Sep 17, 2007 #1 L leonken Full Member level 3 Joined Jun 12, 2004 Messages 169 Helped 0 Reputation 0 Reaction score 0 Trophy points 1,296 Activity points 1,437 Two kind of regulators: NMOS pass device and PMOS pass device. Why the voltage drop across the pass deivce of PMOS is smaller than the NMOS? Added after 37 minutes: The figure
Two kind of regulators: NMOS pass device and PMOS pass device. Why the voltage drop across the pass deivce of PMOS is smaller than the NMOS? Added after 37 minutes: The figure
Sep 17, 2007 #2 P pugongying Member level 3 Joined Apr 2, 2006 Messages 64 Helped 1 Reputation 2 Reaction score 1 Trophy points 1,288 Activity points 2,110 voltage drop using NMOS is determined by VGS while using PMOS it is determined by VDS you can find the difference
voltage drop using NMOS is determined by VGS while using PMOS it is determined by VDS you can find the difference
Sep 18, 2007 #3 W wwwww12345 Member level 1 Joined Sep 6, 2007 Messages 34 Helped 0 Reputation 0 Reaction score 0 Trophy points 1,286 Activity points 1,486 For NMOS, Vin >VG>VS(Vout)+VTN, so VDS=Vin-Vout>VTN, but for PMOS, no such formula is expected.
Sep 18, 2007 #4 L leonken Full Member level 3 Joined Jun 12, 2004 Messages 169 Helped 0 Reputation 0 Reaction score 0 Trophy points 1,296 Activity points 1,437 Because the Vdd(Vin) of the opamp is supplied by Vin. The output of opamp is less than Vdd(Vin).
Sep 20, 2007 #5 C chenmy Junior Member level 2 Joined Aug 18, 2006 Messages 22 Helped 6 Reputation 12 Reaction score 5 Trophy points 1,283 Activity points 1,460 Hi,Leonken: For NMOS , Vin is the supply of OPAMP,so Vin >VG>Vout+VTN should be met. so VDS=Vin-Vout>VTN (about ~700mV in 0.35um process) For PMOS,you only need to make VSD=Vin-Vout larger than Vdsat(usually about 100mV~300mV) So the PMOS LDO headroom is larger than NMOS LDO. It is the NMOS Source follower that reduces your headroom. Best regards! Chenmy
Hi,Leonken: For NMOS , Vin is the supply of OPAMP,so Vin >VG>Vout+VTN should be met. so VDS=Vin-Vout>VTN (about ~700mV in 0.35um process) For PMOS,you only need to make VSD=Vin-Vout larger than Vdsat(usually about 100mV~300mV) So the PMOS LDO headroom is larger than NMOS LDO. It is the NMOS Source follower that reduces your headroom. Best regards! Chenmy
Sep 23, 2007 #6 leo_o2 Advanced Member level 4 Joined Sep 3, 2004 Messages 1,322 Helped 278 Reputation 558 Reaction score 241 Trophy points 1,343 Location China Activity points 5,761 Some LDO add a charge-pump and can also realize low drop-out with NMOS
Sep 25, 2007 #7 paley Member level 4 Joined Mar 3, 2006 Messages 78 Helped 6 Reputation 12 Reaction score 3 Trophy points 1,288 Activity points 1,596 because the nmos' Vgs
Sep 26, 2007 #8 M manissri Full Member level 5 Joined Apr 16, 2005 Messages 281 Helped 9 Reputation 18 Reaction score 3 Trophy points 1,298 Activity points 3,279 if u apply pmos pass devic the it will give u gain and phase shift of 180. when u use nmos as pass device then it will act as a source follower. so unity gain and no phase shift.
if u apply pmos pass devic the it will give u gain and phase shift of 180. when u use nmos as pass device then it will act as a source follower. so unity gain and no phase shift.
Oct 5, 2007 #9 J joe_chen Member level 2 Joined Oct 3, 2007 Messages 46 Helped 3 Reputation 6 Reaction score 1 Trophy points 1,288 Activity points 1,435 one is source follower and the other is simply common source stage