Electronics Fundamentals BIPOLAR TRANSISTORS. Construction, circuit symbols and biasing examples for NPN and PNP junction transistors.

Transcription

1 IPOLA TANSISTOS onstruction, circuit symbols and biasing examples for NPN and PNP junction transistors Slide 1

2 xternal bias voltages create an electric field, which pulls electrons (emitted into the base by the emitter) across the base and into the collector. This results, seemingly paradoxically, in a large flow of electrons through the base-collector junction (reverse-biased). This current is easily controlled by small changes in base voltage. Slide 2

3 Transistors as back-to-back diodes Notations: V, V, V voltage at a terminal (relative to ground); V, V, V voltage between two terminals V, V, V power-supply voltage associated with terminal Slide 3

4 ipolar transistor basic definitions V V V - potential of base relative to emitter V V V - potential of collector relative to base V V V - potential of collector relative to emitter From Kirchhoff s voltage law: V V + V For NPN transistors (in normal mode) all these potential differences are positive I current flowing into collector I current flowing out of emitter I current flowing into base From Kirchhoff s current law: I I + I For NPN transistors (in normal mode) all these currents are positive Slide 4

5 ipolar transistor: bers-moll model ollector current: I I β ase-emitter diode equation (forward-bias): I I s ( ev / kt e 1) Dynamic resistance of emitter: r e V I r e kt e 1 I Slide 5

6 ipolar transistor: bers-moll model Dynamic resistance of base: r V I kt β e 1 I Approximations (at room temperature e/kt 39 V -1 ): r e 25mV I r 25mV β I Slide 6

7 r e a few ohms r a few hundred ohms (β of order 100) The factor β applies also to any resistor that is in series with the emitter: ( r ) e + r β β + typically: tens to hundreds of kiloohms Slide 7

8 The simplest way to analyze transistor circuits If a transistor is on and conducting of miliampers of collector current, its base-emitter voltage difference V is approximately constant at about 700 mv: the emitter voltage follows the base Since β is large: the collector current nearly equals the emitter current Since the collector is a point of high impedance: the collector assumes any voltage required by Ohm s law as applied to the rest of the circuit Slide 8

9 Output characteristics curves for an NPN bipolar transistor (saturation region: V < V ) Slide 9

10 ase width modulation (arly s effect) U 1 U 1 <U 2 U 2 pn-junction depletion layer Slide 10

11 Transfer characteristics curves for an NPN bipolar transistor Slide 11

12 Transistor switch Slide 12

13 V I Z in V V ( β + 1) Similarly,, Z Z I load out 1 I β + 1 Z source β ( β + 1) V mitter follower (typical β ) npn follower cannot sink current, pnp cannot source Slide 13

14 Load (output) resistance V I V I 0,6V V e V 0,6V e For large β collector current is independent from the supply value Transistor current source ( (transistor saturation: V < V Slide 14

15 ommon-emitter amplifier Slide 15

16 ommon-emitter amplifier predictions of quiescent D voltages: V V2 1 + I ( V V ) / V out V /( 2) I How the circuit amplifiers (voltage gain)? V out I I expected drop I V / V / V V V / out in in / Slide 16

17 Driving loudspeaker with push-pull buffer with crossover distortions: if 0.7V < V in < 0.7V then I 1 I 2 0 Slide 17

18 Darlington pair urrent gain: V drop: β β 1 β 2 V V1 + V 2 Slide 18