Presentation on theme: "Bias Voltage Generation. Use Cascode to Increase output Resistance Rout is approximately g m3 r o3 r o2 L1=L2, but L3 need not equal to L2. Design Criteria:"— Presentation transcript:
Bias Voltage Generation
Use Cascode to Increase output Resistance Rout is approximately g m3 r o3 r o2 L1=L2, but L3 need not equal to L2. Design Criteria: Choose V b so that V Y and V X. Question: How do you generate Vb ?
Cascode Current Source Requirement: Choose Vb so that V X =V Y V N =V GS0 +V X =V GS3 +V Y Therefore, V GS3 =V GS0 Since I D1 =I D2, (W/L) 3 =(W/L) 0
Cascode Current Mirror VDS1=249.6 mV VDS6=263.7 mV VDS5=0.675 V VDS0=0.286 V IDS5=20.41uA IDS0=10 uA gmovergds_5=47 gds6=10.35uS Rout=4.5 MOhms (Mismatch) (Close)
Sensitivity of IOUT due to VOUT As VX decreases from VDD, M3 enters the triode region first. M2 enters the triode region
Accuracy and Voltage Headroom Trade-Off Vb is chosen to allow minimum VP. Problem: VX is not equal to VY Iout is not equal to Iref. Vb is chosen to allow VX=VY VP is not minimum. But Iout is equal to Iref.
Design Criteria Desirables: – I OUT should be I REF. (i.e. V X =V Y ) – V out should be minimized. (i.e. V OD2 +V OD3 ) V OUT =V OD3 +V OD4 V A =V B →I OUT =mI REF IREF produces VGS1 and VGS3. If VA is to be defined by Vb, then VD1 must not be connected to VG1, otherwise, it becomes unclear which node defines VA.
Low Voltage Cascode To keep M2 in saturation: V x >V b -V th →V x +V th2 >V b To keep M1 in saturation: V A >V x -V th1 Since V A =V b -V GS2, V b >V x -V th1 +V GS2 Design criteria for M2
Vb Requirement Vb=VOD3+VGS4 to produce a minimum output Voltage of VOD3 and VOD4. By design, VGS4=VGS2, VA=VB Vb=V OD2 +V TH2 +V OD1.
VOD3=0.163 V VOD4=0.056 V VOUT(min)=VOD3+VOD4=0.219 V
Vb Generation (Option 1) Requirement: Vb=V OD2 +V TH2 +V OD1 V GS5 =V GS2 V OD1 =V GS6 -I 1 R b Problem: M5 suffers from no body effect M2 suffers from body effect Rb is not well controlled, unless Rb is off-chip.
Vb Generation (Option 2) Requirement: Vb=V OD2 +V TH2 +V OD1 V GS5 =V GS2 V OD1 =V GS6 -V TH7 Problem: M5 suffers from no body effect M2 suffers from body effect Design M7 (Large W 7 /L 7 ) so that VGS7 is approx. VTH7 Need to have sufficiently large VGS6, otherwise M6=triode since M7=Sat.