EECE488 Set 5 - Current Mirrors 1SM
EECE488: Analog CMOS Integrated Circuit Design
Set 5
Current Mirrors
Shahriar MirabbasiDepartment of Electrical and Computer Engineering
University of British [email protected]
EECE488 Set 5 - Current Mirrors 2SM
Applications of Current Sources
EECE488 Set 5 - Current Mirrors 3SM
Simple Resistive Biasing for Current Source
2
12
2 )(2 THDD
oxnOUT VV
RRR
LWCI −
+≈µ
EECE488 Set 5 - Current Mirrors 4SM
Problems
• Output current depends on:
– Supply
– Process
– Temperature
• What if the bias voltage is independent of supply voltage?
• Is there a way of generating reliable currents?
EECE488 Set 5 - Current Mirrors 5SM
Basic Idea
Iout is a function of gate-source voltage
Typically we assume that one precisely defined current source isavailable and other current sources copy their current from this precise source.
EECE488 Set 5 - Current Mirrors 6SM
Basic Idea
This structure is called current mirror
EECE488 Set 5 - Current Mirrors 7SM
Question
• What happens if the two transistors in the basic current mirror have different sizes?
EECE488 Set 5 - Current Mirrors 8SM
Example
Assuming all the transistors are in saturation region, find Iout:
EECE488 Set 5 - Current Mirrors 9SM
Current Mirrors: Amplifier Bias Example
EECE488 Set 5 - Current Mirrors 10SM
Board Notes
EECE488 Set 5 - Current Mirrors 11SM
Current Mirrors: Signal Amplification Example
• Find the small signal voltage gain of the following circuit.
EECE488 Set 5 - Current Mirrors 12SM
Effect of Channel Length Modulation
EECE488 Set 5 - Current Mirrors 13SM
Cascode Current Mirror
EECE488 Set 5 - Current Mirrors 14SM
Board Notes
EECE488 Set 5 - Current Mirrors 15SM
Cascode Current Mirror
EECE488 Set 5 - Current Mirrors 16SM
Cascode Current Mirror
EECE488 Set 5 - Current Mirrors 17SM
Cascode Current Mirror
EECE488 Set 5 - Current Mirrors 18SM
Board Notes
EECE488 Set 5 - Current Mirrors 19SM
Cascode Current Mirror Biasing
EECE488 Set 5 - Current Mirrors 20SM
Cascode Current Mirror Biasing
EECE488 Set 5 - Current Mirrors 21SM
Current Mirror Biasing
EECE488 Set 5 - Current Mirrors 22SM
Basic Circuit to Generate Supply Independent Current
EECE488 Set 5 - Current Mirrors 23SM
Supply Independent Current
EECE488 Set 5 - Current Mirrors 24SM
Board Notes
EECE488 Set 5 - Current Mirrors 25SM
Supply Independent Current
EECE488 Set 5 - Current Mirrors 26SM
Start-up Problem
EECE488 Set 5 - Current Mirrors 27SM
Start-up Problem
DDGSTHGSDDTHTHTH VVVVandVVVV >++<++ 351351
EECE488 Set 5 - Current Mirrors 28SM
Board Notes
EECE488 Set 5 - Current Mirrors 29SM
Active Current Mirrors
EECE488 Set 5 - Current Mirrors 30SM
Active Current Mirrors in Differential to Single-Ended Amplifiers
EECE488 Set 5 - Current Mirrors 31SM
Differential to Single-Ended Amplifiers
EECE488 Set 5 - Current Mirrors 32SM
Calculation of Gm
ID1 = ID3 = ID 4 = gm1,2Vin / 2 ID2 = −gm1,2Vin / 2
Iout = ID2 − ID4 = −gm1,2Vin ,⇒ Gm = gm1, 2
EECE488 Set 5 - Current Mirrors 33SM
Small-Signal Gain
Av ≈ gm 1,2 (ro2 || ro4 )
EECE488 Set 5 - Current Mirrors 34SM
Common Mode Characteristics
ACM =∆Vout
∆Vin ,CM
EECE488 Set 5 - Current Mirrors 35SM
Common Mode
ACM ≈−
12gm3,4
||ro3,4
21
2gm1,2
+ RSS
=−1
1 + 2gm1,2 RSS
gm1,2
gm 3, 4
EECE488 Set 5 - Current Mirrors 36SM
Common Mode
CMRR =ADM
ACM
= gm1, 2(ro1,2 || ro3,4 )gm 3,4 (1 + 2gm1,2 RSS )
gm1,2
= gm 3,4 (ro1, 2 || ro3,4 )(1 + 2gm1, 2RSS)
