stage5. Cascode Stage6. Differential Pair6. Differential Pair
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1. General Considerations
Miller Effect Miller Effect Association of Poles with Nodes
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Miller Effect
ill ’ h Miller’s Theorem
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Miller Effect
l Example
Z = 1/CFs/ F
Z1 = [1/CFs]/(1+A)
Z2= [1/CFs]/(1+1/A)Z2 [1/CFs]/(1+1/A)
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Association of Poles with Nodes
We can associate each pole with one node pof the circuit.
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Association of Poles with Nodes
The above statement is not valid in general.g
The location of the poles is difficult to calculate because R3 and C3 create interaction between X and Y.
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2. Common-Source Stage
Schematic
2. Common Source Stage
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2. Common-Source Stage (cont.)
Surmise the transfer function by associating
g ( )
poles with nodes and Miller’s theorem.
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2. Common-Source Stage (cont.)2. Common Source Stage (cont.)
Obtain the exact transfer function.
Sum the currents at each node:
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2. Common-Source Stage (cont.)2. Common Source Stage (cont.)
Ob i h f f i Obtain the exact transfer function.
Writing the denominator
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2. Common-Source Stage (cont.)g ( )
If | 1| | 2| If we assume |ωp1|<<|ωp2|
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2. Common-Source Stage (cont.)g ( )
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2. Common-Source Stage (cont.)
S i h f f i b i i
2. Common Source Stage (cont.)
Surmise the transfer function by associating poles with nodes and Miller’s theorem.
Obtain the exact transfer function.
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3. Source Followers3. Source Followers
S F ll i ll l d Source Followers are occasionally employed as level shifters or buffers.
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3. Source Followers (cont.)( )
T f f i Transfer function
Input impedance Zin Input impedance Zin
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3. Source Followers (cont.)3. Source Followers (cont.)
At relatively low frequencies, gmb >>|CLs|
At high frequencies, gmb <<|CLs|
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3. Source Followers (cont.)3. Source Followers (cont.)
Output impedance
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4. Common-Gate Stageg
S h i Schematic
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4. Common-Gate Stage (cont.)4. Common Gate Stage (cont.)
Transfer function Transfer function
Input impedance
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5. Cascode Stageg
C d S b fi i l i Cascode Stage proves beneficial in: Increasing the voltage gain of amplifiers.gain of amplifiers.
Increasing the output impedance of current sources.
Providing shielding as well.
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5. Cascode Stage (cont.)g ( )
U i Mill Eff t d A i ti f Using Miller Effect and Association of Poles with Nodes at node A, X and Y:
Node A:
Node X:
Node Y:
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6. Differential Pair6. Differential Pair
Differential Pair with differential i lsignals.
Differential Pair with active current imirror.
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6. Differential Pair (cont.)6. Differential Pair (cont.)
Differential Pair with differential signals:
The respond is identical to that of a common-
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psource stage.
6. Differential Pair (cont.)( )
Differential Pair with active current mirror:
This circuit has 2 poles:
Mirror pole at node E.
Output pole at output.
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6. Differential Pair (cont.)6. Differential Pair (cont.)
Estimate the frequency response:
CE: the total capacitance at node E, arising
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from CGS3, CGS4, CDB3, CDB1, and the Miller Effect of CGD1, CGD4.
6. Differential Pair (cont.)
Replacing Vin M1 M2 by a Thevenin
6. Differential Pair (cont.)
Replacing Vin, M1, M2 by a Thevenin equivalent, where Vx = gmNrONVin, Rx = 2rON
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6. Differential Pair (cont.)6. Differential Pair (cont.) Assume 1/gmP << rOP. The small-signal voltage
at E is equal to:q
Th ll i l d i t f M4 i V d The small-signal drain current of M4 is gm4VE and – gm4VE –Ix = Vout(CLs + 1/rOP), we have:
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END OF CHAPTER 6
FREQUENCY REPONSE OF AMPLIFIERSAMPLIFIERS
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