Presentation on theme: "Dr. D G Borse B C E. The BJT – Bipolar Junction Transistor Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer."— Presentation transcript:
Dr. D G Borse B C E
The BJT – Bipolar Junction Transistor Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping. The Two Types of BJT Transistors: npnpnp npn E B C pnp E B C Cross Section B C E Schematic Symbol B C E Collector doping is usually ~ 10 9Collector doping is usually ~ 10 9 Base doping is slightly higher ~ – 10 11Base doping is slightly higher ~ – Emitter doping is much higher ~ 10 17Emitter doping is much higher ~ 10 17
Dr. D G Borse BJT Relationships - Equations B C E IEIEIEIE ICICICIC IBIBIBIB - + V BE V BC V CE B C E IEIEIEIE ICICICIC IBIBIBIB - + V EB V CB V EC n p n I E = I B + I C V CE = -V BC + V BE p n p I E = I B + I C V EC = V EB - V CB Note: The equations seen above are for the transistor, not the circuit.
Dr. D G Borse Figure : Current flow (components) for an n-p-n BJT in the active region. NOTE: Most of the current is due to electrons moving from the emitter through base to the collector. Base current consists of holes crossing from the base into the emitter and of holes that recombine with electrons in the base. - Electrons + Holes V BE V CB n+n+n+n+ n p-p-p-p- I ne I pe - I co Bulk-recombination Current I nc
Dr. D G Borse Physical Structure Consists of 3 alternating layers of n- and p-type semiconductor called emitter (E), base (B) and collector (C). Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter. Carrier transport in the active base region directly beneath the heavily doped (n + ) emitter dominates i-v characteristics of BJT.
Dr. D G Borse Recombination - Electrons + Holes + _ + _ C B E n p n + IBIBIBIB IcIcIcIc IEIEIEIE V BE V CB
Dr. D G Borse Figure: An npn transistor with variable biasing sources (common-emitter configuration). I nc I ne I pe For CB Transistor I E = I ne + I pe I c = I nc - I co And I c = - αI E + I Co CB Current Gain, α ═ (I c - I co ). (I E - 0) (I E - 0) For CE Trans., I C = βI b + (1+β) I co where β ═ α, 1- α is CE Gain 1- α is CE Gain I CO Bulk- recombination current
Dr. D G Borse Common-Emitter Circuit Diagram +_ VCCVCCVCCVCC ICICICIC V CE IBIBIBIB Collector-Current Curves V CE ICICICIC Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0 Region of Operation Description ActiveSmall base current controls a large collector current SaturationV CE(sat) ~ 0.2V, V CE increases with I C CutoffAchieved by reducing I B to 0, Ideally, I C will also be equal to 0.
Dr. D G Borse BJT’s have three regions of operation: 1) Active - BJT acts like an amplifier (most common use) 2) Saturation - BJT acts like a short circuit 3) Cutoff - BJT acts like an open circuit BJT is used as a switch by switching between these two regions. When analyzing a DC BJT circuit, the BJT is replaced by one of the DC circuit models shown below. DC Models for a BJT:
Dr. D G Borse DC and DC = Common-emitter current gain = Common-base current gain = I C = I C = I C = I C I B I E I B I E The relationships between the two parameters are: = = = = Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.
Dr. D G Borse Output characteristics: npn BJT (typical) Note: Two key specifications for the BJT are B dc and V o (or assume V o is about 0.7 V) Note: The PE review text sometimes uses dc instead of dc. They are related as follows: Input characteristics: npn BJT (typical) The input characteristics look like the characteristics of a forward-biased diode. Note that V BE varies only slightly, so we often ignore these characteristics and assume: Common approximation: V BE = V o = 0.65 to 0.7V Find the approximate values of dc and dc from the graph.
Dr. D G Borse Figure: Common-emitter characteristics displaying exaggerated secondary effects.
Dr. D G Borse Figure: Common-emitter characteristics displaying exaggerated secondary effects.
Dr. D G Borse Various Regions (Modes) of Operation of BJT Most important mode of operationMost important mode of operation Central to amplifier operationCentral to amplifier operation The region where current curves are practically flatThe region where current curves are practically flatActive: Saturation: Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch)Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch) Cutoff: Current reduced to zeroCurrent reduced to zero Ideal transistor behaves like an open switchIdeal transistor behaves like an open switch * Note: There is also a mode of operation called inverse active mode, but it is rarely used.
