Lect. 22: MOSFET Current Mirror and Active Load

Various bias techniques for MOSFET circuits (Razavi 9.2.3)

How do we make a constant with ?

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

Constant current source: 1 ' W 2 IkDn1  VV GSt 2 L 1 VV II  DD GS DREF1 R

Assuming Q1, Q2 have same properties (kn’),

1 ' W 2 IIOD2 k n VV GStn  2 L 2

I WL/  O  2 IWL/  Current mirror REF 1

Limitation on Vo? VVOGSt V

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

Mismatches between IREF and IO Due to channel-length modulation For Q1 and Q2 IO = IREF only if VDS1= VDS2  V0=VGS

As VO increases, IO increases from IREF

VVOGS IIO REF r0

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

V =3V, Q , Q identical 1 W DD 1 2 I Ik'2() VV  L= 1m, W=100m, V =0.7V, k ’=200A/V2, D1 REF n GS t t n 2 L 1 ro = 200k 1 2 100 200  10  (VVGS  t ) , 1. Determine R for IO=100A. 2 2. What is the lowest value for VO? (VV ) 0.316, V  1.016 3. How much IO changes when VO changes 1V? GS t GS

VV 31.016 R  DD GS 19.84k I REF 0.1mA

VVVOGStmin 0.316V

VO 1V IO  5A rko2 200 

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

(Without r0 consideration)

WL/  II 2 2 REFWL/ 1

WL/  II 3 3 REFWL/ 1

I34 I

WL/  II 5 54WL/ 4

Current-steering circuits: current source (Q5), current sink (Q2)

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

CS

Where is RD ?

Current source as a resistor  Active load

(Remember Q2 has rO)

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

Small-signal model for Q1 and Q2

Gain: -gm1 (rO1 || rO2)

PMOS current mirror provides large resistance (Active Load) as well as bias current!

 Good for IC!

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

2 2 VDD=3V, Vtn=|Vtp|=0.6V, kn’=200A/V , kp’=65A/V , 1. A grmo11( || r o 2 ) L=0.4m, W=4m, ro1=200k, ro2,3 = 100k, IREF=100A. ' W 4 gkmn1 2 I REF 2 200 100 0.63mA/V 1. What is the small-signal voltage gain, vO/vI? L 1 0.4 2. What is the maximum vO for which the above is valid?

 A 0.63(mA/V)  (200 ||100)(k  )  42

2 1 ' W VSD,3 2. For Q3,3 , IkREF p VV SG tp 2 L 3 ro

14 2 VSG,3 100 65VSG  0.6  2 0.4 100K

VVSG ~1.12

For vVOSDSGtp,max, 2,min  VV | | 1.12 0.6 0.52 V

vVVODDSD,max 2,min 2.48 V

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 22: MOSFET Current Mirror and Active Load

Homework:

Determine Icopy in following circuits. Assume all MOS transistors are in saturation and the influence of r0 can be ignored.

Electronic Circuits 1 (13/2) Prof. Woo-Young Choi