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EE210 Digital Electronics
Class Lecture 5
May 24, 2008
Solution to:
QUIZ #1
HW #1
In This Class
Bipolar JunctionTransistors (BJTs)
We Will Discuss Following:
5.10 The Basic BJT Digital Logic Inverter
Bipolar Junction Transistors (BJTs)
Bipolar Junction Transistors (BJTs)
Mode
EBJ
CBJ
Cutoff
Reverse
Reverse
Active
Forward
Reverse
Saturation
Forward
Forward
5.3 BJT as Amplifier and Switch
• Common-Emitter CKT
• Input vI = vBE (bias +
signal) at Base and Emitter
• vO = vCE Collector and
Ground
• RC has Two Functions:
Establish voltage at C and
Convert iC to vO or vCE
• vO = vCE = VCC - RC iC
BJT as Amplifier and Switch (Cont…)
• When vBE = vI < 0.5
V Cut Off iC ≈ 0
vO = vCC Segment XY
• When vI > 0.5 V BJT
Active & Cond., iC
Inc. and vO Dec.,
vO = VCC - RC ISevBE/VT
• Exp. Term Gives Rise
to Steep Slope of YZ
Segment
BJT as Amplifier and Switch (Cont…)
• Active mode ends When
vO falls by 0.4 V below
that of the vI.
• CBJ Turns ON and BJT is
in Saturation region at
point Z.
• In Saturation vO = vCE =
VCEsat ≈ 0.1 – 0.2 V.
• ICsat = (VCC – VCEsat)/RC
• In Saturation BJT exhibit
very small RCEsat b/w C
and E
BJT as Amplifier and Switch (Cont…)
• Saturated BJT Provides
Low Res. Path b/w Node
C and Ground and Can
be Thought of a Closed
Switch.
• Cut Off BJT iC ≈ 0 and
Thus acts as an Open
Switch.
The Basic BJT Digital Logic Inverter
We learned in Chap 1 that
Logic Inverter is Most
Fundamental Component of
Digital System
We will use this BJT Ckt to
Realize Logic inverter
Makes use of Cutoff and
Saturation Modes of BJT to
Work as Logic Inverter
In These Modes the Power
Dissipation is Low
For: RB=10 kΩ,
RC=1k Ω, β = 50,
and VCC=5V
VTC → 3 StraightLine Segments
Correspond to BJT
Cutoff, Active and
Saturation
Regions
1. vI=VOL=VCEsat= 0.2V
vO = VOH = VCC = 5V
2. vI=VIL BJT starts to
turn ON, Thus
VIL=0.7V
3. VOL< vI < VIH : Active. Operate as Amp.
Av ≈ - βRC/RB = - 50*1/10 = - 5 V/V
4. vI=VIH BJT enters into Saturation. VIH is value of vI
at which BJT is at the Edge of Saturation.
IB = ((VCC-VCEsat)/RC)/β = 0.096 mA
VIH = IBRB + VBE =1.66V
5. vI=VOH=5V Deep Saturation vO = VCEsat ≈ 0.2V and
Βforced= 11
6. Noise Margins:
NMH=VOH-VIH=5-1.66=3.34V
NML=VIL-VOL=0.7-0.2=0.5V
Margins Vastly Different
Inverter Less Than Ideal
7. The Gain in Transition Region
(Slope) computed from X and Y
points:
Av= - (5 - 0.2)/(1.66 - 0.7) = - 5 V/V
Saturated vs Nonsaturated BJT Digital Ckts
• Inverter we just discussed belongs to
saturated variety of BJT Digital ckts – TTL
• Some TTL versions are in use, Generally
Saturated Bipolar Digital ckts are no more
Technology of Choice in Digital System
Design
• The Reason Being the Speed of Operation
• LongTtime Delay to Turn OFF a Saturated
BJT
• Minority carrier distribution
in base region of saturated
BJT: Blue Triangle gradient
gives the diffusion current
across base
• Grey Rectangle Causes
Transistor to be Driven
Deeper into Saturation
• To Achieve High Speeds
the BJT Should not
Saturate – Current Mode
Logic or ECL (Chap 11)
Home Work No. 3 (Due May 12, 2007)
1.
2.
3.
4.
Problem 3. 6
Problem 3.10
Problem 5.168
Problem 5.171
In Next Class
We Will Discuss:
Chap 4 MOS Field-Effect Transistors
4.1 Device Structure and Physical
Operation
4.2 Current-Voltage Characteristics