ipolar Junction Transistor (JT ipolar Junction Transistors JT is a three-terminal device: emitter (, collector ( and base (. There are two types: pnp-type and npn-type. npn transistor: emitter & collector are n-doped and base is p-doped. mitter is heavily doped, collector is moderately doped and base is lightly doped and base is very thin. PNP - transistor NPN - transistor p + n p + + n + p n V V V + + V ipolar Junction Transistor (JT JT consists of two pn junctions: emitter-base junction (J or J and collector-base junction (J or J JT Modes of Operation There are four regions of operation of a JT transistor (example for a pnp JT: V active region (emitter-base F, collector-base R Saturation region (both junctions forward biased 3 V utoff region Inverted active region (both junctions reverse biased (emitter-base R, collector-base F Since JT has three terminals, re are three possible amplifier types: types: V p + n p V V p n p + (a ommon-base (b ommon-emitter (c ommon-collector 4 V V p + n p V
n Active mode (npn JT The J is forward-biased and J is reverse biased. 5 p Active mode (npn JT n ase transport factor ( T = n / in ommon-base current gain ( = n / i = T< Terminal currents of JT in active mode: i(emitter current = in(electron injection from to + ip(hole injection from to (collector current = n(electron drift + O(J reverse uration current with emitter open i(base current = i(hole injection from to + i(recombination in base region NTU L. H. Lu emitter into base. bias of J causeslectronics electrons to diffuse from As base region is very thin, majority of se electrons diffuse to edge of depletion region of J, and n are swept to collector by electric field of reverse-biased J. A small fraction of se electrons recombine with holes in base region. Holes are injected from base to emitter region. 6 Large-signal model (npn JT - active mode Active mode (npn JT - Terminal currents = IS ev /VT IS ev /VT A qdn ni NA W Area of base-emitter junction Width of base region Large-signal model Doping concentration in base lectron diffusion constant Intrinsic carrier concentration IS : uration current IS = A : W : NA : Dn : ni : i = and current gain for JT in active region i = i + = ( + i + = = ommon-base current gain ommon-emitter current gain kt VT = 5mV @ 5 q ( i β: common - emitter current gain α: common - base current gain ( + ommon-emitter current gain: i ( D p N W Dn N L p i i 7 i W Dn n /( 8 4-3
are p-type ar to that of npnlarge-signal model (pnp JT - active mode active mode urrent-voltage relationships - active mode Summary of JT current-voltage relationships in active currents for a JT in active mode mode depend The values of terminal e v /VT junction voltage of J. The values of terminaloncurrents for a JT in active mode solely depend on junction vo The ratios of terminal currents for a JT in active mode are The ratios of terminalconstant. currents for a JT in active mode are constant. directions for npn and transistors are opposite. The The current directions for npncurrent and pnp transistors arepnp opposite. currentt gain i for f JT in i active ti region i npn transistor ommon-emitter current gain ( ommon-base current gain i ( + i i i i i pnp transistor I S ev /VT I S e v / VT i e v /VT i IS i I S e v /VT I S e v /VT e v /VT IS i 9 NTU lectronics L. H. Lu Active mode NTU lectronics L. H. Lu x: The JT in circuit has β = and exhibits a v of.7v at = ma. Design circuit so that a current of ma flows through collector and a voltage of +5V at collector. R = 4-9 Active mode x: Find I, I, I and V if voltage at emitter was measured and found to be -.7V and β = 5. V = 5k ma V =.7 + VT ln =.77V ( 5 I = 7.7k R = V
Active mode urrent-voltage characteristics x: Find α and β for JT in following circuit, if The common-emitter : measurement indicates V to be +V and V to be +.7V. What voltage V do you expect at collector? I (ma 8 9 µa 8 µa 7 I (ma 7 µa 6 6 µa 5 µa (Saturation region 5 4 µa V = V V = V 9 V = V (Active region V 3 V (V (utoff region ~ β I O I O = 3 (Active region 6 5 µa 4 3 µa..4.6.8. I = µa V (V 4.3 JT ircuits at D (a 7 8 3 µa I = µa 9 4 5 5 µa 3 µa 4 µa 5 I (µa 6 µa (Saturation region 5 6 µa 7 µa 6 4 5 9 µa 8 µa 7 7 3 µa 8 4 3 8 I (µa V 5 5 V (V. (utoff region ~ β I O I O = 4 (b (a Figure 3.4 haracteristics of a silicon transistor in common-emitter : (a collector characteristics; (b base characteristics. Figure 3.4 haracteristics of a silicon transistor in common-emitter : (a collector characteristics; (b base characteristics. JT operation modes JT ircuits at D The JT operation depends voltages at J and J Note that on characteristics of Fig. 3.4 magnitude of I is in microamnote that on characteristics of Fig. 3.4mode magnitude of I on is in microamsaturation of common-emitter peres, compared to milliamperes of I. onsider also that curves of I are not as peres, compared to milliamperes of I. onsider also that curves of I are not as horizontal as those obtained for I in common-base, indicating that I-Vforcharacteristics are stronglyindicating nonlinear, that horizontal as thosethe obtained I in common-base collector-to-emitter voltage will influence magnitude of collector current. collector-to-emitter voltage will influence magnitude of collector current. The