Electronic Circuits EE359A

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1 Electronic Circuits EE359A Bruce McNair B Lecture 12 1

2 MOSFET vs. BJT current-voltage characteristic i C ( v) i D ( v) v The drain current of a MOSFET follows a square law relationship The collector current of a BJT follows an exponential relationship This means that the BJT can control a current that varies over ~5 orders of magnitude, compared to MOSFET current that varies as v OV 2 ( V), or about 1 order of magnitude. 2

3 MOSFET vs. BJT design parameters Significant parameter MOSFET W/L BJT A E (E-B junction area) Range :1 3

4 MOSFET vs. BJT design tradeoffs Available parameters MOSFET I D, V OV, W, L BJT I C, V BE, I S Useful parameters Pick any 3 I C (V BE is related and not very adjustable) 4

5 Differential and Multistage Amplifiers Ch 8 5

6 Noise issues with single ended systems Noise, interference Ground loops, offset reference 6

7 Noise issues with single ended systems Noise, interference Large effective antenna 7

8 DC offset issues with single ended systems offset reference 8

9 Differential benefits + - Differential signal Independent of ground reference + - 9

10 Differential benefits Differential amplification lends itself to twisted pair wiring + - Alternate loops cancel Small antenna loops

11 MOS differential pair MOSFETs are biased for saturation mode operation (not triode region) Resistive loads (for now) Ideal current source 11

12 Common-mode input voltage Assume identical MOSFETs, identical drain resistors 12

13 Common-mode input voltage Range of common mode voltage is limited: I V + V + V + V V V + V R 2 V CS ss CS t OV CM DD t D 13

14 Differential input voltage For all current to flow through Q 1 : 1 W I = k v V 2 L ( ) 2 ' n GS1 t 14

15 Differential input voltage For all current to flow through Q 1 : This determines limits of v id : 1 W I = k v V 2 L ( ) 2 ' n GS1 t 2V v 2V OV id OV 15

16 Differential input voltage For all current to flow through Q 1 : This determines limits of v id : 1 W I = k v V 2 L ( ) 2 ' n GS1 t 2V v 2V OV id OV Both transistors are in saturation, even though one is not conducting 16

17 Large signal operation 1 W i = k v V 2 L ( ) 2 ' D1 n GS1 t 1 W i = k v V 2 L ( ) 2 ' D2 n GS2 t v = v v = v v id GS1 GS 2 G1 G2 17

18 Large signal operation 1 W i = k v V 2 L ( ) 2 ' D1 n GS1 t 1 W i = k v V 2 L ( ) 2 ' D2 n GS2 t v = v v = v v id GS1 GS 2 G1 G2 1 W i i = k v 2 L I = i + i ' D1 D2 n id D1 D2 18

19 Large signal operation 1 W i = k v V 2 L ( ) 2 ' D1 n GS1 t 1 W i = k v V 2 L ( ) 2 ' D2 n GS2 t i D1 v = v v = v v id GS1 GS 2 G1 G2 I I vid vid /2 = VOV 2 VOV 2 1 W i i = k v 2 L I = i + i ' D1 D2 n id D1 D2 i D1 I I vid vid /2 = 1 2 VOV 2 VOV 2 19

20 Large signal operation 20

21 Large signal operation Nonlinear operating characteristics 21

22 Large signal operation Limited linear operating range v id /2 << V OV 22

23 Large signal operation Linear operating range v id /2 << V OV increase V Ov by using smaller W/L (but g m and gain are decreased) 23

24 Small signal differential gain Differential output v = V + v 1 G1 CM 2 id v = V v 1 G2 CM 2 id 24

25 Small signal differential gain v = V + v 1 G1 CM 2 id v = V v 1 G2 CM 2 id 25

26 Small signal differential gain v = V + v 1 G1 CM 2 id v = V v 1 G2 CM 2 id 26

27 Small signal differential gain v = V + v 1 G1 CM 2 id v = V v 1 G2 CM 2 id v v = g =+ g v R 2 v R 2 A vod = g R v id o1 m D id o2 m D d m D id 27

28 Differential amplifier with current source loads v A = g r r ( ) od d m o1 o3 vid 28

29 Cascode differential amplifier v A = g R R ( ) ( ) on m3 03 o1 op m5 05 o7 ( ) od d m on op vid R = g r r R = g r r R op R on 29

30 Common-mode rejection ratio 30

31 Common-mode rejection ratio Consider mismatched R D : Q 1 has drain resistor R D Q 2 has resistor R D +DR D v v o1 o2 A cm R 2R D SS C+ΔR 2R SS v D icm v icm ΔR D R D ΔR D = = 2RSS 2RSS RD 31

32 Common-mode rejection ratio Consider mismatched g m : Q 1 : g m + 1 / 2 Δg m Q 2: g m - 1 / 2 Δg m ( 1 2) 1 ( 1 2) ( Δg ) R g g R v = v v = v + + m m D od o1 o2 icm gm gm RSS A A cm cm vod m v = 1 + 2g R R D Δg m 2RSS gm D icm m SS 32

33 Common-mode rejection ratio CMRR = A A d cm CMRR 2gmRSS = ΔRD RD CMRR 2gmRSS = Δgm gm 33

34 BJT differential pair Q 1 and Q 2 must not saturate 34

Integrated Circuit Amplifiers. Comparison of MOSFETs and BJTs

Integrated Circuit Amplifiers. Comparison of MOSFETs and BJTs Integrated Circuit Amplifiers Comparison of MOSFETs and BJTs 17 Typical CMOS Device Parameters 0.8 µm 0.25 µm 0.13 µm Parameter NMOS PMOS NMOS PMOS NMOS PMOS t ox (nm) 15 15 6 6 2.7 2.7 C ox (ff/µm 2 )