microelectronic circuits, kyung hee univ. spring, 2016...
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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