Bipolar Junction Transistors (BJTs) 1.

Bipolar Junction Transistors (BJTs) 1

BJT’s are used as amplifiers in analog electronics.
Bipolar Junction Transistors, BJT, a three terminal, non-linear electronic device. BJT’s are used as amplifiers in analog electronics. BJTs are used as switches in digital electronics. Invented in 1948 by Bardeen, Brattain, and Shockley at Bell Telephone Labs. Popular for three decades before being overtaken by Field Effect Transistors, FETs. Microelectronic Circuits - Fifth Edition Sedra/Smith

A simplified structure of the npn transistor.
sedr42021_0501.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

A simplified structure of the pnp transistor

BJTs have the following four modes of operation.
Still used in harsh environment and high frequency applications, also Bi-CMOS applications. BJTs have the following four modes of operation. The cutoff and saturation modes are used in digital electronics to obtain a switch. The active mode is used in analog electronics to obtain an amplifier. Mode EBJ CBJ Cuttoff Reverse Active Forward Reverse-Active Saturation Microelectronic Circuits - Fifth Edition Sedra/Smith

Current flow in an npn transistor biased to operate in the active mode
Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers are not shown.) sedr42021_0503.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Ignore the very small drift currents.
The EBJ is a forward biased pn junction. Holes are injected from the base to the emitter and electrons injected from the emitter to the base. These two components combine to form the emitter current, iE. A highly doped emitter and a lightly doped base keep the hole component of iE small. Microelectronic Circuits - Fifth Edition Sedra/Smith

Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode: vBE > 0 and vCB ≥ 0. sedr42021_0504.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

The concentration np(0), using the Law of the Junction is:
The electron diffusion current, In, points in the positive x-direction. Recombination of electrons with holes in the base cause np(x) to deviate a small amount from ideal. Microelectronic Circuits - Fifth Edition Sedra/Smith

The collector current, ic, points in the negative x-direction and is equal in magnitude to In.
The collector current is directly proportional to the emitter area, AE, and inversely proportional to the effective width of the base, W. Microelectronic Circuits - Fifth Edition Sedra/Smith

The base current, iB, is composed of two components, iB1 and iB2 (iB= iB1+ iB2).
The first component, iB1, is due to holes injected into the emitter from the base. The second component, iB2, is due to the recombination of holes and electrons in the base. Microelectronic Circuits - Fifth Edition Sedra/Smith

The base current is the sum of these two components
Microelectronic Circuits - Fifth Edition Sedra/Smith

The emitter current, iE, (using KCL) is equal to the base current plus the collector current.
α = αF, β = βF forward active mode, αR and βR for the reverse active mode. Microelectronic Circuits - Fifth Edition Sedra/Smith

Summary of the BJT Current-Voltage Relationships in the Active Mode

Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.
sedr42021_0505a.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Cross-section of an npn BJT.
Scale current relations Cross-section of an npn BJT. sedr42021_0506.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Model for the npn transistor when operated in the reverse active mode (i.e., with the CBJ forward biased and the EBJ reverse biased). sedr42021_0507.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

The Ebers-Moll (EM) model of the npn transistor.

The diode equations for iDE and iDC are as follows.
This structure yields positive values of iC, iE, and iB in the forward-active mode and is valid for all modes. The diode equations for iDE and iDC are as follows. Microelectronic Circuits - Fifth Edition Sedra/Smith

Substituting the diode equations into the expressions for iC, iE, and iB and using the scale current relations we obtain the following equations. Microelectronic Circuits - Fifth Edition Sedra/Smith

In the forward active mode vBE is between 0. 6 V and 0
In the forward active mode vBE is between 0.6 V and 0.8 V, and vBC is negative. The exponential terms containing vBC will be negligibly small and we obtain Note that if we ignore the very small second term in these equations we obtain the relations derived earlier. Microelectronic Circuits - Fifth Edition Sedra/Smith

The iC –vCB characteristic of an npn transistor fed with a constant emitter current IE. The transistor enters the saturation mode of operation for vCB < –0.4 V, and the collector current diminishes. sedr42021_0509.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Concentration profile of the minority carriers (electrons) in the base of an npn transistor operating in the saturation mode. Ignoring small terms: Law of the Junction: sedr42021_0510.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Current flow in a pnp transistor biased to operate in the active mode.