Dr. D G Borse BJT Trans-conductance Curve For Typical NPN Transistor 1 V BE ICICICIC 2 mA 4 mA 6 mA 8 mA 0.7 V Collector Current: I C = I ES e V BE / V T Transconductance: (slope of the curve) g m = I C / V BE I ES = The reverse saturation current of the B-E Junction. of the B-E Junction. V T = kT/q = 26 mV T=300 o K) = the emission coefficient and is usually ~1 usually ~1
Dr. D G Borse Three Possible Configurations of BJT Biasing the transistor refers to applying voltages to the transistor to achieve certain operating conditions. 1. Common-Base Configuration (CB) : input = V EB & I E output = V CB & I C 2. Common-Emitter Configuration (CE): input = V BE & I B output= V CE & I C output= V CE & I C 3. Common-Collector Configuration (CC) :input = V BC & I B (Also known as Emitter follower) output = V EC & I E (Also known as Emitter follower) output = V EC & I E
Dr. D G Borse Common-Base BJT Configuration Circuit Diagram: NPN Transistor +_+_ ICICICIC IEIEIEIE IBIBIBIB V CB V BE EC B V CE V BE V CB Region of Operation ICICICIC V CE V BE V CB C-B Bias E-B Bias Active IBIBIBIB =V BE +V CE ~0.7V 0V Rev.Fwd. SaturationMax~0V~0.7V -0.7V
Dr. D G Borse Common-Base (CB) Characteristics Although the Common-Base configuration is not the most common configuration, it is often helpful in the understanding operation of BJT V c - I c (output) Characteristic Curves Saturation Region IEIEIEIE ICICICIC V CB Active Region Cutoff I E = 0 0.8V2V4V6V8V mA I E =1mA I E =2mA Breakdown Reg.
Dr. D G Borse Common-Collector BJT Characteristics Emitter-Current Curves V CE IEIEIEIE Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0 The Common- Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at I C as a function of V CE and I B we are looking at I E. Also, since ~ 1, and = I C /I E that means I C ~I E
Dr. D G Borse n p n Transistor: Forward Active Mode Currents Forward Collector current is I co is reverse saturation current V T = kT/q =25 mV at room temperature Base current is given by Emitter current is given by is forward common-emitter current gain is forward common- base current gain In this forward active operation region, V BE IE=IE=IE=IE= IC=IC=IC=IC= IB=IB=IB=IB=
Dr. D G Borse Various Biasing Circuits used for BJT Fixed Bias Circuit Collector to Base Bias Circuit Potential Divider Bias Circuit
Dr. D G Borse The Thermal Stability of Operating Point S Ico The Thermal Stability Factor : S Ico S Ico = ∂I c ∂I co ∂I co This equation signifies that I c Changes S Ico times as fast as I co Differentiating the equation of Collector Current I C & rearranging the terms we can write S Ico ═ 1+β 1- β (∂I b /∂I C ) 1- β (∂I b /∂I C ) It may be noted that Lower is the value of S Ico better is the stability V be, β
Dr. D G Borse The Fixed Bias Circuit The Thermal Stability Factor : S Ico S Ico = ∂I c ∂I co ∂I co General Equation of S Ico Comes out to be S Ico ═ 1 + β S Ico ═ 1 + β 1- β (∂I b /∂I C ) 1- β (∂I b /∂I C ) V be, β Applying KVL through Base Circuit we can write, I b R b + V be = V cc Diff w. r. t. I C, we get (∂I b / ∂I c ) = 0 S Ico = (1+β) is very large Indicating high un-stability IbIbIbIb RbRbRbRb RCRCRCRC RCRCRCRC
Dr. D G Borse The Collector to Base Bias Circuit The General Equation for Thermal Stability Factor, S Ico = ∂I c S Ico = ∂I c ∂I co ∂I co Comes out to be S Ico ═ 1 + β S Ico ═ 1 + β 1- β (∂I b /∂I C ) 1- β (∂I b /∂I C ) V be, β Applying KVL through base circuit we can write (I b + I C ) R C + I b R b + V be = V cc Diff. w. r. t. I C we get (∂I b / ∂I c ) = - R C / (R b + R C ) Therefore, S Ico ═ (1+ β) 1+ [ βR C / (R C + R b ) ] 1+ [ βR C / (R C + R b ) ] Which is less than (1+β), signifying better thermal stability IcIcIcIc IbIbIbIb V BE + - IEIEIEIE
