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This material, coming into use for various application, has significant potential. The exam cites use in MMICs, Monolithic Microwave Integrated Circuits at the highest frequencies. If you want to read more, this material used in High-electron-mobility transistor (HEMT), aka heterostructure FET (HFET), aka modulation-doped FET (MODFET). Other applications are in power control circuitry for electric vehicles, and a range of high intensity LEDs.
MMICs are Monolithic Microwave Integrated Circuits, which operate as signal and power amplifiers across a set bandwidth, with consistent gain. They typically have have 50 ohms input and output impedance.
Power is typically applied to the output, via a resistor or inductor. The inductor conducts DC power, while preventing RF or microwave energy going to the power supply.
|Image and diagram indicating the use of a MMIC, by WikiMedia user: Appaloosa.|
Similar devices, such as the MAR-6, can be used in TV masthead amplifiers.
Dual In-line Packages are often used to house intergrated circuits. They are termed DIP, or occasionally, DIL. They have a row of leads, usually spaced at 0.1" or 2.54 mm down each side. Plastic is most common, although ceramic material can be used, which is better for dissipating heat.
They are most usually mounted by inserting the pins through a printed ciruit board, and soldering them in place. Socketing is an alternative.
While there are FM broadcast band receiver ICs in these packages, at UHF, the length of the pin, and the internal conductor leading to the silicon die becomes too long.
Opto-couplers are also mounted in these packages, as are arrays of resistors or Darlington transistors.
Almost all modern commercial electronics is built using surface mount components. In many case the components are significantly smaller, and lower in profile, than through-hole ones. Track and lead lengths can be much shorter than on through-hole boards, making the useful frequency extend into the UHF range.
To further extend the frequency range into the microwave region, microstrip boards have tracks with tuned widths, above a ground plane, to ensure the correct impedance.
These are the same thing. The dual names are because they can couple an electrical signal between two circuit sections, while electrically isolating them. They can be used in low voltage equipment controlling mains powered circuits, or between mains powered equipment and telecommunications equipment, to remove the risk of dangerous voltages appearing on the 'phone line, where they could harm technicians and line workers.
Common types consist of a LED, often and infra-red one, which illuminates a photodiode or phototransistor. Current into the LED results in a small voltage or the conduction of the phototransistor on the output side.
While various materials are used for solar cells, