Lecture 4. Reading: Chapter EE105 Spring 2008 Lecture 4, Slide 2 Prof. Wu, UC Berkeley. Structure and Symbol of Bipolar Transistor

Transcription

1 Lecture 4 OULNE Bipolar Junction ransistor (BJ) General considerations Structure Operation in active mode Large signal model and characteristics ransconductance Small signal model he Early effect Reading: hapter EE105 Spring 2008 Lecture 4, Slide 1 Prof. Wu, U Berkeley Structure and Symbol of Bipolar ransistor Bipolar transistor can be thought of as a sandwich of three doped Si regions. he outer two regions are doped with the same polarity, while the middle region is doped with opposite polarity. EE105 Spring 2008 Lecture 4, Slide 2 Prof. Wu, U Berkeley EE105 Fall

2 Forward Active Region Forward active region: BE > 0, B < 0. Figure b) presents a wrong way of modeling Figure a). EE105 Spring 2008 Lecture 4, Slide 3 Prof. Wu, U Berkeley Accurate Bipolar Representation ollector also carries current due to carrier injection from base. EE105 Spring 2008 Lecture 4, Slide 4 Prof. Wu, U Berkeley EE105 Fall

3 onstant urrent Source deally, the collector current does not depend on the collector to emitter voltage. his property allows the transistor to behave as a constant current source when its base emitter voltage is fixed. EE105 Spring 2008 Lecture 4, Slide 5 Prof. Wu, U Berkeley Base urrent β Base current consists of two components: Reverse injection of holes into the emitter and Recombination of holes with electrons coming from the emitter. EE105 Spring 2008 Lecture 4, Slide 6 Prof. Wu, U Berkeley B EE105 Fall

4 Emitter urrent + E E β B B β Applying Kirchoff s current law to the transistor, we can easily find the emitter current. EE105 Spring 2008 Lecture 4, Slide 7 Prof. Wu, U Berkeley Summary of urrents E B S 1 β β + 1 β BE exp S exp S β α β +1 BE exp BE EE105 Spring 2008 Lecture 4, Slide 8 Prof. Wu, U Berkeley EE105 Fall

5 Bipolar ransistor Large Signal Model A diode is placed between base and emitter and a voltage controlled current source is placed between the collector and emitter. EE105 Spring 2008 Lecture 4, Slide 9 Prof. Wu, U Berkeley Example: Maximum R L As R L increases, x drops and eventually forward biases the collector base junction. his will force the transistor out of forward active region. herefore, there exists a maximum tolerable collector resistance. EE105 Spring 2008 Lecture 4, Slide 10 Prof. Wu, U Berkeley EE105 Fall

6 haracteristics of Bipolar ransistor EE105 Spring 2008 Lecture 4, Slide 11 Prof. Wu, U Berkeley Example: haracteristics β μ A 0.25μ A 1.69μ A 0.25μ A EE105 Spring 2008 Lecture 4, Slide 12 Prof. Wu, U Berkeley EE105 Fall

7 ransconductance g m d BE S exp dbe 1 BE S exp ransconductance, g m shows a measure ofhow well the transistor converts voltage to current. t will later be shown that gm is one of the most important parameters in circuit design. g g m m EE105 Spring 2008 Lecture 4, Slide 13 Prof. Wu, U Berkeley isualization of ransconductance g m can be visualized as the slope of versus BE. A large has a large slope and therefore a large g m. EE105 Spring 2008 Lecture 4, Slide 14 Prof. Wu, U Berkeley EE105 Fall

8 Small Signal Model: Derivation Small signalmodel is derived by perturbing voltage difference every two terminals while fixing the third terminal and analyzing the change in current of all three terminals. We then represent these changes with controlled sources or resistors. EE105 Spring 2008 Lecture 4, Slide 15 Prof. Wu, U Berkeley Small Signal Model: BE hange EE105 Spring 2008 Lecture 4, Slide 16 Prof. Wu, U Berkeley EE105 Fall

9 Small Signal Model: E hange deally, E has no effect on the collector current. hus, it will not contribute to the small signal model. t can be shown that B has no effect on the small signal model, either. EE105 Spring 2008 Lecture 4, Slide 17 Prof. Wu, U Berkeley Small Signal Example g r π m Ω β 375 Ω g m Here, small signal parameters are calculated from D operating point and are used to calculate the change in collector current due to a change in BE. EE105 Spring 2008 Lecture 4, Slide 18 Prof. Wu, U Berkeley EE105 Fall

10 Small Signal Example n this example, a resistor is placed between the power supply and collector, therefore, providing an output voltage. EE105 Spring 2008 Lecture 4, Slide 19 Prof. Wu, U Berkeley A Ground Since the power supply voltage does not vary with time, it is regarded as a ground in small signal analysis. EE105 Spring 2008 Lecture 4, Slide 20 Prof. Wu, U Berkeley EE105 Fall

11 Early Effect he claim that collector current does not depend on E is not accurate. As E increases, the depletion region between base and collector increases. herefore, the effective base width decreases, which leads to an increase in the collector current. EE105 Spring 2008 Lecture 4, Slide 21 Prof. Wu, U Berkeley Early Effect llustration With Early effect, collector current becomes larger than usual and a function of E. EE105 Spring 2008 Lecture 4, Slide 22 Prof. Wu, U Berkeley EE105 Fall

12 Early Effect Representation EE105 Spring 2008 Lecture 4, Slide 23 Prof. Wu, U Berkeley Early Effect and Large Signal Model Early effect can be accounted for in large signal model by simply changing the collector current with a correction factor. n this mode, base current does not change. EE105 Spring 2008 Lecture 4, Slide 24 Prof. Wu, U Berkeley EE105 Fall

13 Early Effect and Small Signal Model r o Δ Δ E S A exp BE A EE105 Spring 2008 Lecture 4, Slide 25 Prof. Wu, U Berkeley Summary of deas EE105 Spring 2008 Lecture 4, Slide 26 Prof. Wu, U Berkeley EE105 Fall