phys289 lecture 3 - texas a&m...
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PHYS289Lecture 3
Electronic Circuits
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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
Sinusoidal • Time variable signal
• Characterized by– Frequency
– Phase
– Amplitude
Sinusoidal
• Many sinusoids of top of each other– Many frequencies, phases, amplitudes added
• Fourier transform to sort out
Fourier transforms
Fourier transforms
Other kinds of signals
These have Fourier transforms too
Lots of combinations
Pulses
Machines available to generate these signals
• Function generator
• Pulse generator
• Signal generators
• Generally characterized by frequency, shape of pulse, etc.
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
Capacitors
Battery
Capacitor
Unit = Farad
Pico Farad - pF = 10-12FMicro Farad - uF = 10-6F
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
Capacitors
• Capacitance is determined by 3 factors– Plate surface area– Plate spacing– Insulating material (dielectric)
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
Circuits with capacitors
Potential across capacitor changes when a current flows through it
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
Capacitor Charging
Capacitor Discharge
RC time constant
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
Some applications
Time-delay circuit:Can induce a delay in a signal
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
Unintentional capacitive coupling
Circuits with capacitors
• Integrators
– V << Vin
• Ramp generators
– If provide constant current,
• Voltage continues to increase
• All sometimes useful
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
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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Inductor types
Molded inductor & air-wound inductor Adjustable air-wound inductor
Ferrite core toroidal transformer Iron powder toroidal inductorAir wound inductor
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.
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
Transformer
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