EE 330 Lecture 33. High Gain Amplifiers Current Sources and Mirrors The Cascode Configuration

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1 EE 330 Lecture 33 Hih Gain mplifiers Current Sources and Mirrors The Cascode Confiuration

2 Review from Last Lecture Hih-ain amplifier V DD I B i B V BE π m V BE 0 V EE This ain is very lare (but realistic)! nd no desin parameters affect the ain But how can we make a current source? - m V 0 -ICQ V V - V I /V V F t CQ F t VF 200V V V 25mV t

3 Review from Last Lecture Hih-ain amplifier I B V DD V 8000 V EE How can we build the dc current source? What is the small-sinal model of an actual current source?

4 Review from Last Lecture Current Sources/Mirrors R I 0 I 1 I 1 E0 Q 0 E1 Current Sink Multiple Outputs Possible Can be built for sourcin or sinkin currents lso useful as a current amplifier MOS counterparts work very well and are not plaued by base current

5 Review from Last Lecture Current Sources/Mirrors Biasin Circuit R I 0 I 1 I 1 E0 Q 0 E1 Key Block Current Sink

6 Review from Last Lecture Current Sources/Mirrors I 0 I 1 Q 0 I 2 I n Q n E0 E1 E2 En Multiple-Output Bipolar Current Sink I = I Ek k 0 E0

7 Review from Last Lecture Current Sources/Mirrors V DD Q p0 Q p1 Q p2 Q pn Ep0 Ep1 Ep2 Epn I p1 I p2 I pn I 0 I n0 I n1 I nn Q 0 Q n1 Q n2 Q nn E0 En1 En2 Enn Multiple-Output Bipolar Current Source and Sink I = I Enk nk 0 E0 I = I En1 Epk pk 0 E0 Ep0

8 Current Sources/Mirrors I in I out Q 0 E0 E1 npn Current Mirror I = E1 I out in E0 Termed a current mirror Output current linearly dependent on I in Serves as a current amplifier Widely used circuit But I in and I out must be positive!

9 Current Sources/Mirrors I BS i in i out MI BS Q 0 E0 E1 M= E1 E0 npn current mirror amplifier i out =?

10 Current Sources/Mirrors I BS i in i out MI BS Q 0 E0 E1 M= E1 E0 npn current mirror amplifier i out = E1 E0 i in mplifiers both positive and neative currents (provided i IN >-I BS )

11 Current Sources/Mirrors I 0 I OU T Q 0 I in I out M 0 M 1 W 0,L 0 W 1,L 1 E0 E1 npn Current Mirror n-channel Current Mirror I out=?

12 Current Sources/Mirrors I in I out M 0 M 1 W 0,L 0 W 1,L 1 n-channel Current Mirror μc W I = V -V 2 OX 0 in GS0 T0 2L0 μ C W I = V -V OX 1 2 out GS1 T1 2L1 If process parameters are matched, it follows that W1 L0 I out= I W0 L1 in Current mirror ain can be accurately controlled Layout is important to et accurate ain (for both MOS and BJT)

13 Current Sources/Mirrors Summary I 0 Q 0 I OU T I in I out M 0 M 1 W 0,L 0 W 1,L 1 E0 E1 npn Current Mirror n-channel Current Mirror I = E1 I out in E0 W1 L0 I out= I W0 L1 in

14 Layout of Current Mirrors I in I out Example with M = 2 W 1,L 1 M 1 M 2 W 2,L 2 M= W L W L Standard layout W2 2W L1 2L M= W 1 2 W L 2 2 L 2W1 2W L1 2L M= W 1 2 W L 1 2 L 2 Gate area after fabrication depicted

15 Layout of Current Mirrors Example with M = 2 Standard layout M= W L W L W1 2W L1 2L M= W 1 2 W L 1 2 L 2 2W1 4W L1 2L M= W 1 2 W L 1 2 L 2 Better Layout

16 Layout of Current Mirrors Example with M = 2 Standard layout M= W L W L W1 4W L1 2L M= W 1 2 W L 1 2 L 2 Better Layout 2W1 4W L1 2L M= W 1 2 W L 1 2 L 2 Even Better Layout This is termed a common-centroid layout

17 n-channel current mirror current amplifier I BS i in i out MI BS M 1 M 2 W L M= W L W 1,L 1 W 2,L 2 i out = W L W L i in mplifies both positive and neative currents

18 Current Sources/Mirrors I 0 I 1 M 0 M 1 I 2 M 2 I 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 V DD W I = L I k 0 k 0 W0 L k M 0 M 1 M 2 M n W 0,L 0 W 1,L 1 W 2,L 2 I 0 I 1 I 2 I n W n,l n multiple output p-channel current source array

