Lecture (08) Bipolar Junction Transistor (2)

Transcription

1 Lecture (08) ipolar Junction Transistor (2) y: Dr. Ahmed lshafee 1 JT haracteristic ollector haracteristic urves 2

2 Applying fixed V, increasing V Saturation Assume that V is set to produce a certain value of I and V is zero both the junction and the junction are forwardbiased because the base is at approximately 0.7 V while the emitter and the collector are at 0 V. I is zero. Saturation is both junctions 3 are forward biased As V is increased, V increases as the collector current increases, because V remains less than 0.7 V due to the forward biased base collector junction. 4

3 Active when V exceeds 0.7 V, the base collector junction becomes reverse biased JT become active, or linear, region I levels off and remains essentially constant for a given value of I as V continues to increase 5 breakdown When V reaches a sufficiently high voltage, the reverse biased basecollector junction goes into breakdown; and the collector current increases rapidly as indicated by the part of the curve to the right of point 6

4 cutoff When I =0, the transistor is in the cutoff region although there is a very small collector leakage current as indicated. utoff is the nonconducting state of a transistor. 7 xample 03 8

5 9 Applying fixed V, increasing V utoff when I = 0, the transistor is in the cutoff region I O is extremely small, it will usually be neglected in circuit analysis V = V O represents collector toemitter with the base open. 10

6 Applying fixed V, increasing V Saturation When the base emitter junction becomes forward biased and the base current is increased, the collector current also increases (I =βi ) V decreases as a result of more drop across the collector resistor (V = V I R). V reaches its saturation value, V(sat), base collector junction becomes forwardbiased and At the point of saturation, the (I =βi ) is no longer valid. 11 D Load Line dc load line drawn on a family of curves connecting the cutoff point and the saturation point. The bottom of the load line is at ideal cutoff where I =0 and V =V. 12

7 xample V over the Vsat then transistor is in active mode, 14

8 TH JT AS AN AMPLIFIR Amplification is the process of linearly increasing the amplitude of an electrical signal and is one of the major properties of a transistor. a JT exhibits current gain (β) When a JT is biased in the active (or linear) region, 15 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits D quantities V V I I I V V A quantities Vbe Vce Vb Vc r e 16 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits

9 An ac voltage, Vs, is superimposed on the dc bias voltage V by capacitive coupling as shown. The dc bias voltage V is connected to the collector through the collector resistor, R. 17 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits The ac input voltage produces an ac base current, which results in a much larger ac collector current. The ac collector current produces an ac voltage across R, thus producing an amplified, but inverted, internal ac emitter resistance r e is designated in and appears in series with R. The ac base voltage is The ac collector voltage, Vc, equals the ac voltage drop across R Since the ac collector voltage is 18 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits

10 the ac voltage gain, Av, of the transistor remember that Vb 19 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits 20 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits

11 xample Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits xample Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits

12 Thanks,.. See you next week (ISA), 23