ob York Transistor asics - JTs
ipolar Junction Transistors (JTs) Key points: JTs are current-controlled devices very JT has a base, collector, and emitter The base current controls the collector current Thereare two types of JTS: NPN and PNP The use of JTs is conceptually similar to the use of FTs with some differences in biasing and details of operation. The NPN is analogous to the N-channel FT The PNP is analogous to the P-channel FT n the forward active region the collector current varies linearly with the base current. This linear dependence simplifies bias calculations and also makes JTs attractive for many analog circuit applications NPN ommon-emitter () configuration V ce : c b beta is the current gain V, 0.-0.3V ce sat NPN JT PNP JT ollector mitter ase ase mitter ollector deal NPN haracteristics, 5 3 3 1 1 ncreasing V ob York ack to TO
NPN -V haracteristics: The bers-moll Model The physical construction of JTs is like two PN junction diodes that are very close together The bers-moll Model (right) uses dependent sources to model the physical interaction between the and diodes. One junction is always larger than the other, giving asymmetrical -V characteristics (below) Reverse active region ( diode reverse biased, diode forward biased) ( diode forward biased, diode reverse biased) ncreasing NPN JT v cb v be e c b v ce e n the forward active region, bers-moll (M) NPN Model dc de ach active region can be described by a current gain, β f and β r, where β f >> β r ( 1) c fe e f b f e α F de α R dc f 1 f n the reverse active region, c r b V Reverse active currents are exaggerated here for clarity. Usually β r <<β f c rb e ( r 1) b c re r r 1 r ob York ack to TO
NPN JT: The Forward Active Region Devices are designed to operate in the forward active region where: ase-emitter diode is forward biased ase-collector diode is reverse biased For silicon transistors: v 0.6-0.7 V v 0.V be, cb deal NPN haracteristics V ce >0.-0.3V NPN JT v cb v be v ce M NPN Model in forward active region =α F e e b c 5 3 3 ncreasing Simplified Model for Si NPN biasing calculations in Forward Active Region 0.7V =β f 1 V, 0.-0.3V ce sat 1 V diode replaced by constantvoltage-drop model ob York ack to TO
PNP JTs PNPs are the complementary version of NPNs, just like PMOS is the complementary version of NMOS. Polarity of currents and voltages reversed as compared with NPN. For the forward active region: v 0.6-0.7 V v 0.V V 0.-0.3V eb bc ec PNP JT e v eb v ec M PNP Model in forward active region e b c Generally PNP has smaller current gain than NPN v bc =α F, deal PNP haracteristics 5 3 1 V, 0.-0.3V ce sat 3 1 ncreasing V ec Simplified Model for Si PNP biasing calculations in Forward Active Region ob York ack to TO 0.7V =β f diode replaced by constantvoltage-drop model
Real JTs Real Devices depart from the idealized behavior in a number of ways. Two important ones are illustrated here Most JTs show a significant slope in the -V curves. This is called the arly effect. We will learn how to model this in. Also, the current gain is not exactly constant and varies with device current and temperature as shown below. Note β increases with temperature., 8 6 7 6 3 1 ncreasing The current gain can also vary from device-todevice, so circuit techniques must be used to make the designs insensitive to variations in β V ce,sat V urrent gain vs. collector current at different temperatures (N390 data sheet) ob York ack to TO