19 Current Sources/Mirrors multiple sourcin and sinkin current outputs V DD M p0 M p1 M p2 M pm pj p0 W p0,l p0 W p1,l p1 W p2,l p2 W pm,l pm I = I p0 I p1 I p2 I pm W L MI pj 0 Lpj Wp0 W L M= L W n0 0 n0j 0 I 0 M 0 I n0 M n0 I n1 M n1 I n2 M n2 I nk M nk W I = L nj 0 nj I0 Lnj W0 W 0,L 0 W n0,l n0 W n1,l n1 W n2,l n2 W nk,l nk m and k may be different Often M=1

20 Hih-ain amplifier I B V DD V 8000 V EE How can we build the current source? What is the small-sinal model of an actual current source?

21 Basic Current Sources and Sinks Bipolar Mirror-Based Current Sink Bipolar Mirror-Based Current Source R Q 0 E0 E1 I 0 Q 0 R I 0 E0 E1 Biasin Circuit Biasin Circuit Biasin circuit uses same as amplifier and no other independent sources

22 Hih-ain amplifier V DD I B Q 0 I B EB Q B R E0 Biasin Circuit V EE Biasin Circuit Bias circuitry requires only a sinle independent dc voltae source! Incremental overhead is only one transistor, Q B

23 Basic Current Sources and Sinks Basic Bipolar Current Sinks Basic Bipolar Current Sources V XX V V I t X= JSEe XX R R VCC-0.6V R Very practical methods for biasin the BJTs (or MOSFETs) can be used Current Mirrors often used for eneratin sourcin and sinkin currents Can think of biasin transistors with V XX and in these current sources

24 Hih-ain amplifier I B V DD V 8000 V EE How can we build the current source? What is the small-sinal model of an actual current source?

25 Small-sinal model of all other BJT Sinks and Sources introduced so far are the same Basic Current Sources and Sinks Small-sinal Model of BJT Current Sinks and Sources V XX Not Diode Connected! i B V BE π m V BE 0 0

26 Basic Current Sources and Sinks Small-sinal Model of MOS Current Sinks and Sources m V GS V GS 0 0 Small-sinal model of all other MOS Sinks and Sources introduced thus far are the same

27 Hih-ain amplifier V DD I B V - m 0 V EE i B V BE1 m1 V B π E1 V EE V -m1-2 m

28 Hih-ain amplifier V DD I B V EE V - m 0 m VF = 8000 V 0 t V EE V - 2 m1 Nonideal current source decreased the ain by a factor of 2 01 But the voltae ain is still quite lare (-4000) Can the ain be made even larer?

29 Hih-ain amplifier Can the ain be made even larer? Discuss The Cascode Confiuration VXX VXX M 2 M 1 M 1 M 2

30 The Cascode mplifier (consider npn BJT version) I B Discuss VXX ctually a cascade of a CE stae followed by a CB stae but usually viewed as a sinle-stae structure Cascode structure is widely used

31 Basic mplifier Structures Discuss 1. Common Emitter/Common Source 2. Common Collector/Common Drain 3. Common Base/Common Gate 4. Common Emitter with R E / Common Source with R S 5. Cascode (actually CE:CB or CS:CD cascade) 6. Darlinton (special CE:CE or CS:CS cascade) The first 4 are most popular

32 Cascode Confiuration Discuss Two-port model of cascode amplifier V X V 2 π2 m2 V 2 O2 I 1 V 1 π1 m1 V 1 O1 V +V +V =I X m2 X V -V + = I 1 m π2 X Observin V 1 = and eliminatin V 2 between these two equations, we obtain and Standard Form for mplifier Two-Port 1 =I1 π π2+m2 m1 02+m2 V X=IX -VIN π2 02 π2+01 i 1 V 1 R in i 2 vrv2 R o v0v1 V 2 V = i R + V 1 1 IN VR 2 V = i R + V 2 2 O V0 1

33 Cascode Confiuration Discuss Two-port model of cascode amplifier V X V 2 π2 m2 V 2 O2 V 1 π1 m1 V 1 O π2+m2 m1 02+m2 V X=IX -VIN π2 02 π =I1 π1 It thus follows for the npn bipolar structure that : m1 02+m2 m1 m2 VCC= π π2 0CC π2 02 π2 0CC= π2 + m2 m2 V 1 πcc VCC V 1 = πcc π1

34 Cascode Confiuration Discuss I B VCC - m1 m2 02 π2 VXX 0CC 02 π2 m2 = πcc π1 m1 m1 VCC - β β 0CC β Voltae ain is a factor of β larer than that of the CE amplifier with current source load Output impedance is a factor of β larer than that of the CE amplifier

35 Cascode Confiuration Discuss I B m1 m1 VCC β β VXX 0CC 02 β m1 2VF VCC β= Vt 800, 000 VCC This ain is very lare and only requires two transistors! What happens to the ain if a transistor-level current source is used for I B?

