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Amateur Radio Info & Exams - Components 2 - Logic ICs, PLDs, Inductors, & Piezoelectric devices

Logic families

While early logic was RTL, then DTL (resistor-transistor logic, then diode-transistors logic); has mostly gone by the wayside, TTL, or transistor-transistor logic remains a fairly common form of logic. This uses bipolar transistors. The supply must be 5 volts, and current consumption tends to be high. It can operate at high speed. Off the exam, the part numbers commence with 74 for consumer grade, and 54 for Mil-spec (military specification).

CMOS, standing for Complementary metal–oxide–semiconductor, is the newer family, with low current consumption, and likewise off the exam, part numbers start with 40, sometimes 45.

There is an allusion to ECL, emitter coupled logic, which can use differential signalling.

More modern is BiCMOS. This allows elements which benefit from being CMOS, such as the high impedance (FET) inputs using this technology; while elements needing high speed, and especially low impedance, such as the outputs using bipolar transistors, as TTL does. BiCMOS is also used in Op-Amps, while SiGeC (Silicon-Germanium-Carbon) BiCMOS is in some cases replacing GaAs in RF circuitry.

Tr-state Logic

In many computerised system, there are data buses of multiple lines (say 8 bits wide). At various times the processor and other devices take turns in reading and writing data along the bus. Thus devices have to be able to both assert a 1 or 0; or to remain in a high impedance state while other devices communicate.

This is also used in many micro-controller ICs, where a pin can be switched between being an input, or an output.


This is an IC which compares the voltage of 2 inputs, and switches the output between its two rails depending on the values on the inputs. A typical unit may put the output high if the +ve input exceed the -ve one, and low if it does not. There is normally some hysteresis to prevent noise on a signal close to the reference voltage from causing the unit to switch rapidly between states.

Piezoelectric devices

Certain materials, when an electric current is applied, deform. This is used in devices such as some tweeters (high frequency speakers), beepers, noise-making toys, ultrasonic cleaners, and ultrasonic anti-fouling transducers for boats. In some cases, these are driven with high voltages (above 100 volts).

Such materials also convert force into electricity and so can be can be used as vibration sensors, microphones, energy recovery, vandal resistant switches (Sydney folk will see these in emergency help points on railway stations), the spark generator in some electric cancer-stick lighters, and devices with claim to alleviate mosquito bite pain using an electric pulse.

Relevant Questions

These are the actual questions from the Extra licence exam pool, as published by the NCVEC.

What is the function of hysteresis in a comparator?
A. To prevent input noise from causing unstable output signals
B. To allow the comparator to be used with AC input signal
C. To cause the output to change states continually
D. To increase the sensitivity

This prevents noise causing the input from fluttering between low and high outputs, answer A.

What happens when the level of a comparator’s input signal crosses the threshold?
A. The IC input can be damaged
B. The comparator changes its output state
C. The comparator enters latch-up
D. The feedback loop becomes unstable

When the input changes to a voltage above or below the reference input, the output will change state, answer B.

What is tri-state logic?
A. Logic devices with 0, 1, and high impedance output states
B. Logic devices that utilize ternary math
C. Low power logic devices designed to operate at 3 volts
D. Proprietary logic devices manufactured by Tri-State Devices

This is binary logic, with the usual 0 adn 1 outpuls, plus the ability to not assert a value on the bus, answer A.

Micro-controller ICs often do this, and can also use the same pin as an input.

Which of the following is an advantage of BiCMOS logic?
A. Its simplicity results in much less expensive devices than standard CMOS
B. It is totally immune to electrostatic damage
C. It has the high input impedance of CMOS and the low output impedance of bipolar transistors
D. All of these choices are correct

BiCMOS has the high input impedance of a CMOS (FET) input, and the low output impedance of bipolar transistors, answer C.

What is an advantage of CMOS logic devices over TTL devices?
A. Differential output capability
B. Lower distortion
C. Immune to damage from static discharge
D. Lower power consumption

These have low power consumption, and are more suitable for battery powered circuits, answer D.

Why do CMOS digital integrated circuits have high immunity to noise on the input signal or power supply?
A. Large bypass capacitance is inherent
B. The input switching threshold is about two times the power supply voltage
C. The input switching threshold is about one-half the power supply voltage
D. Bandwidth is very limited

To go from 0 to 1, the input must exceed half the supply voltage, or from 1 to 0, below half the supply, answer C.

In most cases the inputs are fairly close to the negative or positive rails, so it would take a great deal of noise the cause the input to the wrong state.

What best describes a pull-up or pull-down resistor?
A. A resistor in a keying circuit used to reduce key clicks
B. A resistor connected to the positive or negative supply line used to establish a voltage when an input or output is an open circuit
C. A resistor that ensures that an oscillator frequency does not drive lower over time
D. A resistor connected to an op-amp output that prevents signals from exceeding the power supply voltage

This is a resistor to set a level when the input would otherwise be open, say down to ground when the input is an SPDT switch contact to the positive rail; or to pull it up when the input is an open collector output, which goes to ground when the input is active, answer B.

In Figure E6-3, what is the schematic symbol for a NAND gate?
A. 1
B. 2
C. 3
D. 4

Various logic gate symbols

The flat input end, and semi-circular output end means AND, but we want a NAND, with the output is inverted, so we need the one with the circle on the output, item 2, answer B.

What is a Programmable Logic Device (PLD)?
A. A device to control industrial equipment
B. A programmable collection of logic gates and circuits in a single integrated circuit
C. Programmable equipment used for testing digital logic integrated circuits
D. An algorithm for simulating logic functions during circuit design

These consist of a very large number of logic gates and circuits in a single IC, which can be programmed into complex logic systems, answer B.

These can be faster than processors for some tasks.

In Figure E6-3, what is the schematic symbol for a NOR gate?
A. 1
B. 2
C. 3
D. 4

The OR function is indicated by the curved input, and the Gothic arch turned sideways. This is converted to NOR bu the the circle on the output, as in item 4, answer D.

Gothic arches are seen in many traditional church buildings, and some historic public or university buildings.

In Figure E6-3, what is the schematic symbol for the NOT operation (inverter)?
A. 2
B. 4
C. 5
D. 6

These have a single input and a single output, with an inversion circle on output, item 5, answer C.

Triangular, but without the circle here, is a buffer, used to distribute a signal around a board to many inputs, or perhaps to drive a small load, such as an LED.

Why should core saturation of an impedance matching transformer be avoided?
A. Harmonics and distortion could result
B. Magnetic flux would increase with frequency
C. RF susceptance would increase
D. Temporary changes of the core permeability could result

This can cause distortion, resulting in harmonics, answer A.

What is the equivalent circuit of a quartz crystal?
A. Motional capacitance, motional inductance, and loss resistance in series, all in parallel with a shunt capacitor representing electrode and stray capacitance
B. Motional capacitance, motional inductance, loss resistance, and a capacitor representing electrode and stray capacitance all in parallel
C. Motional capacitance, motional inductance, loss resistance, and a capacitor representing electrode and stray capacitance all in series
D. Motional inductance and loss resistance in series, paralleled with motional capacitance and a capacitor representing electrode and stray capacitance

Yikes, lots of similar answers... However, remember that it is a complex arrangement. Motional capacitance, motional inductance, and loss resistance in series; and this is in parallel with the capacitance of the electrodes with connect to the crystal wafer, and stray capacitance, say to the case, answer A.

Which of the following is an aspect of the piezoelectric effect?
A. Mechanical deformation of material by the application of a voltage
B. Mechanical deformation of material by the application of a magnetic field
C. Generation of electrical energy in the presence of light
D. Increased conductivity in the presence of light

The piezoelectric effect is used in some truck reversing bleepers, alarms etc, and are available as either complete beepers with the oscillator and driver; or as just the element, with or without an enclosure. In either case, the material is coated onto a thin metal disc. So, yes, the voltage applied deforms the material, answer A.

Which materials are commonly used as a slug core in a variable inductor?
A. Polystyrene and polyethylene
B. Ferrite and brass
C. Teflon and Delrin
D. Cobalt and aluminum

Ferrite or brass slugs are placed in variable inductors, and can be screwed in or out to vary the inductance, answer B.

What is one reason for using ferrite cores rather than powdered-iron in an inductor?
A. Ferrite toroids generally have lower initial permeability
B. Ferrite toroids generally have better temperature stability
C. Ferrite toroids generally require fewer turns to produce a given inductance value
D. Ferrite toroids are easier to use with surface mount technology

Fewer turns are generally required for a certain inductance, answer C.

What core material property determines the inductance of an inductor?
A. Thermal impedance
B. Resistance
C. Reactivity
D. Permeability

The only answer which relates to magnatism is permeability, answer D.

What is current in the primary winding of a transformer called if no load is attached to the secondary?
A. Magnetizing current
B. Direct current
C. Excitation current
D. Stabilizing current

This is the magnetising current, answer A.

What is one reason for using powdered-iron cores rather than ferrite cores in an inductor?
A. Powdered-iron cores generally have greater initial permeability
B. Powdered-iron cores generally maintain their characteristics at higher currents
C. Powdered-iron cores generally require fewer turns to produce a given inductance
D. Powdered-iron cores use smaller diameter wire for the same inductance

These can handle larger currents, answer B.

The chirpy 28 MHz power oscillator based CW transmitter I built uses an iron-powder toroidal core as its output transformer, despite the author listing it as ferrite core.

What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier?
A. Electrolytic capacitors
B. Butterworth filters
C. Ferrite beads
D. Steel-core toroids

Ferrite beads increase the inductance of the conductors, but the reactance at HF is much lower than at VHF or UHF, so VHF adn UHF signals are attenuated (filtered), while HF ones pass, answer C.

What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor?
A. Toroidal cores confine most of the magnetic field within the core material
B. Toroidal cores make it easier to couple the magnetic energy into other components
C. Toroidal cores exhibit greater hysteresis
D. Toroidal cores have lower Q characteristics

The majority of the magnetic energy is contained within the core, answer A.

Which type of core material decreases inductance when inserted into a coil?
A. Ceramic
B. Brass
C. Ferrite
D. Powdered-iron

Inserting brass (a copper alloy) into a coil reduces its inductance, answer B.

What is inductor saturation?
A. The inductor windings are over coupled
B. The inductor’s voltage rating is exceeded causing a flashover
C. The ability of the inductor’s core to store magnetic energy has been exceeded
D. Adjacent inductors become over-coupled

Saturation happens when the inductor’s core's ability to store magnetic energy has been exceeded, answer C.

What is the primary cause of inductor self-resonance?
A. Inter-turn capacitance
B. The skin effect
C. Inductive kickback
D. Non-linear core hysteresis

Turns in an inductor often consist of copper windings separated only by a thin layer of enamel insulation. Metal layers, separated by thin layers of insulation, we call that a capacitor! Yes, it is the inter-turn capacitance, answer A.

On to: Components 3 - Analogue ICs & Optoelectronics

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, June 2022.

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