EE 330 Lecture 31 urrent Source Biasing urrent Sources and Mirrors
eview from Last Lecture Basic mplifier Gain Table DD DD DD DD in B E out in B E out E B BB in E out in B E E out in 2 D Q EE SS E/S /D B/G EwE/SwS MOS BJT MOS BJT MOS BJT MOS BJT -g EB m r π Q β t Q t 2 DQ m g m g +g E 2 + DQ E EB π E r +β β E Q E + Q E t t Q E 2 D Q 2 g EB DQ EB m -1 g m Q t Q t π EE - E r +β β t Q E E out 2 DQ EB -1 g m an use these equations only when circuit is EXTLY like that shown above!! Q t
B eview from Last Lecture Basic mplifier haracteristics Summary E/S in DD B E EE out Large noninverting gain Low input impedance Moderate (or high) output impedance Used more as current amplifier or, in conjunction with D/S to form two-stage cascode /D in DD B E out E SS Gain very close to +1 (little less) High input impedance for BJT (high for MOS) Low output impedance Widely used as a buffer B/G DD B BB E in out Large noninverting gain Low input impedance Moderate (or high) output impedance Used more as current amplifier or, in conjunction with D/S to form two-stage cascode EwE/ SwS in DD E E out easonably accurate but somewhat small gain (resistor ratio) High input impedance Moderate output impedance Used when more accurate gain is required EE
eview from Last Lecture High-gain BJT amplifier in B DD E EE out -g -g m m g0 G To make the gain large, it appears that all one needs to do is make large! -g m - Q But t is fixed at approx 25m and for good signal swing, Q <( DD- EE )/2 f DD - EE =5, D D 5 2 2 25m t t E E 100 Gain is practically limited with this supply voltage to around 100
eview from Last Lecture in D G High-gain MOS amplifier DD D S SS out -g m -gm D g0 GD To make the gain large, it appears that all one needs to do is make D large! -g m D -2 D Q EB But EB is practically limited to around 100m and for good signal swing, DQ D <( DD- SS )/2 D D EB f DD - SS =5 and EB =100m, 5 50 100m Gain is practically limited with this supply voltage to around 100 re these fundamental limits on the gain of the BJT and MOS mplifiers? D S S
eview from Last Lecture High-gain amplifier DD i B OU B OUT OUT N BE g π g m BE N N EE This gain is very large! -g 0 m Too good to be true! Need better model of MOS device!
eview from Last Lecture High-gain amplifier DD B OUT i B OUT OUT N N BE g π g m BE g 0 N EE This gain is very large (but realistic)! -g m g0 -Q - / - F t Q F t F t 200 8000 25m But how can we make a current source?
High-gain amplifier DD 8000 B OUT N EE How can we build the ideal current source? What is the small-signal model of an actual current source?
urrent Sources/Mirrors 0-0.6 Load f the base currents are neglected 0 1 E0 Q 0 E1
urrent Sources/Mirrors 0-0.6 0 Q 0 E0 Load 1 E1 f the base currents are neglected 0=JSE0e 1=JS E1e BE1 t BE0 since BE1 = BE2 1 E1 E0 Behaves as a current source! t 0 ctually termed a sink current since coming out of load
urrent Sources/Mirrors 0 1 1 E0 Q 0 E1 urrent Sink Multiple Outputs Possible an be built at sourcing or sinking currents lso useful as a current amplifier MOS counterparts work very well and are not plagued by base current
urrent Sources/Mirrors 0 1 Q 0 2 Q 2 n Q n E0 E1 E2 En Multiple-Output Bipolar urrent Sink = Ek k 0 E0
urrent Sources/Mirrors DD Q 0 Q 2 Q n E0 E1 E2 En 0 1 2 n Multiple-Output Bipolar urrent Source = Ek k 0 E0
urrent Sources/Mirrors DD Q p0 Q p1 Q p2 Q pn Ep0 Ep1 Ep2 Epn p1 p2 pn 0 n0 n1 nn Q 0 Q n1 Q n2 Q nn E0 En1 En2 Enn Multiple-Output Bipolar urrent Source and Sink nk =? pk =?