active region for common-emitter is that portion of Simplified models and classifications areofneeded to speed modes: up hand-calculation analysis with a small JT operation In uration region, it behaves as a closed Theswitch active region for common-emitter is that portion upper-right quadrant that has greatest linearity, that is, that region in which upper-right quadrant that has greatest linearity, that is, that region in which resistance R equally spaced. In Fig. 3.4a this region exists curves for I are nearly straight pnpandtransistor npn transistor curves for I are nearly straight and equally spaced. In Fig. 3.4a this region exists to right of vertical dashed line at V and above curve for I equal to J The uration I-V curve can be approximated a Mode straight line linej curve for I equal to to right by of vertical dashed at V and above is called uration region. zero. The region to left of V v v zero. The region to left of V is called uration region. In active region of a common-emitter amplifier collector-base junction intersecting v axis Active Reverse is reverse-biased, while base-emitter junction is forward-biased. In active region of a common-emitter amplifier collector-base junction at Voff Saturation Mode Saturation Mode Inverse Mode Inverse Mode is reverse-biased, while base-emitter junction is forward-biased. You will recall that se were same conditions that existed in active reutoff Reverse Reverse v, v gion of common-base vthe v, v v, v., vregion of common-emitter active The uration voltage You will recall that se were same conditions that existed in active rev v can be employed for voltage, current, or power amplification. Saturation V = Voff + IR gion of common-base. The active region of common-emitter The cutoff region for common-emitter utoff Mode utoff Mode Active Mode Active Mode is not as well defined as can be employed for voltage, current, or power amplification. v on,collector v. v, v for common-base, v Notev v characteristics of Fig. 3.4 V is normally treated, v Inverse Reverse The cutoff region for common-emitter is not as well defined as I is zero. For common-base, when that I is not equalto zero when as a constant of. V input current I was equal to zero, collector current was equal only to refor common-base. Note on collector characteristics of Fig. 3.4 verse uration current IO, so that curve I! and voltage axis were, for that I is not equal to zero when I is zero. For common-base, when for simplicity regardless Simplified models for of re- Simplified and current classifications for operation of npnpurposes, JT all operation practical one.npn JT: to zero, collector was equal only to input current I was equal models value of The reason for this difference in collector characteristics can be derived through verse uration current IO, so that curve I! and voltage axis were, for ut-off mode: v <.5V and v <.4V; i = = i = proper manipulation of qs. (3.3 and (3.6. That is, ut-off all practical purposes, one. mode: Incremental β in uration Active mode: q. (3.6: I! "I # IO The reason for this difference in collector characteristics can be derived through is lower than that in active i = i =(3.6. That is, i = v =.7V and v Substitution proper manipulation of qs.(3.3 and >.3V gives q. (3.3: I! "(I # I # IO region: βforced = I / I < β vq.< (3.6:.5 VIand <.4! "Iv# IO Substitution gives Active q. (3.3: I! " (I # I # IO mode: 5 Rearranging yields "I I V O!$ $ $ #$ v =I.7 V % "and i% ": : i = : v >.3 V i : : i = : β : (Rearranging + β yields (3.8 Saturation mode: v =.7V and v =.V; / i < β 6 : (+(3.8 3.6 ommon-mitter onfiguration "I IO I! $$ $ #$ %" %" 3.6 ommon-mitter onfiguration.4
D analysis of JT circuits D analysis of JT circuits Step : assume operation mode Step : use conditions or model for circuit analysis Step 3: verify solution Step 4: repeat above steps with anor assumption if necessary x: Determine all node voltage and branch currents of circuit, assume that β is specified to be. 7 8 D analysis of JT circuits D analysis of JT circuits x: Determine all node voltage and branch currents of circuit, x: Determine all node voltage and branch currents of circuit, assume β =. assume β =. 9
D analysis of JT circuits D analysis of JT circuits x: Determine all node voltage and branch currents of circuit, x: Determine all node voltage and branch currents of circuit, assume β =. assume β =. D analysis of JT circuits x: Determine all node voltage and branch currents of circuit, assume β = β =. D bias for JT amplifier The amplifiers are operating at a proper dc bias point. The D bias circuit is to ensure JT in active mode with a proper collector current I. 3 4
D bias for JT amplifier D bias for JT amplifier The classical discrete-circuit bias arrangement: The two-power-supply version: A single power supply and resistors are needed. I = 5 6 D bias for JT amplifier iasing using a collector-to-base feedback resistor: D bias for JT amplifier iasing using a constant-current source: R ensures JT in active (V > V =.7V R is chosen to operate JT in active mode. The current source is typically implemented by a JT current mirror. oth JT transistors Q and Q are in active mode. Assume current gain β is very high: I = IRF = 7 V V R + R /( + V + V R V 8