36 Cascode Confiuration I B Q 3 VXX VXX

37 Cascode Confiuration Q 3 Q 3 VXX

38 Hih-ain amplifier comparisons Q 3 0CC V 1 πcc VCC V 1 03 It thus follows that 0CC V VCC CC = But 0CC ; 03 /β V VCC 0CC 03 VCC This is a dramatic reduction in ain compared to what the ideal current source biasin provided β

39 Cascode Confiuration Q 3 V VCC But recall Thus VCC V 0CC 03 m1 01 m1 01 VCC ICQ V V t V F 8000 I CQ Vt VF This is still a factor of 2 better than that of the CE amplifier with transistor current source m1 VCE 201 It only requires one additional transistor But its not nearly as ood as the ain the cascode circuit seemed to provide β

40 Cascode Confiuration Comparisons V DD I B V EE V - m 0 V EE V -m1-2 m I B Q 3 VXX V m1 01 β VXX V m1 m Gain limited by output impedance of current scource!! Can we desin a better current source? In particular, one with a hiher output impedance?

41 Better current sources Need a hiher output impedance than o The output impedance of the cascode circuit itself was very lare! 0CC 01 β Can a current source be built with the cascode circuit?

42 Cascode current sources V XX V XX M 2 Discuss M 1 V DD M 1 V XX V XX M 2

43 Cascode Confiuration Discuss VXX V ZZ Q 3 Q 4 m1β V= V= ,000 2 This ain is very lare and is a factor of 2 below that obtained with an ideal current source biasin lthouh the factor of 2 is not desired, the performance of this circuit is still very ood This factor of 2 ain reduction is that same as was observed for the CE amplifier when a transistorlevel current source was used

44 Cascode Confiuration Comparisons V DD I B I B Discuss V EE V - m 0 V = -8,000 V EE V - 2 m1 01 V = -4,000 VXX V m1 01 V = -800,000 β V ZZ Q 3 Q 3 Q 4 VXX V m1 01 V = -8,000 VXX m1β V= 01 2 V = -400,000 Can we use more cascodin to further increase the ain?

45 The Cascode mplifier (consider n-ch MOS version) Discuss I B VXX M 2 VCC - 0CC m1 m m2 M 1 Same issues for biasin with current source as for BJT case With cascode current source, ain only drops by a factor of 2

46 The Cascode mplifier (consider n-ch MOS version) V DD V DD Discuss I B V ZZ M 3 M 4 V ZZ M 3 VXX M 2 VXX M 2 M 1 M 1 VXX M 2 M 1 VCC - m1 m VCC - m m1 m2 VCC

47 Current Source Summary (BJT) Basic Cascode V XX V XX / CC 01 β

48 Current Source Summary (MOS) Basic Cascode V DD M 1 V DD VXX M 2 M 1 M 1 M 1 V ZZ M m2

49 Hih Gain mplifier Comparisons ( n-ch MOS) I B V DD V DD I B V DD M 1 V ZZ M 2 M 1 V XX M 2 V ZZ M 3 V ZZ M 4 M 3 V - m m1 V M 1 VCC - m1 m M 2 V XX M 1 VCC - m1 01 M 2 VXX M 1 1 m1 m2 VCC

50 Hih Gain mplifier Comparisons (BJT) V DD I B I B VXX V V EE - m 0 V V EE 2 1 m1 01 V m1 01 β VXX Q 3 VXX V ZZ Q 3 Q 4 Sinle-ended hih-ain amplifiers inherently difficult to bias (because of the hih ain) Biasin becomes practical when used in differential applications These structures are widely used but usually with differential inputs V m1 01 m1β V= 01 2

51 End of Lecture 33

EE 330 Class Seating

EE 330 Class Seating 1 2 3 4 5 6 EE 330 Class Seatin 1 2 3 4 5 6 7 8 Zechariah Daniel Liuchan ndrew Brian Difen imee Julien Di Pettit Borerdin Li Mun Crist Liu Salt Tria Erik Nick Bijan Win Yi Panzhou Travis Wentai Hisham