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4.1.3 Structure of Actual Transistors• Figure 4.7 shows a more realistic BJT cross-section• Collector virtually surrounds entire emitter region

• This makes it difficult for electrons injected into base to escape collection• Unity α, large β

• Device is not symmetrical• As such, emitter and collector cannot be interchanged• Device is uni-directional• 𝐼𝐼𝑆𝑆𝑆𝑆 is 10~100 times larger than 𝐼𝐼𝑆𝑆𝑆𝑆

Figure 4.7: Cross-section of an npn BJT

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4.1.4 Operation in Saturation Mode

• For BJT to operate in active mode, CBJ must be reverse biased

• However, for small values of forward-bias, a pn-junction does not operate effectively

• As such, active mode operation of npn-transistor may be maintained for vCB down to approximately -0.4V

• Only after this point will “diode” begin to really conduct

• Fig 4.8

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4.1.4 Operation in Saturation Mode

this termsplays biggerrole as

exceeds 0.

/

4V

/collector current (eq6.14) :

in saturation region

base current (eq6.15)

in saturation

BC TB

BC

E

C

T

SI

v Vv VC S SC

v

i I I

=

− − − − − − − − − − − − − − − − − − − − − − − − −

= −

− − −

e e

As is increased, the value of is forced lower and lowe

/

r.

/: region

(eq6.16) forced :

B

BC TBE T

C

v Vv VSB SC

Cforced

B saturation

v

Ii I

ii

β

β

β

β β

− − − − − − − − − − −

= +

=

− − − − − − − − − − − − − − −

≤

− −

e e

(eq4.14)

(eq4.15)

(eq4.16)

Fig 4.5(c)Fig 4.9

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4.1.4 Operation in Saturation Mode

• Two questions must be asked to determine whether BJT is in saturation mode, or not:

• Is the CBJ forward-biased by more than 0.4V?• Is the ratio iC/iB less than β.?

• Collector-to-emitter voltage (𝑣𝑣𝑆𝑆𝑆𝑆) of a saturated transistor• 𝑉𝑉𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑉𝑉𝐵𝐵𝑆𝑆 − 𝑉𝑉𝐵𝐵𝑆𝑆• 𝑉𝑉𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 ≅ 0.1 𝑡𝑡𝑡𝑡 0.3 𝑉𝑉 : CBJ has a larger area than the EBJ• 𝑉𝑉𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 = 0.3 𝑉𝑉 at the edge of saturation• 𝑉𝑉𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 = 0.2 𝑉𝑉 in deep saturation

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4.1.5 The pnp Transistor

Figure 6.10: Current flow in a pnp transistor biased to operate in the active mode.

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4.1.5 The pnp Transistor

Figure 4.11: Two large-signal models for the pnp transistor operating in the active mode

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4.2 Current-Voltage Characteristics

Figure 4.12: Circuit symbols for BJTs.

Figure 4.13: Voltage polarities and current flow in transistors

biased in the active mode.

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4.2.1 Circuit Symbols and Conventions

Figure 4.14 Graphical representation of the conditions for operating the BJT in the active mode and in the saturation mode.

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4.2.1 Circuit Symbols and Conventions

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Collector-Base Reverse Current (ICB0)• Previously, small reverse current was ignored

• Carried by thermally-generated minority carriers

• However, it does deserve to be addressed• The collector-base junction current (ICBO) is normally in the nano-

ampere range• Many times higher than its theoretically-predicted value• Contains a substantial leakage component• Dependent on vCB

• Depend strongly on temperature (doubling every 10 °C rise)

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4.2.2 Graphical Representation of Transistor Characteristics

Figure 4.16/17: (left) The iC-vBE characteristic for an npn transistor. (right) Effect of temperature on the iC-vBE characteristic. Voltage polarities and current flow in

transistors biased in the active mode.

/

BE Tv VC Si I= e -2mV for each rise of 1 °C

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4.2.3 Dependence of iC on Collector Voltage – The Early Effect

• When operated in active region, practical BJT’s show some dependence of collector current on collector voltage

• As such, iC-vCB characteristic is not “straight”

Common emittercharacteristics

Early voltage(10-100V)

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4.2.4 Common-Emitter Characteristics• The Common-Emitter Current Gain

• A second way to quantify β is changing base current by ∆iB and measuingincremental ∆iC

• The Saturation Voltage VCEsat and Saturation Resistance (𝑅𝑅𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠 ≡ �𝜕𝜕𝑣𝑣𝐶𝐶𝐶𝐶𝜕𝜕𝑖𝑖𝐶𝐶 𝑖𝑖𝐵𝐵=𝐼𝐼𝐵𝐵,𝑖𝑖𝐶𝐶=𝐼𝐼𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶

)

• 𝑅𝑅𝑆𝑆𝑆𝑆𝑠𝑠𝑠𝑠𝑠𝑠: few ohms to a few tens of ohms

Figure 4.20: Common-emitter characteristics. (a) Basic CE circuit; note that in (b) the horizontal scale is expanded around the origin to show the saturation region in some detail. A much greater expansion of the saturation region is shown in (c).

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Figure 4.21: A simplified equivalent-circuit model of the saturated transistor.

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4.3 BJT Circuits at DC• Apply only dc voltages• 𝑉𝑉𝐵𝐵𝑆𝑆 : 0.7V for conducting transistor• 𝑉𝑉𝑆𝑆𝑆𝑆 : 0.2V for saturated transistor• Neglect the Early effect

Figure 4.21: A simplified equivalent-circuit model of the saturated transistor.

• In which mode is the transistor operating?• Is the CBJ forward-biased by more than 0.4V?• Is the ratio iC/iB less than β.?

• Important!!!• Example 4.4~4.12

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4.3 BJT Circuits at DC