Transcript Chapter 3
Electronic Troubleshooting Chapter 3 Bipolar Transistors • Most devices still require some individual (discrete) transistors • Used to customize operations • Interface to external devices • Understanding their operation is critical to understanding the operation of digital or analog ICs Construction & Symbols • They use two PN junctions • Two versions, but both have narrow center regions (aka Base) • NPN transistors • Base is P material (holes ) are the primary charge carriers and the collector and emitter regions are made fro n materials Construction & Symbols • They use two PN junctions • Two versions ( continued ) • NPN transistors • Usually used with positive power supplies • The arrow on the symbol indicates the direction of conventional current flow • PNP transistors • Usually used with negative power supplies • The arrow on the symbol indicates the direction of conventional current flow • Construction • Most are made from silicon, but some are made from germanium NPN Transistor Current Flow NPN Current Flow • With the base emitter PN junction forward biased • Current flows • Emitter – Collector flow is much larger than Emitter-Base current • The Emitter-Base current controls the amount of EmitterCollector current. • Key Equations: β = hFE = IC/IB IE = IC + IB IC = β IB β values range from 20 - 200 if β is large, IE ~ IC NPN Input Characteristics NPN Current Flow • Sample problem • IB = 10µA, β = 99 • Find: IC and IE I C I B 99 10A 990 A I E I C I B 990A 10A • Thus with a large β, IC ~ IE w/error = 1% • In class Exercises • If a transistor has β = 150 and IB = 6mA, what is IC. • What value of base current will make IE = 2mA if β = 80 Typical Transistor Packages Acts like a Variable Resistor • Walk-through • Set R = to the lamp’s resistance and assume the lamp lights dimly • Reducing R brightens the lamp & increases I • Increasing R dims the lamp & decreases I • Replace R with a transistor • Lamp brightness is controlled by IB which controls IC • Decreasing VBB to 0v will decrease IC to approx 0A • Small leakage current will still exist and it varies with temperature Equivalent Circuits • Transistor cutoff and saturated equivalent circuits • In cutoff IC = 0A and all of VCC is dropped across the transistor • In Saturation only an insignificant amount of VCC is dropped across the transistor. Biasing the Transistor • Biased Transistors • When used as an amplifier the transistor is operated between cutoff and saturation • Output signals shouldn’t go to saturation or cutoff • Base-biased Circuit • If RB is adjusted for IC = 1mA VCE VCC I C RC 12 1mAx5000 7v • Near ½ VCC Biasing the Transistor • Base-biased Circuit • Apply small input signal through a Cap • Causes changes in IB around the level established by RB the and VCC. • The changes in ΔIC = Δ IB x β • The changes around the DC level set by RB and VCC will equal Δ IC x RC • Finding IB is critical • For base biased circuits like on this and the next slide with lower supply voltages VBE is critical Biasing the Transistor • Base-biased Circuit • Finding IB is critical • Assume RB = 1MΩ, β = 80, RC = 3KΩ • Find: IB , IC , VCE IB VCC VBE 12v 0.7v 11.3A RB 1M I B 12 A VCC RB I C I B 80 11.3A 0.904mA 0.904mA I C I B 80 12A 0.96mA VCE VCC RC I C 12v 0.904mA 3K 9.29V 9.3V VCE VCC RC I C 12v 0.96m 3K 9.12V 9.1V Biasing the Transistor • Base-biased Circuit • Distorted output • The quiescent value of VCE is critical to having non distorted outputs • Positive clipping indicates that the quiescent level of IC is too low • Negative clipping indicates that the quiescent level of IC is too high • Such circuits are unstable to variations due to temperature and age Biasing the Transistor • Base-biased Circuit • In Class Exercises • Problems 3-5 and 3-6 on page 58 of the textbook • 3-5 » Given: VCC = 15V, RB = 750 K ohms β = 60, RC = 5000 ohms » Find: IB, IC, VCE • 3-6 » Given: VCC = 10V, RB = 470 K ohms, β = 90, RC = 33000 ohms » Find: IB, IC, VCE Biasing the Transistor • Voltage Divider & Emitter Resister Bias • Provides a more stable quiescent value for VB • Small changes in one component don’t get multiplied by β V VBE I E IC B if VBE negligible RE IC VB RE • If IE increases in value due to heat and VE increases, VB should be more stable and VB shouldn’t change much since it depends on a ratio. Then VBE decreases and then decreases IC , thus the resulting change is less than expected from the initial change Biasing the Transistor • Voltage Divider & Emitter Resister Bias • Provides a more stable quiescent value for VB • β isn’t required to determine IC V VBE I E IC B RE VB VCC R2 R1 R2 VB IC RE VCC R2 IC R1 R2 RE • Thus even if β changes the gain of the transistor amplifier circuit is stable Biasing the Transistor • Voltage Divider & Emitter Resister Bias • Example Problem • Given: VCC=20v, R1=30KΩ, R2=10KΩ, RE= 2KΩ, RC=3KΩ • Find: IC and VC VCC R2 20V 10 K IC 2.5mA R1 R2 RE 30 K 10 K2 K VCE VCC RC I C 20v 2.5mA 3K 12.5V • In Class Problems: 3-9 to 3-12 on page 60 • Input AC Signal Amplifier • Characteristics • AC signal coupled through a cap • ib depends upon the resistance of the BE junction and β • re = resistance of the BE junction = 25mV/IE • rin = β(rE+re)= resistance seen by the input signal • rin = βre when rE= 0 ohms • Higher quiescent IE yields lower re and rin , and higher quiescent IB and larger Δib • Sample Problem • Given: IC = 1mA and β =100, RE= 0 ohms • Find: rin AC Signal Amplifier • Input • Sample Problem re 25mV 25 1mA rin re 100 25 2500 • Circuit Analysis vin 5mVP • Biasing Values (DC values) IB VCC 10 A RB I C I B 1mA VC VCC I C RC 7v • AC values – See Top of slide plus ib 5mV 2A 2500 ic peak ib peak 2 A 100 200 A AC Signal Amplifier • Circuit Analysis • AC values vo peak ic peak RC 200A 5,000 1v Av vo 1v 200 vin 5mv AC Signal Amplifier • Circuit Analysis • Add a load on the output • Under AC analysis VCC appears to the output AC signal as a ground • Thus RC and RL form a current divider • rL = RC || RL • Thus and vo ic rL vo ic rL rL Av vin ib re re Assumes rE = 0 ohms AC Signal Amplifier • Circuit Analysis • Voltage Divider and RE Variation • Add a AC By-Pass Cap across RE • The AC signal will see a short to ground • The DC bias voltages will see the Cap as an open • An open bypass Cap will result in significantly less gain – rE goes from 0 ohms to RE is usually much larger than re . See equation to the right. rL rL Av re RE RE AC Signal Amplifier • Typical Problems • Transistor Problems • Failures • They don’t become weaker • They open or short • Open Transistor • No IC » DC voltage at the collector will = VCC • Shorted Transistor • Will have 0 volts across the transistor • Other Component problems • Resistors • Values can change with age and heat • Yields abnormal bias voltages • Coupling Capacitors • Can become leaky, short or open • Leads to circuit malfunctions, i.e., bias voltage not blocked AC Signal Amplifier • Typical Problems Multistage amplifiers • Coupling Capacitor • An open would stop the AC output of the first stage from reaching the second stage • A short would allow the DC biasing voltages on the first stage to send the second stage into saturation • Manufacturing/Repair • Solder Bridges • As the one shown indicates a shorted transistor – Cured by removing it • Break/Opens in foil tracks • Will appear as open components Identifying the Basic Amplifier • Characteristics • Best practice when done on actual equipment • Sample Commercial circuits used • Part of Sony Tape recorder Model CF-320 • Part of Panasonic Tape recorder Model RF-7400 • Sample Circuit Analysis • Sony Tape recorder Model CF-320 • See circuit on the next slide & page 53 of the textbook • Driver stage using Q4 • Single stage audio amplifier • Voltage divider in the base » R35, R36, R37 » C29 from junction of R35 and R36 to ground serves to smooth power supply variations » C28 servers to filter out high frequencies and to prevent oscillations Sony Model CF-320 Identifying the Basic Amplifier • Sample Circuit Analysis • Sony Tape recorder Model CF-320 • Driver stage using Q4 • Emitter Circuit » R38 key to determining the VE » R43 provides feedback from the secondary of the output transformer. Has minimal effect on DC biasing levels • Collector Circuit » Doesn’t have a RC, but uses the primary of transformer T2 » Drives a push-pull power amplifier that consists of Q8 and Q9 » R39 and C30 are connected across the primary of T2 to filter out high frequency noise • Panasonic Tape recorder Model RF-7400 • Circuit is on the next slide and page 54 of the textbook • Analysis that focuses upon another amplifier circuit starts with the third from the bottom paragraph on page 53 Panasonic Model RF-7400 Finding Single Stage Amp Problems • Characteristics • Usually uses a Voltage Divider with RE configuration • Troubleshooting • Check Power supply voltages • Check VC • If normal –Transistor and all biasing resisters are normal • Causes of low VC are: • • • • Larger RC value Larger IC value causes a lower VC Larger VB value causes larger IC and lower VC Cause of a high VB • • VC VCC I C RC Lower R1 value Larger R2 value Finding Single Stage Amp Problems • Troubleshooting • Key points • • • Understand the operation of the circuit Take some measurements to determine current operation The above steps will lead to the problem. • The flow chart on the next slide demonstrates the measurement and thought process for troubleshooting a single stage amplifier • Flowchart • Walk through all the branches • Homework • Problems 3-17 through 3-22 Troubleshooting Flowchart The values in the flowchart are per the above circuit.