Transistors Transfer Resistor Chapter 9 Bipolar Transistors Base
Transistors Transfer Resistor Chapter 9
Bipolar Transistors Base Collector Emitter Two PN junctions joined together Two types available – NPN and PNP The regions (from top to bottom) are called the collector (C), the base (B), and the emitter (E)
Operation q Begin by reverse biasing the CB junction Ø Here we are showing an NPN transistor as an example q Now we apply a small forward bias on the emitter-base junction Ø Electrons are pushed into the base, which then quickly flow to the collector Ø The result is a large emitter-collector electron current (conventional current is C-E) which is maintained by a small E-B voltage q Some of the electrons pushed into the base by the forward bias E-B voltage end up depleting holes in that junction Ø This would eventually destroy the junction if we didn’t replenish the holes Ø The electrons that might do this are drawn off as a base current
Currents
Conventional View
Origin of the names q the Emitter 'emits' the electrons which pass through the device q the Collector 'collects' them again once they've passed through the Base q. . . and the Base? . . .
Original Manufacture
Base Thickness q The thickness of the unmodified Base region has to be just right. Ø Too thin, and the Base would essentially vanish. The Emitter and Collector would then form a continuous piece of semiconductor, so current would flow between them whatever the base potential. Ø Too thick, and electrons entering the Base from the Emitter wouldn't notice the Collector as it would be too far away. So then, the current would all be between the Emitter and the Base, and there'd be no Emitter. Collector current.
Amplification Properties q The C-B voltage junction operates near breakdown. Ø This ensures that a small E-B voltage causes avalanche Ø Large current through the device
Common Base NPN
Common Emitter NPN
Common Collector NPN How does IC vary with VCE for various IB? Note that both dc sources are variable Set VBB to establish a certain IB
Collector Characteristic Curve q If VCC = 0, then IC = 0 and VCE = 0 q As VCC ↑ both VCE and IC ↑ q When VCE 0. 7 V, base-collector becomes reverse-biased and IC reaches full value (IC = b. IB) q IC ~ constant as VCE ↑. There is a slight increase of IC due to the widening of the depletion zone (BC) giving fewer holes for recombinations with e¯ in base. q Since IC = b. IB, different base currents produce different IC plateaus.
NPN Characteristic Curves
PNP Characteristic Curves
Load Line Slope of the load line is 1/RL For a constant load, stepping IB gives different currents (IC) predicted by where the load line crosses the characteristic curve. IC = b. IBworks so long as the load line intersects on the plateau region of the curve.
Saturation and Cut-off Note that the load line intersects the 75 m. A curve below the plateau region. This is saturation and IC = b. IB doesn’t work in this region.
Example q We adjust the base current to 200 m. A and note that this transistor has a b = 100 Ø Then IC = b. IB = 100(200 X 10 -6 A) = 20 m. A q Notice that we can use Kirchhoff’s voltage law around the right side of the circuit Ø VCE = VCC – ICRC = 10 V – (20 m. A)(220 W) = 10 V – 4. 4 V = 5. 6 V
Example q Now adjust IB to 300 m. A Ø Now we get IC = 30 m. A Ø And VCE = 10 V – (30 m. A)(220 W) = 3. 4 V q Finally, adjust IB = 400 m. A Ø IB = 40 m. A and VCE = 1. 2 V
Plot the load line VCE IC 5. 6 V 20 m. A 3. 4 V 30 m. A 1. 2 V 40 m. A
Gain as a function of IC As temperature increases, the gain increases for all current values.
Operating Limits q There will be a limit on the dissipated power Ø PD(max) = VCEIC Ø VCE and IC were the parameters plotted on the characteristic curve. w If there is a voltage limit (VCE(max)), then you can compute the IC that results w If there is a current limit (IC(max)), then you can compute the VCE that results
Example q Assume PD(max) = 0. 5 W VCE(max) = 20 V IC(max) = 50 m. A PD(max) VCE 0. 5 W IC 5 V 100 m. A 10 50 15 33 20 25
Operating Range
Voltage Amplifiers Common Base PNP Now we have added an ac source The biasing of the junctions are: BE is forward biased by VBB - thus a small resistance BC is reverse biased by VCC – and a large resistance Since IB is small, IC IE
Equivalent ac Circuit r. E = internal ac emitter resistance IE = Vin/r. E (Ohm’s Law) Vout = ICRC IERC Recall the name – transfer resistor
Current Gains q Common Base Ø a = IC/IE < 1 q Common Emitter Ø b = IC/IB
Example q If b = 50, then a = 50/51 = 0. 98 Ø Recall a < 1 q Rearranging, b = a + ab b(1 -a) = a b = a/(1 -a)
Transistors as Switches
The operating points We can control the base current using VBB (we don’t actually use a physical switch). The circuit then acts as a high speed switch.
Details q In Cut-off Ø All currents are zero and VCE = VCC q In Saturation Ø IB big enough to produce IC(sat) b. IB q Using Kirchhoff’s Voltage Law through the ground loop Ø VCC = VCE(sat) + IC(sat)RC Ø but VCE(sat) is very small (few tenths), so Ø IC(sat) VCC/RC
Example a) What is VCE when Vin = 0 V? Ans. VCE = VCC = 10 V b) What minimum value of IB is required to saturate the transistor if b = 200? Take VCE(sat) = 0 V IC(sat) VCC/RC = 10 V/1000 W = 10 m. A Then, IB = IC(sat)/b = 10 m. A/200 = 0. 05 m. A
Example LED If a square wave is input for VBB, then the LED will be on when the input is high, and off when the input is low.
Transistors with ac Input Assume that b is such that IC varies between 20 and 40 m. A. The transistor is constantly changing curves along the load line.
Pt. A corresponds to the positive peak. Pt. B corresponds to the negative peak. This graph shows ideal operation.
Distortion q The location of the point Q (size of the dc source on input) may cause an operating point to lie outside of the active range. Driven to saturation Driven into Cutoff
Base Biasing q It is usually not necessary to provide two sources for biasing the transistor. The red arrows follow the base-emitter part of the circuit, which contains the resistor RB. The voltage drop across RB is VCC – VBE (Kirchhoff’s Voltage Law). The base current is then… and IC = b. IB
Base Biasing q Use Kirchhoff’s Voltage Law on the black arrowed loop of the circuit VCC = ICRC + VCE So, VCE = VCC – ICRC VCE = VCC – b. IBRC q Disadvantge Ø b occurs in the equation for both VCE and IC Ø But b varies – thus so do VCE and IC Ø This shifts the Q-point (b-dpendent)
Example q Let RC = 560 W RB = 100 k. W VCC = +12 V @ 25 °C b = 100 @ 75 °C b = 150 @ 75 °C IB is the same IC = 16. 95 m. A VCE = 2. 51 V IC increases by 50% VCE decreases by 56%
Transistor Amplifiers q Amplification Ø The process of increasing the strength of a signal. Ø The result of controlling a relatively large quantity of current (output) with a small quantity of current (input). q Amplifier Ø Device use to increase the current, voltage, or power of the input signal without appreciably altering the essential quality.
Class A q Entire input waveform is faithfully reproduced. q Transistor spends its entire time in the active mode Ø Never reaches either cutoff or saturation. Ø Drive the transistor exactly halfway between cutoff and saturation. Ø Transistor is always on – always dissipating power – can be quite inefficient
Class A
Class B q No DC bias voltage Ø The transistor spends half its time in active mode and the other half in cutoff
Push-pull Pair Transistor Q 1 "pushes" (drives the output voltage in a positive direction with respect to ground), while transistor Q 2 "pulls" the output voltage (in a negative direction, toward 0 volts with respect to ground). Individually, each of these transistors is operating in class B mode, active only for one-half of the input waveform cycle. Together, however, they function as a team to produce an output waveform identical in shape to the input waveform.
Class AB q Between Class A (100% operation) and Class B (50% operation).
Class C IC flows for less than half then cycle. Usually get more gain in Class B and C, but more distortion
Common Emitter Transistor Amplifier Notice that VBB forward biases the emitter-base junction and dc current flows through the circuit at all times The class of the amplifier is determined by VBB with respect to the input signal. Signal that adds to VBB causes transistor current to increase Signal that subtracts from VBB causes transistor current to decrease
Details q At positive peak of input, VBB is adding to the input q Resistance in the transistor is reduced q Current in the circuit increases q Larger current means more voltage drop across RC (VRC = IRC) q Larger voltage drop across RC leaves less voltage to be dropped across the transistor q We take the output VCE – as input increases, VCE decreases.
More details q As the input goes to the negative peak Ø Transistor resistance increases Ø Less current flows Ø Less voltage is dropped across RC Ø More voltage can be dropped across C-E q The result is a phase reversal Ø Feature of the common emitter amplifier q The closer VBB is to VCC, the larger the transistor current.
PNP Common Emitter Amplifier
NPN Common Base Transistor Amplifier Signal that adds to VBB causes transistor current to increase Signal that subtracts from VBB causes transistor current to decrease • At positive peak of input, VBB is adding to the input • Resistance in the transistor is reduced • Current in the circuit increases • Larger current means more voltage drop across RC (VRC = IRC) • Collector current increases • No phase reversal
PNP Common Base Amplifier
NPN Common Collector Transistor Amplifier Also called an Emitter Follower circuit – output on emitter is almost a replica of the input Input is across the C-B junction – this is reversed biased and the impedance is high Output is across the B-E junction – this is forward biased and the impedance is low. Current gain is high but voltage gain is low.
