Lecture 4. Reading: Chapter EE105 Fall 2007 Lecture 4, Slide 1 Prof. Liu, UC Berkeley

Transcription

1 Lecture 4 OUTLNE Bipolar Junction Transistor (BJT) General considerations Structure Operation in active mode Large-signal model and - characteristics Reading: Chapter EE105 Fall 2007 Lecture 4, Slide 1 Prof. Liu, UC Berkeley

2 oltage-dependent Current Source A voltage-dependent current source can act as an amplifier. f KR L is greater than 1, then the signal is amplified. A out in = KR L EE105 Fall 2007 Lecture 4, Slide 2 Prof. Liu, UC Berkeley

3 oltage-dependent Current Source with nput Resistance The magnitude of amplification is independent of the input resistance r in. EE105 Fall 2007 Lecture 4, Slide 3 Prof. Liu, UC Berkeley

4 Exponential oltage-dependent Current Source deally, a bipolar junction transistor (BJT) can be modeled as a three-terminal exponential voltagedependent current source: EE105 Fall 2007 Lecture 4, Slide 4 Prof. Liu, UC Berkeley

5 Reverse-Biased PN Junction as a Current Source PN junction diode current is ~independent of the reverse-bias voltage. t depends only on the rate at which minority carriers are introduced into the depletion region. We can increase the reverse current by injecting minority carriers near to the depletion region. EE105 Fall 2007 Lecture 4, Slide 5 Prof. Liu, UC Berkeley

6 BJT Structure and Circuit Symbol A bipolar junction transistor consists of 2 PN junctions that form a sandwich of three doped semiconductor regions. The outer two regions are doped the same type; the middle region is doped the opposite type. EE105 Fall 2007 Lecture 4, Slide 6 Prof. Liu, UC Berkeley

7 NPN BJT Operation (Qualitative) n the forward active mode of operation: The collector junction is reverse biased. The emitter junction is forward biased. current gain: β C B EE105 Fall 2007 Lecture 4, Slide 7 Prof. Liu, UC Berkeley

8 Base Current The base current consists of two components: 1) njection of holes into the emitter, and 2) Recombination of holes with electrons injected from the emitter. = β C B EE105 Fall 2007 Lecture 4, Slide 8 Prof. Liu, UC Berkeley

9 BJT Design mportant features of a well-designed BJT (large β ): njected minority carriers do not recombine in the quasi-neutral base region. Emitter current is comprised almost entirely of carriers injected into the base (rather than carriers injected into the emitter). EE105 Fall 2007 Lecture 4, Slide 9 Prof. Liu, UC Berkeley

10 Carrier Transport in the Base Region Since the width of the quasi-neutral base region (W B = x 2 -x 1 ) is much smaller than the minority-carrier diffusion length, very few of the carriers injected (from the emitter) into the base recombine before they reach the collector-junction depletion region. Minority-carrier diffusion current is ~constant in the quasi-neutral base The minority-carrier concentration at the edges of the collectorjunction depletion region are ~0. EE105 Fall 2007 Lecture 4, Slide 10 Prof. Liu, UC Berkeley

11 Diffusion Example Redux Linear concentration profile constant diffusion current x p = N 1 L Non-linear concentration profile varying diffusion current x p = N exp L d J p, diff dp = qdp dx N = qdp L J p, diff dp = qdp dx qdpn = exp L d x L d EE105 Fall 2007 Lecture 4, Slide 11 Prof. Liu, UC Berkeley

12 Collector Current C = A E qd N W B n n B 2 i exp BE T 1 C S BE exp where S = T A E qd N W B n n B 2 i The equation above shows that the BJT is indeed a voltage-dependent current source; thus it can be used as an amplifier. EE105 Fall 2007 Lecture 4, Slide 12 Prof. Liu, UC Berkeley

13 Emitter Current Applying Kirchhoff s Current Law to the BJT, we can easily find the emitter current. E = 1 + = 1 C B C + β EE105 Fall 2007 Lecture 4, Slide 13 Prof. Liu, UC Berkeley

14 Summary of BJT Currents exp 1 exp = = β BE S B T BE S C EE105 Fall 2007 Lecture 4, Slide 14 Prof. Liu, UC Berkeley 1 exp = β β α β β β T BE S E T S B

15 Parallel Combination of Transistors When two transistors are connected in parallel and have the same terminal voltages, they can be considered as a single transistor with twice the emitter area. EE105 Fall 2007 Lecture 4, Slide 15 Prof. Liu, UC Berkeley

16 Simple BJT Amplifier Configuration Although the BJT converts an input voltage signal to an output current signal, an (amplified) output voltage signal can be obtained by connecting a load resistor (with resistance R L ) at the output and allowing the controlled current to pass through it. EE105 Fall 2007 Lecture 4, Slide 16 Prof. Liu, UC Berkeley

17 BJT as a Constant Current Source deally, the collector current does not depend on the collector-to-emitter voltage. This property allows the BJT to behave as a constant current source when its base-to-emitter voltage is fixed. EE105 Fall 2007 Lecture 4, Slide 17 Prof. Liu, UC Berkeley

18 Constraint on Load Resistance f R L is too large, then X can drop to below ~0.8 so that the collector junction is forward biased. n this case, the BJT is no longer operating in the active mode, and so < β C B There exists a maximum tolerable load resistance. EE105 Fall 2007 Lecture 4, Slide 18 Prof. Liu, UC Berkeley

19 BJT -Characteristics EE105 Fall 2007 Lecture 4, Slide 19 Prof. Liu, UC Berkeley

20 Example EE105 Fall 2007 Lecture 4, Slide 20 Prof. Liu, UC Berkeley

21 BJT Large Signal Model A diode is placed between the base and emitter terminals, and a voltage-controlled current source is placed between the collector and emitter terminals. EE105 Fall 2007 Lecture 4, Slide 21 Prof. Liu, UC Berkeley

22 BJT vs.back-to-back Diodes Figure (b) presents a wrong way of modeling the BJT. EE105 Fall 2007 Lecture 4, Slide 22 Prof. Liu, UC Berkeley