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NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 1 Topic 5 Bipolar Junction Transistors ECE 271 Electronic Circuits I.

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Presentation on theme: "NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 1 Topic 5 Bipolar Junction Transistors ECE 271 Electronic Circuits I."— Presentation transcript:

1 NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 1 Topic 5 Bipolar Junction Transistors ECE 271 Electronic Circuits I

2 NJIT ECE271 Dr. Serhiy Levkov Chapter Goals Explore physical structure of bipolar transistor Understand bipolar transistor action and importance of carrier transport across base region Study terminal characteristics of BJT. Explore differences between npn and pnp transistors. Define four operation regions of BJT. Explore simplified models for each operation region. Study Q-point Biasing of BJT. Chap 5 - 2

3 NJIT ECE271 Dr. Serhiy Levkov Bipolar Transistor: Physical Structure Consists of 3 alternating layers of n- and p- type semiconductor called emitter (E), base (B) and collector (C). Chap 5 - 3

4 NJIT ECE271 Dr. Serhiy Levkov Bipolar Transistor: Physical Structure Consists of 3 alternating layers of n- and p- type semiconductor called emitter (E), base (B) and collector (C). Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter. Chap 5 - 4

5 NJIT ECE271 Dr. Serhiy Levkov Bipolar Transistor: Physical Structure Consists of 3 alternating layers of n- and p- type semiconductor called emitter (E), base (B) and collector (C). Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter. Carrier transport in the active base region directly beneath the heavily doped (n + ) emitter dominates i-v characteristics of BJT. Chap 5 - 5

6 NJIT ECE271 Dr. Serhiy Levkov npn Transistor and pn-junctions Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor Chap 5 - 6

7 NJIT ECE271 Dr. Serhiy Levkov npn Transistor and pn-junctions Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor They are said to be positive when they forward-bias their respective pn junctions. Chap 5 - 7

8 NJIT ECE271 Dr. Serhiy Levkov npn Transistor and pn-junctions Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor They are said to be positive when they forward-bias their respective pn junctions. The terminal currents are collector current(i C ), base current (i B ) and emitter current (i E ). Chap 5 - 8

9 NJIT ECE271 Dr. Serhiy Levkov npn Transistor and pn-junctions Base-emitter voltage v BE and base-collector voltage v BC determine currents in transistor They are said to be positive when they forward-bias their respective pn junctions. The terminal currents are collector current(i C ), base current (i B ) and emitter current (i E ). Primary difference between BJT and FET is that i B is significant while i G = 0. Chap 5 - 9

10 NJIT ECE271 Dr. Serhiy Levkov npn Transistor: How it Works ( common emitter ) Left pn junction is forward biased – open. Right pn junction is reverse biased – closed. If those would be regular diodes – no current would exist btw emitter and collector. But the width of the base is very narrow, two back-to-back pn junctions are tightly coupled. Electrons injected from emitter into base region rush through it and are removed by collector, creating collector current I C. Some of the electrons will travel to the base, creating base current I B. Base current is usually quite smaller: where  is the common-emitter current gain usually is in the range 50 to 200. Thus transistor works as a current amplifier: Chap 5 - 10 Simulation: http://learnabout-electronics.org/bipolar_junction_transistors_05.phphttp://learnabout-electronics.org/bipolar_junction_transistors_05.php Look for relationship btw i B and i C.

11 NJIT ECE271 Dr. Serhiy Levkov npn Transistor: How it Works ( common base ) Left pn junction is forward biased – open. Right pn junction is reverse biased – closed. Similarly, since the width of the base is very narrow, electrons injected from emitter into the base region rush through it and are removed by collector, creating collector current I C. Some of the electrons will travel to the base, creating base current I B. Base current is usually quite small. Considering transistor as a super node: where is common-base current gain. Chap 5 - 11 Look for relationship btw i E and i C.

12 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) Chap 5 - 12

13 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) 2)when BC is forward biased and BE is zero biased (reverse characteristics). Chap 5 - 13

14 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics Forward transport current is Where I S is saturation current V T = kT/q =0.025 V at room temperature Chap 5 - 14 BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) 2)when BC is forward biased and BE is zero biased (reverse characteristics).