Large-signal model for the pnp transistor operating in the active mode.

Circuit symbols for BJTs.

Voltage polarities and current flow in transistors biased in the active mode.

sedr42021_0515a.jpg Figure 5.15 Circuit for Example 5.1.

sedr42021_e0510.jpg Figure E5.10 Microelectronic Circuits - Fifth Edition Sedra/Smith

The iC –vBE characteristic for an npn transistor.

Effect of temperature on the iC–vBE characteristic
Effect of temperature on the iC–vBE characteristic. At a constant emitter current (broken line), vBE changes by –2 mV/°C. sedr42021_0517.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

Use vBE ≈ 0.7 V for rapid calculation.
Under normal forward-active and saturation modes vBE is between 0.6 V and 0.8 V. Use vBE ≈ 0.7 V for rapid calculation. Microelectronic Circuits - Fifth Edition Sedra/Smith

The iC–vCB characteristics of an npn transistor, common-base characteristic.

Not quite horizontal lines, some increase in iC as vCB is increased.
CBJ will breakdown at BVCB0, typically greater than 50 V. The collector current in the saturation region (vCB < −0.4 V) can be obtained using the EM equations. Microelectronic Circuits - Fifth Edition Sedra/Smith

Common-Emitter Characteristics.
sedr42021_0519.jpg Early Voltage (a) Conceptual circuit for measuring the iC –vCE characteristics of the BJT. (b) The iC –vCE characteristics of a practical BJT. Microelectronic Circuits - Fifth Edition Sedra/Smith

The output resistance, ro
The linear dependence of the collector current, iC, on the collector-emitter voltage, vCE, is called “The Early Effect”. The output resistance, ro Microelectronic Circuits - Fifth Edition Sedra/Smith

Large-signal equivalent-circuit models of an npn BJT operating in the active mode in the common-emitter configuration. sedr42021_0520a.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0521.jpg Figure Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail. Microelectronic Circuits - Fifth Edition Sedra/Smith

Large-signal or dc CE current gain, βdc, (hFE in data books)
Incremental or ac CE current gain, βac, (hfe in data books) βdc and βac differ by 10% to 20% Microelectronic Circuits - Fifth Edition Sedra/Smith

Typical dependence of β on IC and on temperature in a modern integrated-circuit npn silicon transistor intended for operation around 1 mA. sedr42021_0522.jpg Microelectronic Circuits - Fifth Edition Sedra/Smith

An expanded view of the common-emitter characteristics in the saturation region.

βac is high in the active region and low in the saturation region.
IC,sat < βFIB βForced = IC,sat /IB < βF Overdrive factor: βF / βForced Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0524a.jpg Figure (a) An npn transistor operated in saturation mode with a constant base current IB. (b) The iC–vCE characteristic curve corresponding to iB = IB. The curve can be approximated by a straight line of slope 1/RCEsat. (c) Equivalent-circuit representation of the saturated transistor. (d) A simplified equivalent-circuit model of the saturated transistor. Microelectronic Circuits - Fifth Edition Sedra/Smith

