ESE 319 MT Review

Transcription

1 ESE 319 MT Review 1)--> Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)Design of PNP transistor circuits. 5)Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)CC & CB Amplifier Configurations 1

2 2

3 Physical Operation of Forward Active NPN BJT i E =i C i B Forward Active => i C = F i E i C = F i B 3

4 Saturation forwardactive 4

5 ESE 319 MT Review cont. 1)Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)--> Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)Design of PNP transistor circuits. 5)Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)CC & CB Amplifier Configurations 5

6 i C i B R F i E i DE i DC I SC I SE 6

7 Ebers-Moll Mode in Forward-Active Mode. 1 i C =I S e v B E V T I S 1 R 1 i E = I v BE S V e T I S 1 1 F F V BE > 0 V BC < 0 i B = 1 F F I S e v BE V T 1 1 R R I S = 1 F e v BE V T I S 1 F 1 R 7

8 Conclusions Ebers-Moll model is accurate large signal model. Used for all operation modes. includes bias + ac. Too complex for pencil-and-paper computations for most applications, e.g. nonlinear. Has been used in computer simulation, e.g. SPICE. 8

9 ESE 319 MT Review cont. 1)Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)--> Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)Design of PNP transistor circuits. 5)Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)CC & CB Amplifier Configurations 9

10 Small Signal Analysis Origin of r π, r e & r o I C VCCS 10

11 Origin of r π, r e & r o cont. r e r e = r 1 Note: i e = 1 i b = 1 v be r = v be r e 11

12 Origin of r π, r e & r o cont. 12

13 Conclusions Resistors g m, r π, r e, r o are equivalent circuit model parameters, not parasitic resistances. Resistor r x, the ohmic base resistance, is a parasitic resistance. v BE V I S e term T from Ebers-Moll model linearized by limiting the ac signal amplitude. 13

14 ESE 319 MT Review cont. 1)Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)--> Design of PNP transistor circuits. 5)Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)CC & CB Amplifier Configurations 14

15 Amplifier Design with PNP Transistors Bipolar process are optimized for NPN performance: a. PNP have lower β than NPN transistors. b. PNP have lower f T than NPN transistors. Example: Intersil CA3096 NPN & PNP array (Data Sheet) NPN: h fe = β = 390, f T = 280 Mhz (I C = 1mA) PNP: h fe = β = 47, f T < 10 Mhz (I C = 1mA) c. NPN transistors are typically used where high gain and f T are needed, e.g. amplifier transistor d. PNP transistors are used where gain and f T are not as needed, e.g. current sources and active loads. e. When NPNs and PNPs are used together, two power supplies are needed. 15

16 NPN vs. PNP CE Design I C I C NPN PNP I I B B IE I E v O Forward Active: V BE 0V V BC 0V Forward Active: V EB 0V V CB 0V 16

17 NPN and PNP Small Signal Models v O v O NPN: dc and ac currents in same direction Small signal model is identical for NPN and PNP transistors! PNP: dc and ac currents in opposite direction due to vsig polarity 17

18 NPN vs. PNP CE Design cont. I 1 I Let C I 1 Let =100 =100 v O I C v O I B I E I B I E V B = R 2 R 2 R 1 V CC R B =R 1 R 2 V B = R 2 R 2 R 1 V CC 0 R B =R 1 R 2 I C = V CC V C R C I C = I B = V V B BE R B 1 R E V E =V B V BE =I E R E I C = V C V CC = V C V CC => V C < 0 R C R C I C = I B = V B V EB R B 1 R E V E =V EB V B =I E R E 0 18

19 Base DC Thevenin Equivalent Circuit I B I C v O => I E V B =I B R B V EB I E R E = R B 1 R E I B V EB V B V EB I B = R B 1 R E I C = I B = V B V EB R B 1 R E 19

20 NPN vs. PNP CE Design Conclusions DC Bias design same for both NPN & PNP. Be careful! PNP DC I & V are inverse of NPN DC I & V. PNP DC supply voltage V CC polarity is inverse of NPN. AC design the same for both NPN & PNP. Small signal models are identical. Be careful! PNP ac current directions are inverse of DC current directions. 20

21 ESE 319 MT Review cont. 1)Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)Design of PNP transistor circuits. 5)--> Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)CC & CB Amplifier Configurations 21

22 Rs 50 Ohm AC Small-Signal CE Voltage Gain b c R S e v c = v o R S v o = R C i c = R S R C v R S r 1 R s E If 1 R S v o R C 1 R E R S A v = v o R C R E 22

23 I REF I REF 23

24 Conclusions Approximations are encouraged when conditions are satisfied, i.e. If R E = 0: v o = R C i c = R C R S r 1 R E => A v = v o R C R E If r π >> R S => A V = v o r R C = g m R C 24

25 ESE 319 MT Review cont. 1)Physical operation of a BJT (layout, why currents are related, npn vs. pnp). 2)Cover the Eber's Mole Model for forward and reverse active configurations. (large signal model) 3)Cover the various resistances in the small signal model e.g. r(e), r(pi), r(o) etc. 4)Design of PNP transistor circuits. 5)Consider the gain formulae, should we use the -Rc/Re approximation for a CE amplifier, or should we work with the small signal model and consider all the other resistances in parallel. (approximations) 6)--> CC & CB Amplifier Configurations 25

26 CC Amplifier R in R out R in R out 26

27 CB Amplifier v O v O R in R out R in R out 27

28 Conclusions low (r e R E ) low (r e ) high (R C r o ) **All parameters for ac small signal. 28