15 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics Forward transport current is Where I S is saturation current V T = kT/q =0.025 V at room temperature is forward common-emitter current gain Chap 5 - 15 Base current: BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) 2)when BC is forward biased and BE is zero biased (reverse characteristics).

16 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics Forward transport current is Where I S is saturation current V T = kT/q =0.025 V at room temperature Emitter current is given by is forward common-emitter current gain is forward common- base current gain Chap 5 - 16 Base current: BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) 2)when BC is forward biased and BE is zero biased (reverse characteristics).

17 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Forward Characteristics Forward transport current is Where I S is saturation current V T = kT/q =0.025 V at room temperature Emitter current is given by is forward common-emitter current gain is forward common- base current gain In the forward active operation region: Chap 5 - 17 Base current: BJT is almost symmetrical, except that usually emitter is more heavily doped then collector. Thus we consider two models: 1)when BE is forward biased and BC is zero biased (forward characteristics) 2)when BC is forward biased and BE is zero biased (reverse characteristics).

18 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Reverse Characteristics Reverse transport current is Chap 5 - 18

19 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Reverse Characteristics Reverse transport current is Base current is given by Chap 5 - 19 is reverse common-emitter current gain

20 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Reverse Characteristics Reverse transport current is is reverse common-emitter current gain Base current is given by Base currents in forward and reverse modes are different due to asymmetric doping levels in emitter and collector regions. Chap 5 - 20

21 NJIT ECE271 Dr. Serhiy Levkov npn Transistor Model: Reverse Characteristics Reverse transport current is Emitter current is given by is reverse common-emitter current gain is reverse common- base current gain Base current is given by Base currents in forward and reverse modes are different due to asymmetric doping levels in emitter and collector regions. Chap 5 - 21 In the reverse active operation region:

22 NJIT ECE271 Dr. Serhiy Levkov pnp Transistor: Structure Voltages v EB and v CB are positive when they forward bias their respective pn junctions. Collector current and base current exit transistor terminals and emitter current enters the device. Chap 5 - 22

23 NJIT ECE271 Dr. Serhiy Levkov pnp Transistor: Forward Characteristics Forward transport current is Base current is given by Emitter current is given by Chap 5 - 23

24 NJIT ECE271 Dr. Serhiy Levkov pnp Transistor: Reverse Characteristics Reverse transport current is Base current is given by Emitter current is given by Chap 5 - 24

25 NJIT ECE271 Dr. Serhiy Levkov Operation Regions of Bipolar Transistors Chap 5 - 25 Binary Logic States

26 NJIT ECE271 Dr. Serhiy Levkov i-v Characteristics of BJT (Recall MOSFET) Chap 5 - 26

27 NJIT ECE271 Dr. Serhiy Levkov i-v Characteristics of BJT (npn): Common-Emitter Output Characteristics For i B = 0, transistor is cutoff. When i B > 0, and increases, i C also increases. For v CE > v BE, npn transistor is in forward-active region, i C =  F i B is independent of v CE. For v CE < v BE, transistor is in saturation (the voltage btw collector and emitter is small, base collector diode conducts). For v CE < 0, roles of collector and emitter reverse. Chap 5 - 27 npn For pnp, i C vs. v EC Circuit to measure output characteristic:

28 NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 28 Circuit to measure output characteristic: i-v Characteristics of BJT (pnp): Common-Emitter Output Characteristics

29 NJIT ECE271 Dr. Serhiy Levkov i-v Characteristics of BJT (npn): Common-Emitter Transfer Characteristic Defines relation between collector current and base-emitter voltage of transistor. Almost identical to transfer characteristic of pn junction diode Setting v BC = 0 in the collector-current expression yields Chap 5 - 29 Collector current expression has the same form as that of the diode equation

30 NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 30 Simplified Cutoff Region Model The full BJT model is the so called Gummel-Poon transport model, which is relatively complicated. For our purpose, it will be enough to use the simplified model.