Saturation Resistance, RCEsat (~ 3 to 30 ohms)
Model of the saturated transistor Using the EM equations for the saturated BJT one obtains the following expression for RCEsat. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0525.jpg Figure Plot of the normalized iC versus vCE for an npn transistor with bF = 100 and aR = 0.1. This is a plot of Eq. (5.47), which is derived using the Ebers-Moll model. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_tb0503a.jpg Table 5.3 Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_tb0503h.jpg Table 5.3 (Continued)

sedr42021_0526a.jpg Figure (a) Basic common-emitter amplifier circuit. (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a small voltage signal vi is superimposed on the dc bias voltage VBE. The resulting output signal vo appears superimposed on the dc collector voltage VCE. The amplitude of vo is larger than that of vi by the voltage gain Av. Microelectronic Circuits - Fifth Edition Sedra/Smith

Amplification using the common-emitter (CE) configuration.
Lowercase letters with lowercase subscripts refer to the ac signal or small–signal component. Capital letters with capital subscripts refer to the dc or constant component of the signal. Lowercase letters with capital subscripts refer to the total signals, dc signal plus the ac signal (small –signal). Microelectronic Circuits - Fifth Edition Sedra/Smith

The output voltage of the amplifier using the CE configuration biased to operate at the quiescent point, Q, in the active region. Microelectronic Circuits - Fifth Edition Sedra/Smith

Small-signal Amplification gain, Av

Figure 5.27 Circuit whose operation is to be analyzed graphically.
sedr42021_0527.jpg Figure Circuit whose operation is to be analyzed graphically. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0528.jpg Figure Graphical construction for the determination of the dc base current in the circuit of Fig Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0529.jpg Figure Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE in the circuit of Fig Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0530a.jpg Figure Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB (see Fig. 5.27). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0531.jpg Figure Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0532.jpg Figure A simple circuit used to illustrate the different modes of operation of the BJT. Microelectronic Circuits - Fifth Edition Sedra/Smith

Switching using the CE configuration as an inverter.
The BJT will be cutoff with vI < 0.5 V. The BJT at the edge of saturation (EOS). The BJT at the edge of saturation (EOS) will have vCB = −0.4 V and vBE = 0.7 V, therefore vC will be 0.3 V (using KVL). Microelectronic Circuits - Fifth Edition Sedra/Smith

The collector and base currents at the edge of saturation (EOS).
The corresponding value of vI required to drive the transistor to the edge-of-saturation. The ratio of IB to IB(EOS) is known as the overdrive factor. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0533.jpg Figure 5.33 Circuit for Example 5.3.

sedr42021_0534a.jpg Figure Analysis of the circuit for Example 5.4: (a) circuit; (b) circuit redrawn to remind the reader of the convention used in this book to show connections to the power supply; (c) analysis with the steps numbered. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0535a.jpg Figure Analysis of the circuit for Example 5.5. Note that the circled numbers indicate the order of the analysis steps. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0536a.jpg Figure Example 5.6: (a) circuit; (b) analysis with the order of the analysis steps indicated by circled numbers. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0537a.jpg Figure Example 5.7: (a) circuit; (b) analysis with the steps indicated by circled numbers. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0538a.jpg Figure Example 5.8: (a) circuit; (b) analysis with the steps indicated by the circled numbers. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0539a.jpg Figure Example 5.9: (a) circuit; (b) analysis with steps numbered. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0540a.jpg Figure 5.40 Circuits for Example 5.10.

sedr42021_0541a.jpg Figure 5.41 Circuits for Example 5.11.

sedr42021_0542a.jpg Figure Example 5.12: (a) circuit; (b) analysis with the steps numbered. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0543a.jpg Figure Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB. Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad.” Neither scheme is recommended. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0544a.jpg Figure Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage divider supplying the base replaced with its Thévenin equivalent. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0545.jpg Figure Biasing the BJT using two power supplies. Resistor RB is needed only if the signal is to be capacitively coupled to the base. Otherwise, the base can be connected directly to ground, or to a grounded signal source, resulting in almost total b-independence of the bias current. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0546a.jpg Figure (a) A common-emitter transistor amplifier biased by a feedback resistor RB. (b) Analysis of the circuit in (a). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0547a.jpg Figure (a) A BJT biased using a constant-current source I. (b) Circuit for implementing the current source I. Microelectronic Circuits - Fifth Edition Sedra/Smith

