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PHYS289 Lecture 3 - Texas A&M Universitypeople.physics.tamu.edu/depoy/phys289/Notes/lecture3.pdf ·...
Transcript of PHYS289 Lecture 3 - Texas A&M Universitypeople.physics.tamu.edu/depoy/phys289/Notes/lecture3.pdf ·...
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PHYS289Lecture 3
Electronic Circuits
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Course Web Page
• http://people.physics.tamu.edu/depoy/PHYS225.html
• Up now
• Contains
– All lecture notes (PDF)
– Useful links
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Last lecture
• Devices not like a resistor– Zener diode
– Tunnel diode
– Capacitor
• Signals– Sinusoid
• Frequency, phase, and amplitude
– Fourier transform• Can be used to characterize complex signals
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Sinusoidal • Time variable signal
• Characterized by– Frequency
– Phase
– Amplitude
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Sinusoidal
• Many sinusoids of top of each other– Many frequencies, phases, amplitudes added
• Fourier transform to sort out
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Fourier transforms
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Fourier transforms
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Other kinds of signals
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These have Fourier transforms too
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Lots of combinations
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Pulses
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Machines available to generate these signals
• Function generator
• Pulse generator
• Signal generators
• Generally characterized by frequency, shape of pulse, etc.
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Circuits with capacitors• Capacitors
– Q = CV– I = C dV/dT
• Current is proportional to rate of change of potential• Change in potential proportional to current
– Power stored as energy in internal electric field• Can get it back again later
• Parallel capacitance add– C = C1 + C2 + C3 + …
• Serial capacitors add like parallel resistors– 1/C = 1/C1 + 1/C2 + 1/C3 + …
• Many different kinds of capacitors– Each has unique and useful properties
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Capacitors
Battery
Capacitor
Unit = Farad
Pico Farad - pF = 10-12FMicro Farad - uF = 10-6F
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Capacitor types
Ceramic disk
Monolithic ceramic
Dipped silvered-mica
Mylar or polyester
Aluminum electrolytic (+/-)
Tantalum (+/-)
Ceramic disk Monolithic ceramic Dipped siver-mica Mylar Mylar
Solid tantalum, polarized Radial aluminum electrolytic Axial aluminum electrolytic
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Capacitors
• Capacitance is determined by 3 factors– Plate surface area– Plate spacing– Insulating material (dielectric)
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Capacitor ratings
Physical size of capacitors is related to voltage handling ability – WVDC – working voltage DC
Temperature coefficient may also be important –can be + or – or nearly zero
Temperature coefficient depends upon dielectric material
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Circuits with capacitors
Potential across capacitor changes when a current flows through it
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Circuits with capacitors
• C dV/dt = I = -V/R
• V = A e-t/RC
• Capacitors will “charge up” over time after application of an initial voltage
– Approaches the applied potential
• Will also “discharge” over time if the applied potential is reduced
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Capacitor Charging
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Capacitor Discharge
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RC time constant
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RC time constant• Product of RC in a simple circuit
– For R in ohms and C in farads, RC is in seconds
• 1 µF across 1KΩ = 1 ms
– Characteristic time of response for the circuit
• Sets “frequency response” of circuit
– How quickly circuit responds
– How much of which frequencies get through the circuit
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Some applications
Time-delay circuit:Can induce a delay in a signal
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Another application
I = C d/dt(Vin – V) = V/R
V = RC d/dt(Vin – V)
For small changes in dV/dtV ≈ RC dVin/dt
Circuit differentiates the incoming signal
For square wave input, output is a series of pulses
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Unintentional capacitive coupling
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Circuits with capacitors
• Integrators
– V << Vin
• Ramp generators
– If provide constant current,
• Voltage continues to increase
• All sometimes useful
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Inductors
• V = L dI/dt; L is inductance– A simple coil of wire!
• Putting a voltage across an inductor causes the current to ramp
• Power stored in as energy in the magnetic field• 1 V across 1 henry produces 1 amp• Rare to use, but useful in some circumstances
– RF “chokes”– Transformers
• Two closely coupled inductors
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Inductors
Values specified in henries (H), millihenries (mH) and microhenries (μH)
A coil of wire that may be wound on a core of air or other non-magnetic material, or on a magnetic core such as iron powder or ferrite.
Two coils magnetically coupled form a transformer.
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31
Inductor types
Molded inductor & air-wound inductor Adjustable air-wound inductor
Ferrite core toroidal transformer Iron powder toroidal inductorAir wound inductor
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Inductor ratings
Wire gauge and physical size of the coil determine the current handling capacity.
Core material will have a temperature dependence. Air is best, followed by iron powder, then ferrites.
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Transformers
• Two closely coupled coils
• AC voltage applied across one will appear across the other at a different voltage
– Change depends on ratio of the number of turns in the coil
• Power is conserved
– So if voltage goes up, current will go down
• Generally very efficient
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Transformer
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Transformers
• Useful to change “line” power to something else
– At the heart of everything used to power computers, cell phones, etc.
• Isolate circuit from actual connection to the power line