31 NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 31 Simplified Cutoff Region Model The full BJT model is the so called Gummel-Poon transport model, which is relatively complicated. For our purpose, it will be enough to use the simplified model. In cutoff region both junctions are reverse-biased, transistor is off state: v BE < 0, v BC < 0 If we assume that, where V T = kT/q =0.025 and -4kT/q = -0.1 V, then, and the transport model terminal current equations simplifies:

32 NJIT ECE271 Dr. Serhiy Levkov Chap 5 - 32 Simplified Cutoff Region Model As will be shown in the example (next slide) those currents are so small that for practical purposes, they are essentially zero. Thus, equivalent circuit: The full BJT model is the so called Gummel-Poon transport model, which is relatively complicated. For our purpose, it will be enough to use the simplified model. In cutoff region both junctions are reverse-biased, transistor is off state: v BE < 0, v BC < 0 If we assume that, where V T = kT/q =0.025 and -4kT/q = -0.1 V, then, and the transport model terminal current equations simplifies:

33 NJIT ECE271 Dr. Serhiy Levkov Simplified Cutoff Region Model (Example) Problem: Estimate terminal currents using simplified transport model Given data: I S = 10 -16 A,  F = 0.95,  R = 0.25, V BE = 0 V, V BC = -5 V Assumptions: Simplified transport model assumptions Analysis: From given voltages, we know that transistor is in cutoff. Chap 5 - 33 For practical purposes, all three currents are essentially zero.

34 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region: Model In forward-active region, emitter-base junction is forward-biased and collector- base junction is reverse-biased: v BE > 0, v BC < 0 The simplified transport model terminal current equations: Conclusion. All currents are independent of the base-collector voltage v BC. The collector current can be modeled as a current source that is controlled by the base-emitter voltage. Chap 5 - 34 and

35 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region: Circuit Current in base-emitter diode is amplified by common-emitter current gain  F and appears at collector; base and collector currents are exponentially related to base-emitter voltage. Chap 5 - 35 NLCVD

36 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region: Circuit Current in base-emitter diode is amplified by common-emitter current gain  F and appears at collector; base and collector currents are exponentially related to base-emitter voltage. For simplicity, base-emitter diode can be replaced by constant voltage drop model (V BE = 0.7 V) since it is forward-biased in forward-active region. Chap 5 - 36 NLCVD

37 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region: Circuit Current in base-emitter diode is amplified by common-emitter current gain  F and appears at collector; base and collector currents are exponentially related to base-emitter voltage. For simplicity, base-emitter diode can be replaced by constant voltage drop model (V BE = 0.7 V) since it is forward-biased in forward-active region. Like with the diode, using NL model circuit, requires solving nonlinear diode equation in combination with other equations for the circuit in order to find v BE, i B and i C. Chap 5 - 37 NLCVD or

38 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region: Circuit Current in base-emitter diode is amplified by common-emitter current gain  F and appears at collector; base and collector currents are exponentially related to base-emitter voltage. For simplicity, base-emitter diode can be replaced by constant voltage drop model (V BE = 0.7 V) since it is forward-biased in forward-active region. Like with the diode, using NL model circuit, requires solving nonlinear diode equation in combination with other equations for the circuit in order to find v BE, i B, i C, and i E. When using CVD model, v BE is postulated as 0.7V, and i B, i C, and i E are found in combination with other equations for the circuit. Chap 5 - 38 NLCVD

39 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 1) Problem: Estimate terminal currents and base-emitter voltage Given data: I S =10 -16 A,  F = 0.95, V BC = V B - V C = -5 V, I E = 100  A Assumptions: Simplified transport model assumptions, room temperature operation, V T = 25.0 mV Chap 5 - 39 Do example on the board

40 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 1) Problem: Estimate terminal currents and base-emitter voltage Given data: I S =10 -16 A,  F = 0.95, V BC = V B - V C = -5 V, I E = 100  A Assumptions: Simplified transport model assumptions, room temperature operation, V T = 25.0 mV Analysis: Current source forward-biases base-emitter diode, V BE > 0, V BC < 0, we know that transistor is in forward-active operation region. Chap 5 - 40

41 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 2) Problem: Estimate terminal currents, base-emitter and base-collector voltages. Given data: I S = 10 -16 A,  F = 0.95, V C = +5 V, I B = 100  A Assumptions: Simplified transport model assumptions, room temperature operation, V T = 25.0 mV Chap 5 - 41 Do example on the board