Small-signal circuit. sedr42021_0548a.jpg Figure (a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis. Microelectronic Circuits - Fifth Edition Sedra/Smith

dc bias point or quiescent point
Q-(VBE, IC ), dc bias point or quiescent point sedr42021_0549.jpg Figure Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimposed on the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Here, ic = gmvbe, where gm is the slope of the iC–vBE curve at the bias point Q. Microelectronic Circuits - Fifth Edition Sedra/Smith

DC currents and voltages, capital letters with capital subscripts.
DC voltages: VBE, VC, VCE, and VCC. DC currents: IC, IE, and IB. AC (small-signal) currents and voltages, lowercase letter with lowercase subscripts. AC voltages: vbe, vc, and vce AC currents: ic, ie, and ib Total currents and voltages, lowercase letters with capital subscripts. Total voltages: vBE = VBE + vbe, vC = VC + vc, and vCE = VCE + vce. Total currents: iC = IC + ic, iE = IE +ie, and iB = IB + ib. Microelectronic Circuits - Fifth Edition Sedra/Smith

Small-signal approximation, valid for vbe < 10 mV.

Transconductance, gm, slope of the iC − vBE characteristic at the quiescent point.
Small-signal input resistance between the base and emitter, rπ, looking into the base. Microelectronic Circuits - Fifth Edition Sedra/Smith

Relationship between re and rπ
Small-signal resistance between base and emitter, looking into the emitter, re. Relationship between re and rπ Microelectronic Circuits - Fifth Edition Sedra/Smith

Voltage gain, Av Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0550.jpg Figure The amplifier circuit of Fig. 5.48(a) with the dc sources (VBE and VCC) eliminated (short circuited). Thus only the signal components are present. Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0551a.jpg Figure Two slightly different versions of the simplified hybrid-p model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier), and that in (b) represents the BJT as a current-controlled current source (a current amplifier). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0552a.jpg Figure Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance re rather than the base resistance rp featured in the hybrid-p model. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0553a.jpg Figure Example 5.14: (a) circuit; (b) dc analysis; (c) small-signal model. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0554a.jpg Figure Signal waveforms in the circuit of Fig Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0555a.jpg Figure Example 5.16: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0556.jpg Figure Distortion in output signal due to transistor cutoff. Note that it is assumed that no distortion due to the transistor nonlinear characteristics is occurring. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0557.jpg Figure Input and output waveforms for the circuit of Fig Observe that this amplifier is noninverting, a property of the common-base configuration. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0558a.jpg Figure The hybrid-p small-signal model, in its two versions, with the resistance ro included. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0559.jpg Figure Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_e0541a.jpg Figure E5.41 Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0560a.jpg Figure (a) A common-emitter amplifier using the structure of Fig (b) Equivalent circuit obtained by replacing the transistor with its hybrid-p model. Microelectronic Circuits - Fifth Edition Sedra/Smith

Input resistance, Rin Input Voltage, vi

Output voltage, vo Voltage Gain, Av

Open-circuit voltage Gain, Avo
Output resistance, Rout Microelectronic Circuits - Fifth Edition Sedra/Smith

Overall voltage gain, Gv

Overall voltage gain, Gv, is highly dependent on β if Rsig >> r π, an undesirable property.
If Rsig << rπ then the overall voltage gain is almost independent of β. Microelectronic Circuits - Fifth Edition Sedra/Smith

Short-circuit current gain, Ais
The common-emitter configuration provides large current and voltage gains, but Rin is relatively low and Rout is relatively high. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0561a.jpg Figure (a) A common-emitter amplifier with an emitter resistance Re. (b) Equivalent circuit obtained by replacing the transistor with its T model. Microelectronic Circuits - Fifth Edition Sedra/Smith