Dr. D G Borse The Potential Devider Bias Circuit The General Equation for Thermal Stability Factor, S Ico ═ 1 + β 1- β (∂I b /∂I C ) 1- β (∂I b /∂I C ) Applying KVL through input base circuit we can write I b R Th + I E R E + V be = V Th Therefore, I b R Th + (I C + I b ) R E + V BE = V Th Diff. w. r. t. I C & rearranging we get (∂I b / ∂I c ) = - R E / (R Th + R E ) Therefore, This shows that S I co is inversely proportional to R E and It is less than (1+β), signifying better thermal stability Thevenin Equivalent Ckt ICICICIC IbIbIbIb ICICICIC IbIbIbIb ICICICIC Thevenins Equivalent Voltage Self-bias Resistor R th = R 1 *R 2 & Vth = Vcc R 2 R 1 +R 2 R 1 +R 2 R 1 +R 2 R 1 +R 2
Dr. D G Borse A Practical C E Amplifier Circuit Input Signal Source
Dr. D G Borse BJT Amplifier (continued) An 8 mV peak change in v BE gives a 5 A change in i B and a 0.5 mA change in i C. The 0.5 mA change in i C gives a 1.65 V change in v CE. If changes in operating currents and voltages are small enough, then I C and V CE waveforms are undistorted replicas of the input signal. A small voltage change at the base causes a large voltage change at the collector. The voltage gain is given by: The minus sign indicates a phase shift between input and output signals.
Dr. D G Borse A Practical BJT Amplifier using Coupling and Bypass Capacitors AC coupling through capacitors is used to inject an ac input signal and extract the ac output signal without disturbing the DC Q-point Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc. In a practical amplifier design, C 1 and C 3 are large coupling capacitors or dc blocking capacitors, their reactance (X C = |Z C | = 1/ C) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered. C 2 is a bypass capacitor. It provides a low impedance path for ac current from emitter to ground. It effectively removes R E (required for good Q-point stability) from the circuit when ac signals are considered.
Dr. D G Borse D C Equivalent for the BJT Amplifier (Step1) All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting v I, R I, and R 3 from the circuit and leaving R E intact. The the transistor Q will be replaced by its DC model. DC Equivalent Circuit
Dr. D G Borse A C Equivalent for the BJT Amplifier (Step 2) Coupling capacitor C C and Emitter bypass capacitor C E are replaced by short circuits. DC voltage supply is replaced with short circuits, which in this case is connected to ground. R 1 II R 2 =R B R in RoRoRoRo
Dr. D G Borse A C Equivalent for the BJT Amplifier (continued) By combining parallel resistors into equivalent R B and R, the equivalent AC circuit above is constructed. Here, the transistor will be replaced by its equivalent small-signal AC model (to be developed). All externally connected capacitors are assumed as short circuited elements for ac signal
Dr. D G Borse A C Analysis of CE Amplifier 1) Determine DC operating point and calculate small signal parameters 2) Draw the AC equivalent circuit of Amp. DC Voltage sources are shorted to ground DC Current sources are open circuited Large capacitors are short circuits Large inductors are open circuits 3) Use a Thevenin circuit (sometimes a Norton) where necessary. Ideally the base should be a single resistor + a single source. Do not confuse this with the DC Thevenin you did in step 1. 4) Replace transistor with small signal model 5) Simplify the circuit as much as necessary. Steps to Analyze a Transistor Amplifier 6) Calculate the small signal parameters and gain etc. Step 1 Step2 Step3 Step 4 Step 5 π-model used
Dr. D G Borse Hybrid-Pi Model for the BJT The hybrid-pi small-signal model is the intrinsic low-frequency representation of the BJT. The small-signal parameters are controlled by the Q-point and are independent of the geometry of the BJT. Transconductance: Input resistance: Rin Output resistance: Where, V A is Early Voltage (V A =100V for npn)
Dr. D G Borse Hybrid Parameter Model Linear Two port Device ViViViVi IiIiIiIi IoIoIoIo VoVoVoVo