PNP Common Collector Transistor Amplifier
Gain Factors Usually given for common base amplifier Usually given for common emitter amplifier Usually given for common collector amplifier
Gamma q Recall from Kirchhoff’s Current Law Ø IB + I C = I E Ex. For b = 100 a = b/(1+b) = 0. 99 g = 1 + b = 101
Bringing it Together Type Common Base Common Emitter Common Collector Relation between input/output phase 0° 180° 0° Voltage Gain High Medium Low Current Gain Low (a) Medium (b) High (g) Power Gain Low High Medium Input Z Low Medium High Output Z High Medium Low
Hybrid Parameters Condition hi Input resistance Output shorted hr Voltage feedback ratio Input open hf Forward current gain Output shorted ho Output conductance Input open Second subscript indicates common base (b), common emitter (e), or common collector (c)
Hybrid Parameters =b = Slope of curve
Hybrid Parameters hie = VB/IB Ohm’s Law hie =input impedance hre = VB/VC
Hybrid Parameters hfe = IC/IB Equivalent of b hoe = IC/VC
Various Forms Common Emitter (e) Common Base (b) Common Collector (c) hi (ohms) VB/IB VE/IB VB/IB hr (unitless) VB/VC VE/VC VB/VE hf (unitless) IC/IB IC/IE IE/IB ho (watts) IC V C IE V E
Pin-outs No standard – look at the spec sheet or the case
Loudness q When the energy (intensity) of the sound increases by a factor of 10, the loudness increases by 1 bel Ø Named for A. G. Bell Ø One bel is a large unit and we use 1/10 th bel, or decibels q When the energy (intensity) of the sound increases by a factor of 10, the loudness increases by 10 d. B
Decibel Scale q For intensities Ø L = 10 log(I/Io) q For energies Ø L = 10 log(E/Eo) q For amplitudes Ø L = 20 log(A/Ao)
Threshold of Hearing q The Io or Eo or Ao refers to the intensity, energy, or amplitude of the sound wave for the threshold of hearing Ø Io = 10 -12 W/m 2 Ø Loudness levels always compared to threshold w Relative measure
Common Loud Sounds 160 Jet engine - close up 150 Snare drums played hard at 6 inches away Trumpet peaks at 5 inches away 140 Rock singer screaming in microphone (lips on mic) 130 Pneumatic (jack) hammer Planes on airport runway Cymbal crash 120 Threshold of pain - Piccolo strongly played Fender guitar amplifier, full volume at 10 inches away Power tools 110 Subway (not the sandwich shop) 100 Flute in players right ear - Violin in players left ear
Common Quieter Sounds 90 Heavy truck traffic Chamber music 80 Typical home stereo listening level Acoustic guitar, played with finger at 1 foot away Average factory 70 Busy street Small orchestra 60 Average office noise 50 Quiet conversation 40 Quiet office 30 Quiet living room 20 Conversational speech at 1 foot away 10 Quiet recording studio 0 Threshold of hearing for healthy youths
The Math l 1 = 10 log(I 1/Io) l 2 = 10 log(I 2/Io) l 2 – l 1 = Dl = 10(log I 2 – log Io – log I 1 + log Io) = 10(log I 2 – log I 1) l 2 – l 1 = Dl = 10 log(I 2/I 1) Threshold of Hearing when I = Io l = 0 d. B Threshold of Pain when I 1012 Io l = 120 d. B
Example q A loudspeaker produces loudness rated at 90 d. B (l 1) at a distance of 4 ft (d 1). How far can the sound travel (d 2) and still give a loudness at the listener’s ear of 40 d. B (l 2 - conversation at 3 ft. )? Sound follows the inverse square law I 1/I 2 = d 22/d 12 Dl = 50 d. B = 10 log(I 2/I 1) = 5 which means I 2/I 1 = 105 If d 1 = 4 ft, then d 22 = (I 1/I 2) d 12 = 105 (4 ft)2 d 2 = 1260 ft (about ¼ mile)
Common Emitter Current Gain 0 d. B -3 d. B hfe Frequency q For the -3 d. B point Ø Dl = 3 d. B = 10 log (I 1/I 2) w I 1/I 2 = P 1/P 2 w so 3 d. B below initial level mean half the power
Why do Frequency limits occur? q It takes a certain time for e- to travel from emitter to collector (transit time) q If frequency is too high, applied current varies too rapidly q Electrons may be unable to dislodge rapidly enough to move from E to C before current surges in the other direction. Making the base thinner reduces transit time and improves frequency response
Interelement Capacitance q As reverse bias increases on the C-B junction, the depletion zone increases and C decreases (C = e. A/d and d increasing). q As emitter current increases, C increases (d decreasing). q If capacitance changes, so does capacitive reactance Ø Increasing C decreases XC
Feedback q Small base current provides a path back to input Ø If the feedback voltage aids the input voltage, then it is positive (regenerative) feedback Ø If the feedback is too large, the amplifier will oscillate
Superheterodyne Receiver
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