Input resistance, Rin The input resistance looking into the base, Rib, is (β + 1) times the total resistance in the emitter. This is known as the resistance-reflection rule. Microelectronic Circuits - Fifth Edition Sedra/Smith

The voltage gain, Av Microelectronic Circuits - Fifth Edition Sedra/Smith

The open-circuit voltage gain, Avo, found by setting RL = ∞
The open-circuit voltage gain, Avo, is reduced by the factor (1 + gmRe). The factor by which Rib is increased. This allows the designer to trade-off gain for input resistance. Microelectronic Circuits - Fifth Edition Sedra/Smith

The output resistance, Rout, is Rout = RC.
The amplifier is unilateral so Ro = Rout The short-circuit current gain, Ais Microelectronic Circuits - Fifth Edition Sedra/Smith

The overall voltage gain, Gv
The gain is lower than that of the CE amplifier because of the term (β+1)Re in the denominator. However, the gain is also less sensitive to changes in β, a desirable property. Microelectronic Circuits - Fifth Edition Sedra/Smith

Recall that vπ = vi for the CE amplifier
The CE amplifier with emitter resistor can handle larger input signal without nonlinear distortion, since only a fraction of the input signal at the base appears between the base and emitter. Thus, for the same vπ, the signal at the input terminal, vi, can be greater than for the CE amplifier by the factor (1+gmRe) Recall that vπ = vi for the CE amplifier Microelectronic Circuits - Fifth Edition Sedra/Smith

Summary of the characteristics of the CE amplifier with emitter resistor.
The input resistance Rib is increased by the factor (1 + gmRe). The voltage gain from base to collector, Av, is reduced by the factor (1 + gmRe). For the same nonlinear distortion, the input signal vi can be increased by the factor (1 + gmRe). The overall voltage gain is less dependent on the value of β. The high-frequency response is significantly improved. The reduction in gain is the tradeoff for the other improvements. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0562a.jpg Figure (a) A common-base amplifier using the structure of Fig (b) Equivalent circuit obtained by replacing the transistor with its T model. Microelectronic Circuits - Fifth Edition Sedra/Smith

The common-base (CB) amplifier, ignoring Early effect.
The input resistance, Rin = Ri, since unilateral if ro is ignored. The voltage gain, Av Same as CE amplifier without minus sign. Microelectronic Circuits - Fifth Edition Sedra/Smith

The output resistance, Rout = Ro, ignoring Early effect.
The short-circuit current gain Ais. The overall voltage gain, Gv. Microelectronic Circuits - Fifth Edition Sedra/Smith

Summary of Common-Base amplifiers
Very low input resistance. Short-circuit current gain nearly equal to one. Open-circuit voltage gain positive with same magnitude as CE. Relatively high output resistance, compared to input resistance. Excellent high-frequency performance. A significant application of the CB amplifier is as a current buffer. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0563a.jpg Figure (a) An emitter-follower circuit based on the structure of Fig (b) Small-signal equivalent circuit of the emitter follower with the transistor replaced by its T model augmented with ro. (c) The circuit in (b) redrawn to emphasize that ro is in parallel with RL. This simplifies the analysis considerably. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0564.jpg Figure (a) An equivalent circuit of the emitter follower obtained from the circuit in Fig. 5.63(c) by reflecting all resistances in the emitter to the base side. (b) The circuit in (a) after application of Thévenin theorem to the input circuit composed of vsig, Rsig, and RB. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0565.jpg Figure (a) An alternate equivalent circuit of the emitter follower obtained by reflecting all base-circuit resistances to the emitter side. (b) The circuit in (a) after application of Thévenin theorem to the input circuit composed of vsig, Rsig / (b 1 1), and RB / (b 1 1). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0566.jpg Figure Thévenin equivalent circuit of the output of the emitter follower of Fig. 5.63(a). This circuit can be used to find vo and hence the overall voltage gain vo/vsig for any desired RL. Microelectronic Circuits - Fifth Edition Sedra/Smith

Summary of the common-collector properties
The common-collector amplifier exhibits a high input resistance. The common-collector amplifier exhibits a low output resistance. The CC amplifier has a voltage gain smaller than, but very close to unity. The CC amplifier is often used as the last stage in a multi-stage amplifier (voltage buffer). The CC amplifier is also used to connect a high-resistance source to a low-resistance load. Microelectronic Circuits - Fifth Edition Sedra/Smith

Summary and Comparisons
The CE configuration is the best suited for realizing the bulk of the gain required in an amplifier. Including a resistor in the emitter lead of the CE amplifier provides a number of performance improvements at the expense of gain reduction. The low input resistance of the CB amplifier makes it useful only in specific applications, such as high-frequency applications and as a current buffer. The CC is used as a voltage buffer for connecting a high-resistance source to a low-resistance load and as the output stage in a multistage amplifier. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0567.jpg Figure 5.67 The high-frequency hybrid-p model.

sedr42021_0568.jpg Figure Circuit for deriving an expression for hfe(s) ; Ic/Ib. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0569.jpg Figure 5.69 Bode plot for uhfeu.

sedr42021_0570.jpg Figure 5.70 Variation of fT with IC.

sedr42021_0571a.jpg Figure (a) Capacitively coupled common-emitter amplifier. (b) Sketch of the magnitude of the gain of the CE amplifier versus frequency. The graph delineates the three frequency bands relevant to frequency-response determination. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0572a.jpg Figure Determining the high-frequency response of the CE amplifier: (a) equivalent circuit; (b) the circuit of (a) simplified at both the input side and the output side; (c) equivalent circuit with Cm replaced at the input side with the equivalent capacitance Ceq; (d) sketch of the frequency-response plot, which is that of a low-pass STC circuit. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0573a.jpg Figure Analysis of the low-frequency response of the CE amplifier: (a) amplifier circuit with dc sources removed; (b) the effect of CC1 is determined with CE and CC2 assumed to be acting as perfect short circuits; Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0573c.jpg Figure (Continued) (c) the effect of CE is determined with CC1 and CC2 assumed to be acting as perfect short circuits; (d) the effect of CC2 is determined with CC1 and CE assumed to be acting as perfect short circuits; Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0573e.jpg Figure (Continued) (e) sketch of the low-frequency gain under the assumptions that CC1, CE, and CC2 do not interact and that their break (or pole) frequencies are widely separated. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0574.jpg Figure 5.74 Basic BJT digital logic inverter.

sedr42021_0575.jpg Figure Sketch of the voltage transfer characteristic of the inverter circuit of Fig for the case RB 5 10 kW, RC 5 1 kW, b 5 50, and VCC 5 5 V. For the calculation of the coordinates of X and Y, refer to the text. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0576.jpg Figure The minority-carrier charge stored in the base of a saturated transistor can be divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results from driving the transistor deeper into saturation. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_s0576.jpg Figure The transport form of the Ebers-Moll model for an npn BJT. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0577.jpg Figure The SPICE large-signal Ebers-Moll model for an npn BJT. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0578.jpg Figure The PSpice testbench used to demonstrate the dependence of bdc on the collector bias current IC for the Q2N3904 discrete BJT (Example 5.20). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0579.jpg Figure Dependence of bdc on IC (at VCE 5 2 V) in the Q2N3904 discrete BJT (Example 5.20). Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0580.jpg Figure Capture schematic of the CE amplifier in Example 5.21. Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_0581.jpg Figure Frequency response of the CE amplifier in Example 5.21 with Rce = 0 and Rce = 130 . Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_p05020a.jpg Figure P5.20 Microelectronic Circuits - Fifth Edition Sedra/Smith

sedr42021_p05026.jpg Figure P5.26 Microelectronic Circuits - Fifth Edition Sedra/Smith

Bipolar Junction Transistors (BJTs) 1.

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