Dr. D G Borse h-Parameters h 11 = h i = Input Resistance h 12 = h r = Reverse Transfer Voltage Ratio h 21 = h f = Forward Transfer Current Ratio h 22 = h o = Output Admittance
Dr. D G Borse The Mid-frequency small-signal models Three Small signal Models of CE Transistor
Dr. D G Borse BJT Mid-frequency Analysis using the hybrid- model: A common emitter (CE) amplifier The mid-frequency circuit is drawn as follows: the coupling capacitors (C i and C o ) and the bypass capacitor (C E ) are short circuits short the DC supply voltage (superposition) replace the BJT with the hybrid- model The resulting mid-frequency circuit is shown below. An a c Equivalent Circuit rorororo
Dr. D G Borse Details of Small-Signal Analysis for Gain Av ( Using Π-model )Rs Rs From input circuit
Dr. D G Borse C-E Amplifier Input Resistance The input resistance, the total resistance looking into the amplifier at coupling capacitor C 1, represents the total resistance presented to the AC source.
Dr. D G Borse C-E Amplifier Output Resistance The output resistance is the total equivalent resistance looking into the output of the amplifier at coupling capacitor C 3. The input source is set to 0 and a test source is applied at the output. But v be =0. since r o is usually >> R C.
Dr. D G Borse High-Frequency Response – BJT Amplifiers Capacitances that will affect the high-frequency response: Cbe, Cbc, Cce – internal capacitances Cwi, Cwo – wiring capacitances C S, C C – coupling capacitors C E – bypass capacitor
Dr. D G Borse Frequency Response of Amplifiers The voltage gain of an amplifier is typically flat over the mid-frequency range, but drops drastically for low or high frequencies. A typical frequency response is shown below. For a CE BJT: (shown on lower right) low-frequency drop-off is due to C E, C i and C o high-frequency drop-off is due to device capacitances C p and C m (combined to form C total ) Each capacitor forms a break point (simple pole or zero) with a break frequency of the form f=1/(2pR Eq C), where R Eq is the resistance seen by the capacitor C E usually yields the highest low-frequency break which establishes f Low.
Dr. D G Borse Amplifier Power Dissipation Static power dissipation in amplifiers is determined from their DC equivalent circuits. Total power dissipated in C-B and E-B junctions is: where Total power supplied is: The difference is the power dissipated by the bias resistors.
Dr. D G Borse
Figure An Emitter follower.
Dr. D G Borse Figure Emitter follower. Very high input Resistance Very low out put Resistance Unity Voltage gain with no phase shift High current gain Can be used for impedance matching or a circuit for providing electrical isolation An Emitter Follower (CC) Amplifier
Dr. D G Borse Figure An Emitter follower.
Dr. D G Borse Figure: An Emitter follower.
Dr. D G Borse Capacitor Selection for the CE Amplifier The key objective in design is to make the capacitive reactance much smaller at the operating frequency f than the associated resistance that must be coupled or bypassed.
Dr. D G Borse Summary of Two-Port Parameters for CE/CS, CB/CG, CC/CD
Dr. D G Borse A Small Signal h-parameter Model of C E - Transistor = h 11 V ce *h 12
Dr. D G Borse Small-signal Current Gain and Amplification Factor of the BJT o > F for i C I M, however, o and F are usually assumed to be about equal. The amplification factor is given by: For V CE << V A, F represents the maximum voltage gain an individual BJT can provide, independent of the operating point.
Dr. D G Borse A Simple MOSFET Amplifier The MOSFET is biased in the saturation region by dc voltage sources V GS and V DS = 10 V. The DC Q-point is set at (V DS, I DS ) = (4.8 V, 1.56 mA) with V GS = 3.5 V. Total gate-source voltage is: A 1 V p-p change in v GS gives a 1.25 mA p-p change in i DS and a 4 V p-p change in v DS. Notice the characteristic non-linear I/O relationship compared to the BJT.
