Lect. 22: MOSFET Current Mirror and Active Load Various bias techniques for MOSFET circuits (Razavi 9.2.3) How do we make a constant current source with MOSFETs? 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 IIOD2 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= 1m, W=100m, V =0.7V, k ’=200A/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=100A. 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 54WL/ 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 amplifier 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’=200A/V , kp’=65A/V , 1. A grmo11( || r o 2 ) L=0.4m, W=4m, ro1=200k, ro2,3 = 100k, IREF=100A. ' 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 65VSG 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.