urrent Sources/Mirrors DD Q p0 Q p1 Q p2 Q pn Ep0 Ep1 Ep2 Epn p1 p2 pn 0 n0 n1 nn Q 0 Q n1 Q n2 Q nn E0 En1 En2 Enn Multiple-Output Bipolar urrent Source and Sink = E nk nk 0 E0 = E n1 E pk pk 0 E 0 E p0
urrent Sources/Mirrors in out Q 0 E0 E1 npn urrent Mirror E1 out = E0 Termed a current mirror Output current linearly dependent on in Serves as a current amplifier Widely used circuit in
urrent Sources/Mirrors BS i in i out M BS Q 0 E0 E1 M= E1 E0 npn current mirror amplifier i out =?
urrent Sources/Mirrors BS i in i out M BS Q 0 E0 E1 M= E1 E0 npn current mirror amplifier i out = E1 E0 i in mplifiers both positive and negative currents
urrent Sources/Mirrors 0 OU T Q 0 in out M 0 M 1 W 0,L 0 W 1,L 1 E0 E1 npn urrent Mirror n-channel urrent Mirror out=?
urrent Sources/Mirrors in out M 0 M 1 W 0,L 0 W 1,L 1 n-channel urrent Mirror μ W = - O X 1 in G S1 T1 2L2 μ W = - O X 2 out G S2 T2 2L2 2 2 f process parameters are matched, it follows that W2 L1 out = W L 1 2 in urrent mirror gain can be accurately controlled Layout is important to get accurate gain (for both MOS and BJT)
Layout of urrent Mirrors Example with M = 2 W L M= W L 2 1 1 2 Standard layout W 2 2 W L 1 2 L M= W 2 W L 2 L 1 2 M= 2W 2 W L 2 L 1 1 W 2 W L 2 L 1 1 2 Gate area after fabrication depicted
Layout of urrent Mirrors Example with M = 2 W L M= W L 2 1 1 2 Standard layout M= 2W 2 W L 2 L 1 1 W 2 W L 2 L 1 1 2 M= 2W 4 W L 2 L 1 1 W 2 W L 2 L 1 1 2 Better Layout
Layout of urrent Mirrors Example with M = 2 Standard layout W L M= W L 2 1 1 2 M= 2W 4 W L 2 L 1 1 W 2 W L 2 L 1 1 2 Better Layout M= 2W 4 W L 2 L 1 1 W 2 W L 2 L 1 1 2 Even Better Layout This is termed a common-centroid layout
urrent Sources/Mirrors BS i in i out M BS M 1 M 2 W L M= W L 2 1 1 2 W 1,L 1 W 2,L 2 n-channel current mirror current amplifier i out W L = W L 2 1 1 2 i in mplifiers both positive and negative currents
urrent Sources/Mirrors 0 1 M 0 M 1 2 M 2 n M n W 0,L 0 W 1,L 1 W 2,L 2 W n,l n multiple output n-channel current sink array DD W L k 0 k 0 W0 Lk = M 0 M 1 M 2 M n W 0,L 0 W 1,L 1 W 2,L 2 0 1 2 n W n,l n multiple output p-channel current source array
High-gain amplifier DD 8000 B OUT N EE How can we build the current source? What is the small-signal model of an actual current source?
Basic urrent Sources and Sinks Basic Bipolar urrent Sinks Basic Bipolar urrent Sources X XX X t X= JSEe XX YY X X X X X X X -0.6 ery practical methods for biasing the BJTs can be used urrent Mirrors often used for generating sourcing and sinking currents
Basic urrent Sources and Sinks Small-signal Model of BJT urrent Sinks and Sources X XX i B BE g π g m BE g 0 g 0 Small-signal model of all other BJT Sinks and Sources are the same
Basic urrent Sources and Sinks Small-signal Model of BJT urrent Sinks and Sources X XX i B BE g π g m BE g 0 g 0 Small-signal model of all other BJT Sinks and Sources are the same
Basic urrent Sources and Sinks Small-signal Model of BJT urrent Sinks and Sources g m GS GS g 0 g 0 Small-signal model of all other MOS Sinks and Sources are the same
High-gain amplifier DD B OUT N OUT N -g g m 0 EE i B OUT YY Q 2 N BE1 g m1 B g π1 g 01 g 02 E1 N EE OUT N OUT -g -g m 1 m 1 g g 2g 01 02 01
High-gain amplifier DD B YY OUT OUT N N EE EE -g g m 0 -g 2g m 1 01 Nonideal current source decreased the gain by a factor of 2 But the voltage gain is still quite large an the gain be made even larger?