Dr. D G Borse Eber-Moll BJT Model The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model: E C B IRIRIRIR IFIFIFIF IEIEIEIE ICICICIC IBIBIBIB RIERIERIERIE RICRICRICRIC
Dr. D G Borse Eber-Moll BJT Model R = Common-base current gain (in forward active mode) F = Common-base current gain (in inverse active mode) I ES = Reverse-Saturation Current of B-E Junction I CS = Reverse-Saturation Current of B-C Junction I C = F I F – I R I B = I E - I C I E = I F - R I R I F = I ES [exp(qV BE /kT) – 1]I R = I C [exp (qV BC /kT) – 1] If I ES & I CS are not given, they can be determined using various BJT parameters. BJT parameters.
Dr. D G Borse Small Signal BJT Equivalent Circuit The small-signal model can be used when the BJT is in the active region. The small-signal active-region model for a CB circuit is shown below: iBiBiBiB rrrr iEiEiEiE iCiCiCiC iBiBiBiB BC E r = ( + 1) * V T I E I = 1 and T = 25 C r = ( + 1) * I E I E Recall: = I C / I B
Dr. D G Borse The Early Effect (Early Voltage) V CE ICICICIC Note: Common-Emitter Configuration -V A IBIBIBIB Green = Ideal I C Orange = Actual I C (I C ’) I C ’ = I C V CE + 1 V A V A
Dr. D G Borse Early Effect Example Given:The common-emitter circuit below with I B = 25 A, V CC = 15V, = 100 and V A = 80. Find: a) The ideal collector current b) The actual collector current Circuit Diagram +_ V CC ICICICIC V CE IBIBIBIB b = 100 = I C /I B a) I C = 100 * I B = 100 * (25x10 -6 A) I C = 2.5 mA b) I C ’ = I C V CE + 1 = 2.5x = 2.96 mA V A 80 V A 80 I C ’ = 2.96 mA
Dr. D G Borse Breakdown Voltage The maximum voltage that the BJT can withstand. BV CEO =The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~ Volts. BV CBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BV CEO and has a minimum value of ~60 Volts. Breakdown Voltage is Determined By: The Base WidthThe Base Width Material Being UsedMaterial Being Used Doping LevelsDoping Levels Biasing VoltageBiasing Voltage
Dr. D G Borse Potential-Divider Bias Circuit with Emitter Feedback Most popular biasing circuit. Problem: dc can vary over a wide range for BJT’s (even with the same part number) Solution: Adding the feedback resistor R E. How large should R E be? Let’s see. Substituting the active region model into the circuit to the left and analyzing the circuit yields the following well known equation: I CEO has little effect and is often neglected yielding the simpler relationship: Test for stability: For a stable Q-point w.r.t. variations in dc choose: Why? Because then Voltage divider biasing circuit with emitter feedback Replacing the input circuit by a Thevenin equivalent circuit yields:
Dr. D G Borse PE-Electrical Review Course - Class 4 (Transistors) Example : Find the Q-point for the biasing circuit shown below. The BJT has the following specifications: dc = 100, r sat = 100 (V o not specified, so assume V o = 0.7 V) Example : Repeat Example 3 if R C is changed from 1k to 2.2k.
Dr. D G Borse PE-Electrical Review Course - Class 4 (Transistors) Example Determine the Q-point for the biasing circuit shown. The BJT has the following specifications: dc varies from 50 to 400, V o = 0.7 V, I CBO = 10 nA Solution: Case 1: dc = 50 Case 2: dc = 400 Similar to Case 1 above. Results are: I C = mA, V CE = 6.14 V Summary:
Dr. D G Borse BJT Amplifier Configurations and Relationships: Using the hybrid- model. Note: The biasing circuit is the same for each amplifier.
Dr. D G Borse Figure 4.16 The pnp BJT.
Dr. D G Borse Figure : Common-emitter characteristics for a pnp BJT.
Dr. D G Borse Figure 4.18 Common-emitter amplifier for Exercise 4.8.
Dr. D G Borse Figure : BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices at a temperature of 300K.
Dr. D G Borse Figure 4.19b BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices at a temperature of 300K.
Dr. D G Borse Figure: BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices at a temperature of 300K.