EECE488: Analog CMOS Integrated Circuit Design Set 5 Current ...
EECE488: Analog CMOS Integrated Circuit Design Set 5 Current ...
EECE488: Analog CMOS Integrated Circuit Design Set 5 Current ...
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<strong>EECE488</strong>: <strong>Analog</strong> <strong>CMOS</strong> <strong>Integrated</strong> <strong>Circuit</strong> <strong>Design</strong><br />
<strong>Set</strong> 5<br />
<strong>Current</strong> Mirrors<br />
Shahriar Mirabbasi<br />
Department of Electrical and Computer Engineering<br />
University of British Columbia<br />
shahriar@ece.ubc.ca<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 1
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Applications of <strong>Current</strong> Sources<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 2
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Simple Resistive Biasing for <strong>Current</strong> Source<br />
nCox<br />
W R<br />
IOUT ≈ ( −<br />
2 L R +<br />
µ<br />
2<br />
2 2<br />
VDD<br />
VTH<br />
)<br />
R1<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 3
• Output current depends on:<br />
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– Supply<br />
– Process<br />
– Temperature<br />
Problems<br />
• What if the bias voltage is independent of supply voltage?<br />
• Is there a way of generating reliable currents?<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 4
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Basic Idea<br />
Typically we assume that one precisely defined current source is<br />
available and other current sources copy their current from this precise<br />
source.<br />
I out is a function of gate-source voltage<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 5
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Basic Idea<br />
This structure is called current mirror<br />
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Question<br />
• What happens if the two transistors in the basic current mirror<br />
have different sizes?<br />
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Example<br />
Assuming all the transistors are in saturation region, find I out:<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 8
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<strong>Current</strong> Mirrors: Amplifier Bias Example<br />
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Board Notes<br />
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<strong>Current</strong> Mirrors: Signal Amplification Example<br />
• Find the small signal voltage gain of the following circuit.<br />
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Effect of Channel Length Modulation<br />
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Cascode <strong>Current</strong> Mirror<br />
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Board Notes<br />
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Cascode <strong>Current</strong> Mirror<br />
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Cascode <strong>Current</strong> Mirror<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 16
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Cascode <strong>Current</strong> Mirror<br />
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Board Notes<br />
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Cascode <strong>Current</strong> Mirror Biasing<br />
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Cascode <strong>Current</strong> Mirror Biasing<br />
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<strong>Current</strong> Mirror Biasing<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 21
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Basic <strong>Circuit</strong> to Generate Supply Independent <strong>Current</strong><br />
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Supply Independent <strong>Current</strong><br />
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Board Notes<br />
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Supply Independent <strong>Current</strong><br />
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Start-up Problem<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 26
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Start-up Problem<br />
V > V<br />
TH1<br />
+ VTH<br />
5 + VTH<br />
3 < VDD<br />
and VGS1<br />
+ VTH<br />
5 + VGS<br />
3<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 27<br />
DD
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Board Notes<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 28
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Active <strong>Current</strong> Mirrors<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 29
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Active <strong>Current</strong> Mirrors in Differential to Single-Ended<br />
Amplifiers<br />
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Differential to Single-Ended Amplifiers<br />
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Calculation of G m<br />
I D1 = I D3 = I D 4 = g m1,2V in / 2 I D2 = −g m1,2V in /2<br />
I out = I D2 − I D4 = −g m1,2V in ,⇒ G m = g m1, 2<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 32
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Small-Signal Gain<br />
A v ≈ g m 1,2 (r o2 || r o4 )<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 33
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Common Mode Characteristics<br />
A CM = ∆V out<br />
∆V in,CM<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 34
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A CM ≈<br />
Common Mode<br />
1<br />
− ||<br />
2gm3,4 ro3,4 2<br />
1<br />
+ RSS 2gm1,2 =<br />
−1<br />
1 + 2g m1,2 R SS<br />
g m1,2<br />
g m 3, 4<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 35
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Common Mode<br />
CMRR = A DM<br />
A CM<br />
= g m1, 2(r o1,2 || r o3,4) g m 3,4 (1 + 2g m1,2 R SS )<br />
g m1,2<br />
= g m 3,4 (r o1, 2 || r o3,4 )(1 + 2g m1, 2 R SS )<br />
<strong>EECE488</strong> <strong>Set</strong> 5 - <strong>Current</strong> Mirrors 36
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