Diode as Clamper
• A clamping circuit is used to place either the positiveor negative peak of a signal at a desired level.
• The dc component is simply added or subtractedto/from the input signal. The clamper is also referredto as an IC restorer and ac signal level shifter.
• A clamp circuit adds the positive or negative dccomponent to the input signal so as to push it eitheron the positive side (positive clamper) or negativeside (negative clamper).
• For a clamping circuit at least three components — adiode, a capacitor and a resistor are required.Sometimes an independent dc supply is alsorequired to cause an additional shift
• The shape of the waveform will be the same, butits level is shifted either upward or downward.
• The values of the resistor R and capacitor Caffect the waveform.
• The values for the resistor R and capacitor Cshould be determined from the time constantequation of the circuit, t = RC. The values mustbe large enough to make sure that the voltageacross the capacitor C does not changesignificantly during the time interval the diode isnon-conducting. In a good clamper circuit, thecircuit time constant t = RC should be at leastten times the time period of the input signalvoltage
RC charging circuit
Vc=Vs (1-e-t/RC)
Negative clamper
• Discuss the physical structure and operation of the bipolar junction transistor.
• Understand the dc analysis and design techniques of bipolar transistor circuits.
• Investigate various dc biasing schemes of bipolar transistor circuits, including integrated circuit biasing.
BJT: Bipolar Junction Transistor
• Two individual signal diodes back-to-back,give two PN-junctions connected together inseries that share a common p or n terminal.The fusion of these two diodes produces athree layer, two junction, three terminaldevice Bipolar Junction Transistor, or BJT.
• Revolutionized electronics industry from1950-1990
• First BJT was invented in 1947• Responsible for computer age as well as
modern era communication.• Transistors are three terminal active
devices made from different semiconductormaterials.
• Base is very narrow– Emitter ~ 1019 /cm3
– Base ~ 1017 /cm3
– Collector ~ 1015 /cm3
• Electrically unsymmetrical• Four biasing conditions
– Depending on forward or reverse bias
Transistor Resistance Values
Between Transistor Terminals PNP NPN
Collector Emitter RHIGH RHIGH
Collector Base RLOW RHIGH
Emitter Collector RHIGH RHIGH
Emitter Base RLOW RHIGH
Base Collector RHIGH RLOW
Base Emitter RHIGH RLOW
Cross Section of Integrated Circuit npn Transistor
Complex structure
npn BJT in Forward-Active
Understanding the current flow in npn transistor
TBE
TBE
Vv
SV
v
Sc eIeIi ≈
−= 1
Emitter current
Depends on cross-section (10-12 -10-15 A)
TBE
TBE
Vv
EV
v
EE eIeIi 00 1 ≈
−=
• Due to large concentration gradient,electrons injected from emitter diffuse acrossthe base in to B-C space region where EFsweeps them into collector, forming collectorcurrent which depends on the B-E voltage
Collector current
Collector
Base
Emitter
Movement of electrons and holes in npn transistor
• ic <iE ; ic = α iE , where α is called as common base current gain and is less than unity.
• Base Current:– Two components
• Flow of holes• Recombination with majority carriers
(recombination current)
TBE
Vv
B ei ∝
Electrons and Holes in pnp BJT
Circuit Symbols and Current Conventions
Common emitter current gain• Defined as ratio of collector and base current
• Key parameter– Assumed as constant for any given transistor
• 50<β<300• Highly dependent on fabrication
β=B
c
ii
Current Relationships
ααβ
αβ
β
−=
=+=
=+=
1
)1(
EC
BE
BC
BCE
iiii
iiiii
ββα+
=1
• Three possible ways to connect it within anelectronic circuit with one terminal beingcommon to both the input and output.
• Each method of connection respondingdifferently to its input signal within a circuit asthe static characteristics of the transistor varywith each circuit arrangement.– Common Base Configuration – has Voltage Gain
but no Current Gain.
– Common Emitter Configuration – has both Currentand Voltage Gain.
– Common Collector Configuration – has CurrentGain but no Voltage Gain.
Common-Emitter Configurations
Common-Base Configuration
• Current source provide emitter current
Modes of Operation
• Forward-Active– B-E junction is forward biased– B-C junction is reverse biased
• Saturation– B-E and B-C junctions are forward biased
• Cut-Off– B-E and B-C junctions are reverse biased
• Inverse-Active (or Reverse-Active)– B-E junction is reverse biased– B-C junction is forward biased
Current-Voltage Characteristics of a Common-Base Circuit
• B-C/C-B junction forward biased, transistor no longer in forward active mode• B-C/C-B, reverse biased, ic ~ iE• common-base: ideal constant current-source
Cut-off
Saturation
Current-Voltage Characteristics of a Common-Emitter Circuit
VCE ≥ VBE (on)
Finite slope to the curves-due to base-width modulation
- observed by J.M. Early- called as Early effect
Common-base Common-emitter
Early Voltage/Finite Output Resistance
+=
A
CEVv
sc VveIi T
BE
1.
1
constvCE
c
oBE
vi
r=
∂∂
=
C
Ao I
Vr ≅
DC analysis of transistor circuits
• Use of piecewise linear model of pn junction• Assume transistor in forward active mode• Common emitter configuration• VCE>VBE (on)
DC Equivalent Circuit for npn Common Emitter
Example 1.
DC Equivalent Circuit for pnp Common Emitter
Load LineHelps to visualize the characteristics of a transistor circuit
Saturation mode
VBB IB Q-point
As base current continue toincrease, a point is reachedwhere the collector current ICcan no longer increase.
- Transistor is biased insaturation mode
IC/IB < β
Example 2:
circuit Circuit showing values with an assumption of forward active mode
Circuit showing values with an assumption of saturation mode
Problem-Solving Technique: Bipolar DC Analysis
1. Assume that the transistor is biased in forward active modea. VBE = VBE(on), IB > 0, & IC = βIB
2. Analyze ‘linear’ circuit.3. Evaluate the resulting state of transistor.
a. If VCE > VCE(sat), assumption is correctb. If IB < 0, transistor likely in cutoffc. If VCE < 0, transistor likely in saturation
4. If initial assumption is incorrect, make new assumption and return to Step 2.
Voltage Transfer Characteristic for npn Circuit
Voltage Transfer Characteristic for pnp Circuit
BJT Biasing
• Single base resistor biasing• Voltage divider biasing
– Biasing stability
Single Base Resistor Biasing
Common Emitter with Voltage Divider Biasing and Emitter Resistor
• RB is replaced by R1 and R2.
• Emitter resistor is added. AC signal is coupled through Cc.
• Analyzed by Thevenin equivalent circuit for the base circuit.
CCTH VRRRV )/([ 212 +=
21 || RRRTH = (1)
Use of KVL in B-E loop
( ) EEQBETHBQTH RIonVRIV ++=
If the transistor is biased in forward-active mode
( ) BQEQ II β+= 1
(2)
(3)
From (2), the base current can be calculated. Hence, the collector current can be found
The design requirement for bias stability is
The collector current is therefore,
( ) ETH RR β+<< 1
( )( )( ) E
BETHCQ R
onVVIβ
β+−
≅1
( )( )E
BETHCQ R
onVVI −≅
β >> 1, β/(1+β) ~ 1
Example: