Exercises 6.1, 6.2, 6.3 (page 315 on 7 th edition textbook)

Transcription

1 Exercises 6.1, 6.2, 6.3 (page 315 on 7 th edition textbook)

2 Recapitulation and Equivalent Circuit Models Previous slides present first order BJT model. Assumes npn transistor in active mode. Basic relationship is collector current (i C ) is related exponentially to forward bias voltage (v BE ). It remains independent of v CB as long as this junction remains reverse biased. v CB > 0

3 Figure 6.5: Large signal equivalent circuit models of the npn BJT operating in the forward active mode.

4 Example 6.1 (Pages on 7 th edition textbook.) An npn transistor having I S = A and β = 100 is connected as follows: The emitter is grounded, the base is fed with a constant-current source supplying a dc current of 10 μa, and the collector is connected to a 5-V dc supply via a resistance R C of 3 kω. Assuming that the transistor is operating in the active mode, find V BE and V CE. Use these values to verify active-mode operation. Replace the current source with a resistance connected from the base to the 5-V dc supply. What resistance value is needed to result in the same operating conditions?

5 Example 6.1. (Pages on 7 th edition textbook.) Figure 6.6 Circuits for Example 6.1.

6 Structure of Actual Transistors Figure 6.7 shows a more realistic BJT cross section. Collector virtually surrounds entire emitter region. This makes it difficult for electrons injected into base to escape collection. Device is not symmetrical. As such, emitter and collector cannot be interchanged. Device is uni directional.

7 Figure 6.7: Cross section of an npn BJT.

8 Operation in Saturation Mode For BJT to operate in active mode, CBJ must be reverse biased. However, for small values of forward bias, a pnjunction does not operate effectively. As such, active mode operation of npn transistor may be maintained for v CB down to approximately 0.4V. Only after this point will diode begin to really conduct.

9 Operation in Saturation Mode collector current (eq6.14) : in saturation region BC I S C vbe / VT vbc / VT ic ISe ISCe base current (eq6.15) : i in saturation region (eq6.16) forced : C forced ib saturation As v is increased, the value of is forced lower and lower. i B IS e v BE / V T I this terms plays bigger role as vbc exceeds 0. 4V SC e v BC / V T

10 Operation in Saturation Mode Two questions must be asked to determine whether BJT is in saturation mode, or not: Is the CBJ forward biased by more than 0.4V? Is the ratio i C /i B less than?

11 The pnp Transistor Figure 6.10: Current flow in a pnp transistor biased to operate in the active mode.

12 The pnp Transistor Figure 6.11: Two large signal models for the pnp transistor operating in the active mode.

13 Summary of Section 6.1 Device Structure and Physical Operation Important sub sections in Section Simplified Structure and Modes of Operation Operation of the npn Transistor in the Active Mode Structure of Actual Transistors Operation in the Saturation Mode The pnp Transistor Important equations in Section 6.1 (necessary to be remembered) Eqs. 6.3, 6.5, 6.6,

14 Summary of Section 6.1 Device Structure and Physical Operation 2 E n i IS I W N S v / V A ib e ni intrinsic carrier density NA doping concentration of base this expression is generated through combination of (6.5) and (6.7) 1 1 vbe / VT (eq6.8/6.9) ie ic ISe vbe / VT (eq6.3) ic ISe ic transistor parameter AE qdnnp0 saturation current: IS (eq6.10) ic ie ic W (eq6.5) ib this parameter is reffered to as common base current gain AqD n (eq6.4) (eq6.11), (eq6.13) (eq6.6) BE T 1 1 (eq6.12) i E IS e v BE / V T