42 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 2) Problem: Estimate terminal currents, base-emitter and base-collector voltages. Given data: I S = 10 -16 A,  F = 0.95, V C = +5 V, I B = 100  A Assumptions: Simplified transport model assumptions, room temperature operation, V T = 25.0 mV Analysis: Current source causes base current to forward-bias base- emitter diode, V BE > 0, V BC <0, we know that transistor is in forward-active operation region. Chap 5 - 42

43 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 3) Problem: Find Q-point Given data:  F = 50,  R = 1 V BC = V B - V C = -9 V Assumptions: Forward-active region of operation, V BE = 0.7 V Chap 5 - 43 Do example on the board

44 NJIT ECE271 Dr. Serhiy Levkov Simplified Forward-Active Region Model (Example 3) Problem: Find Q-point Given data:  F = 50,  R = 1 V BC = V B - V C = -9 V Assumptions: Forward-active region of operation, V BE = 0.7 V Analysis: Chap 5 - 44

45 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Model In reverse-active region, base-collector junction is forward-biased and base- emitter junction is reverse-biased: v BE 0 The simplified transport model terminal current equations are: Conclusion. All currents are independent of the base-collector voltage v BE. The emitter current can be modeled as a current source that is controlled by the base-collector voltage. Chap 5 - 45 and

46 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Circuit Current in base-collector diode is amplified by the gain  R and appears at collector; base and collector currents are exponentially related to base- collector voltage. Chap 5 - 46 NL CVD or

47 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Circuit Current in base-collector diode is amplified by the gain  R and appears at collector; base and collector currents are exponentially related to base- collector voltage. Base-collector diode can be replaced by constant voltage drop model (V BC = 0.7 V) since it is forward-biased in reverse-active region. Chap 5 - 47 NL CVD or

48 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Circuit Current in base-collector diode is amplified by the gain  R and appears at collector; base and collector currents are exponentially related to base- collector voltage. Base-collector diode can be replaced by constant voltage drop model (V BC = 0.7 V) since it is forward-biased in reverse-active region. Like with the diode, using NL model circuit, requires solving nonlinear diode equation in combination with other equations for the circuit in order to find v BC, i B, i C and i E. Chap 5 - 48 NL CVD or

49 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Circuit Current in base-collector diode is amplified by the gain  R and appears at collector; base and collector currents are exponentially related to base- collector voltage. Base-collector diode can be replaced by constant voltage drop model (V BC = 0.7 V) since it is forward-biased in reverse-active region. Like with the diode, using NL model circuit, requires solving nonlinear diode equation in combination with other equations for the circuit in order to find v BC, i B, i C and i E. When using CVD model, v BC is postulated as 0.7V, and i B, i C and i E are found in combination with other equations for the circuit. Chap 5 - 49 NL CVD or

50 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Example Problem: Find Q-point Given data:  F = 50,  R = 1 V BE = V B - V E = -9 V. Combination of R and the voltage source forward biases base-collector junction. Chap 5 - 50 Do example on the board

51 NJIT ECE271 Dr. Serhiy Levkov Simplified Reverse-Active Region: Example Problem: Find Q-point Given data:  F = 50,  R = 1 V BE = V B - V E = -9 V. Combination of R and the voltage source forward biases base-collector junction. Assumptions: Reverse-active region of operation, V BC = 0.7 V Analysis: Chap 5 - 51

52 NJIT ECE271 Dr. Serhiy Levkov Simplified Saturation Region Model In saturation region, both junctions are forward-biased, and the transistor operates with a relatively large current and a small voltage between collector and emitter. This is v CESAT - the saturation voltage for the npn BJT. No simplified expressions exist for terminal currents other than i C + i B = i E. They are determined by external circuit elements. Chap 5 - 52 Simplified Model

