The shape of the waveform will be the same, but its level is shifted either upward or downward. The values of the resistor R and capacitor C affect

Transcription

1 Diode as Clamper A clamping circuit is used to place either the positive or negative peak of a signal at a desired level. The dc component is simply added or subtracted to/from the input signal. The clamper is also referred to as an IC restorer and ac signal level shifter. A clamp circuit adds the positive or negative dc component to the input signal so as to push it either on the positive side (positive clamper) or negative side (negative clamper). For a clamping circuit at least three components a diode, a capacitor and a resistor are required. Sometimes an independent dc supply is also required to cause an additional shift

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3 The shape of the waveform will be the same, but its level is shifted either upward or downward. The values of the resistor R and capacitor C affect the waveform. The values for the resistor R and capacitor C should be determined from the time constant equation of the circuit, t = RC. The values must be large enough to make sure that the voltage across the capacitor C does not change significantly during the time interval the diode is non-conducting. In a good clamper circuit, the circuit time constant t = RC should be at least ten times the time period of the input signal voltage

4 RC charging circuit V c =V s (1-e -t/rc )

5 Negative clamper

6 BJT: Bipolar Junction Transistor 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.

7 Two individual signal diodes back-to-back, give two PN-junctions connected together in series that share a common p or n terminal. The fusion of these two diodes produces a three layer, two junction, three terminal device Bipolar Junction Transistor, or BJT. Revolutionized electronics industry from 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 semiconductor materials.

8 Base is very narrow Emitter ~ /cm 3 Base ~ /cm 3 Collector ~ /cm 3 Electrically unsymmetrical Four biasing conditions Depending on forward or reverse bias

9 Transistor Resistance Values Between Transistor Terminals PNP NPN Collector Emitter R HIGH R HIGH Collector Base R LOW R HIGH Emitter Collector R HIGH R HIGH Emitter Base R LOW R HIGH Base Collector R HIGH R LOW Base Emitter R HIGH R LOW

10 Cross Section of Integrated Circuit npn Transistor Complex structure

11 npn BJT in Forward-Active

12 Understanding the current flow in npn transistor Emitter current BE VT i = I 0 e 1 I 0 E E v E e v BE V T Depends on cross-section ( A) Due to large concentration gradient, electrons injected from emitter diffuse across the base in to B-C space region where EF sweeps them into collector, forming collector current which depends on the B-E voltage i c v BE VT = I e 1 S Collector current I S e v BE V T

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14 Collector Base Emitter Movement of electrons and holes in npn transistor

15 i c <i E ; i c = α i E, 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) i B e v BE VT

16 Electrons and Holes in pnp BJT

17 Circuit Symbols and Current Conventions

18 Common emitter current gain Defined as ratio of collector and base current i i c B = β Key parameter Assumed as constant for any given transistor 50<β<300 Highly dependent on fabrication

19 Current Relationships i i i i E C E C = = = i C βi (1 + βi = αi + B E i α β = 1 α B B ) β α = 1+ β

20 Three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement. Common Base Configuration has Voltage Gain but no Current Gain. Common Emitter Configuration has both Current and Voltage Gain. Common Collector Configuration has Current Gain but no Voltage Gain.

21 Common-Emitter Configurations

22 Common-Base Configuration Current source provide emitter current

23 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

24 Current-Voltage Characteristics of a Common-Base Circuit Saturation Cut-off B-C/C-B junction forward biased, transistor no longer in forward active mode B-C/C-B, reverse biased, i c ~ i E common-base: ideal constant current-source

25 Current-Voltage Characteristics of a Common- Emitter Circuit V CE V BE (on) Finite slope to the curves -due to base-width modulation - observed by J.M. Early - called as Early effect

26 Common-base Common-emitter

27 Early Voltage/Finite Output Resistance r o V I A C i c vbe v = I e VT CE s VA = i c r v o CE v = const. BE

28 DC analysis of transistor circuits Use of piecewise linear model of pn junction Assume transistor in forward active mode Common emitter configuration V CE >V BE (on)

29 DC Equivalent Circuit for npn Common Emitter

30 Example 1.

31 DC Equivalent Circuit for pnp Common Emitter

32 Load Line Helps to visualize the characteristics of a transistor circuit

33 Saturation mode V BB I B Q-point As base current continue to increase, a point is reached where the collector current I C can no longer increase. - Transistor is biased in saturation mode I C /I B < β

34 Example 2: circuit Circuit showing values with an assumption of forward active mode Circuit showing values with an assumption of saturation mode

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36 Problem-Solving Technique: Bipolar DC Analysis 1. Assume that the transistor is biased in forward active mode a. V BE = V BE (on), I B > 0, & I C = βi B 2. Analyze linear circuit. 3. Evaluate the resulting state of transistor. a. If V CE > V CE (sat), assumption is correct b. If I B < 0, transistor likely in cutoff c. If V CE < 0, transistor likely in saturation 4. If initial assumption is incorrect, make new assumption and return to Step 2.

37 Voltage Transfer Characteristic for npn Circuit

38 Voltage Transfer Characteristic for pnp Circuit

39 BJT Biasing Single base resistor biasing Voltage divider biasing Biasing stability

40 Single Base Resistor Biasing

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42 Common Emitter with Voltage Divider Biasing and Emitter Resistor R B is replaced by R 1 and R 2. Emitter resistor is added. AC signal is coupled through C c. Analyzed by Thevenin equivalent circuit for the base circuit. TH = [ R2 /( R1 + R2 V CC V ) R TH = R 1 R 2 (1)

43 Use of KVL in B-E loop V = I R + V + TH BQ TH BE ( on) I EQRE (2) If the transistor is biased in forward-active mode I EQ ( β ) I BQ = 1+ (3) From (2), the base current can be calculated. Hence, the collector current can be found The design requirement for bias stability is R TH ( β ) R E << 1+ The collector current is therefore, I CQ β ( V ( )) TH VBE on ( 1+ β ) R E

44 β >> 1, β/(1+β) ~ 1 I CQ ( V V ( on) ) TH R BE E

45 Example: