Lecture (08) ipolar Junction Transistor (2) y: Dr. Ahmed lshafee 1 JT haracteristic ollector haracteristic urves 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
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
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
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
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saturation value, V(sat), base collector junction becomes forwardbiased and At the point of saturation, the (I =βi ) is no longer valid. 11 D Load Line dc load line drawn on a family of curves connecting the cutoff point and the saturation point.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
xample 04 12 13 12 V over the Vsat then transistor is in active mode, 14
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
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
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
xample 06 21 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits xample 06 22 Dr. Ahmed lshafee, AU : Fall 2017, lectronic ircuits
Thanks,.. See you next week (ISA), 23