53 NJIT ECE271 Dr. Serhiy Levkov Nonideal BJT Behavior Junction Breakdown Voltages. If reverse voltage across either of the two pn junctions in the transistor is too large, the corresponding diode will break down. Minority Carrier Transport effects in the Base Region. Base transit time ( associated with storing minority-carrier charge Q required to establish career gradient in base region ) places upper limit on useful operating frequency of transistor. Diffusion Capacitance: for v BE and hence i C to change, charge stored in base region must also change.  -Cutoff Frequency, Transconductance and Transit Time - forward- biased diffusion and reverse-biased pn junction capacitances of BJT cause current gain to be frequency-dependent. Early Effect and Early Voltage - in a practical BJT, output characteristics have a positive slope in forward-active region; collector current is not independent of v CE. Chap 5 - 53

54 NJIT ECE271 Dr. Serhiy Levkov Biasing for BJT Digital logic circuits and linear amplifiers use very different operating points of transistors. Chap 5 - 54 This circuit can be either a logic inverter or a linear amplifier depending on a choice of Q- point

55 NJIT ECE271 Dr. Serhiy Levkov Biasing for BJT Goal of biasing is to establish known Q-point which in turn establishes initial operating region of the transistor. For a BJT, the Q-point is represented by (I C, V CE ) for an npn transistor or (I C, V EC ) for a pnp transistor. In general, during circuit analysis, we use simplified mathematical relationships derived for a specified operation region, and the Early voltage is assumed to be infinite. Two practical biasing circuits used for a BJT are: –Four-Resistor Bias network –Two-Resistor Bias network The constant V BE is not practical because of very steep iv curve and strong dependence on temperature. Much better circuits are those of 4-resistor and 2-resistor biasing. Chap 5 - 55

56 NJIT ECE271 Dr. Serhiy Levkov Two-Resistor Bias Network for BJT: Problem: Find Q-point for pnp transistor in 2-resistor bias circuit with given parameters. Given data:  F = 50, V CC = 9 V Assumptions: Forward-active operation region, V EB = 0.7 V Chap 5 - 56 Do example on the board

57 NJIT ECE271 Dr. Serhiy Levkov Two-Resistor Bias Network for BJT: Problem: Find Q-point for pnp transistor in 2-resistor bias circuit with given parameters. Given data:  F = 50, V CC = 9 V Assumptions: Forward-active operation region, V EB = 0.7 V Analysis: Forward-active region operation is correct Q-point is : (6.01 mA, 2.88 V) Chap 5 - 57

58 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT Chap 5 - 58 One of the best circuits for stabilizing the Q-point. R 1 and R 2 - a voltage divider used to establish a fixed voltage at the base. R E and R C a- define the emitter current and CE voltage. Do example on the board

59 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT Chap 5 - 59 One of the best circuits for stabilizing the Q-point. R 1 and R 2 - a voltage divider used to establish a fixed voltage at the base. R E and R C a- define the emitter current and CE voltage. Transform the left (input) part of the circuit with using Thevenin equivalent.

60 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT Chap 5 - 60 Left loop:

61 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT Chap 5 - 61 Left loop: Right loop:

62 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT F. A. region correct: Q-point is (201  A, 4.32 V) Chap 5 - 62 Left loop: Right loop: All calculated currents are > 0 V BC = V BE - V CE = 0.7 - 4.32 = - 3.62 V Hence, base-collector junction is reverse- biased, and assumption of forward-active region operation is correct.

63 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT (load line analysis) Load-line for the circuit is: The two points needed to plot the load line are (0, 12 V) and (314  A, 0). Resulting load line is plotted on common-emitter output characteristics. I B = 2.7  A, intersection of corresponding characteristic with load line gives Q-point. Chap 5 - 63

64 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT: Design Objectives The input loop : Then: Also: Thus design objectives: The value of R EQ is usually designed small, to neglect the voltage drop in it. Then I C, I E, are set by V EQ, V BE, R E Also, V EQ is designed to be large enough that small variations in the assumed value of V BE won’t affect I E. This implies that I B << I R2, so that I R1 = I R2. So base current doesn’t disturb voltage divider action. This implies that I B << I E, so that Q-point is independent of base current. Chap 5 - 64

65 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT: Design Objectives From the good engineering approximation for the 4 resistor biasing: From the input loop: The value of R EQ is usually designed small, to neglect the voltage drop in it. Then I C, I E, are set by V EQ, V BE, R E Also, V EQ is designed to be large enough that small variations in the assumed value of V BE won’t affect I E. This implies that I B << I R2, so that I R1 = I R2. So base current doesn’t disturb voltage divider action. This implies that I B << I E, so that Q-point is independent of base current. Chap 5 - 65

66 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT: Design Guidelines Choose Thévenin equivalent base voltage Select R 1 to set I 1 = 9I B. Select R 2 to set I 2 = 10I B. R E is determined by V EQ and desired I C. R C is determined by desired V CE. Chap 5 - 66

67 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT: Design Example Problem: Design 4-resistor bias circuit with given parameters. Given data: I C = 750  A,  F = 100, V CC = 15 V, V CE = 5 V Assumptions: Forward-active operation region, V BE = 0.7 V Analysis: Divide (V CC - V CE ) equally between R E and R C. Thus, V E = 5 V and V C = 10 V Chap 5 - 67

68 NJIT ECE271 Dr. Serhiy Levkov Four-Resistor Bias Network for BJT Saturation region Chap 5 - 68 Consider the first example, where R C is replaced with 56 kOhm Do example on the board

69 NJIT ECE271 Dr. Serhiy Levkov Tolerances - Worst-Case Analysis: Example (1) Typically, tolerance of discrete transistors is 10%. 5%, 1%. In IC – 30%. Power supply – (5-10)%. Current gain even more. Thus: the problem of tolerance analysis. Problem: Find worst-case values of I C and V CE. Given data:  FO = 75 with 50% tolerance, V A = 50 V, 5 % tolerance on V CC, 10% tolerance for each resistor. Analysis: To max I C, V EQ should be maximized, R E should be minimized and opposite for minimizing I C. Extremes of R E are: 14.4 k  and 17.6 k  To maximize V EQ, V CC and R 1 should be maximized, R 2 should be minimized and opposite for minimizing V EQ. Chap 5 - 69

70 NJIT ECE271 Dr. Serhiy Levkov Extremes of V EQ are: 4.78 V and 3.31 V  Using these values, extremes for I C are: 283  A and 148  A. To maximize V CE, I C and R C should be minimized, and opposite for minimizing V EQ. Extremes of V CE are: 7.06 V and 0.471 V The min is actually a saturated region, hence calculated values for V CE and I C actually not correct  Chap 5 - 70 Tolerances - Worst-Case Analysis: Example (2)

71 NJIT ECE271 Dr. Serhiy Levkov Tolerances - Monte Carlo Analysis In real circuits, it is unlikely that various components will reach their extremes at the same time, instead they will have some statistical distribution. Hence worst-case analysis over-estimates extremes of circuit behavior. In Monte Carlo analysis, values of each circuit parameter are randomly selected from possible distributions of parameters and used to analyze the circuit. Random parameter sets are generated, and the statistical behavior of circuit is built up from the analysis of many test cases. Chap 5 - 71

72 NJIT ECE271 Dr. Serhiy Levkov Tolerances - Monte Carlo Analysis: Example Full results of Monte Carlo analysis of 500 cases of the 4-resistor bias circuit yields mean values of 207  A and 4.06 V for I C and V CE respectively which are close to values originally estimated from nominal circuit elements. Standard deviations are 19.6  A and 0.64 V respectively. The worst-case calculations lie well beyond the extremes of the distributions Chap 5 - 72

73 NJIT ECE271 Dr. Serhiy Levkov BJT SPICE Model Besides capacitances associated with the physical structure, additional components are: diode current i S and substrate capacitance C JS related to the large area pn junction that isolates the collector from the substrate and one transistor from the next. R B is resistance between external base contact and intrinsic base region. Collector current must pass through R C on its way to active region of collector- base junction. R E models any extrinsic emitter resistance in device. Chap 5 - 73

74 NJIT ECE271 Dr. Serhiy Levkov BJT SPICE Model Typical Values Saturation Current IS = 3x10 -17 A Forward current gain BF = 100 Reverse current gain BR = 0.5 Forward Early voltage VAF = 75 V Base resistance RB = 250  Collector Resistance RC = 50  Emitter Resistance RE = 1  Forward transit time TT = 0.15 ns Reverse transit time TR = 15 ns Chap 5 - 74


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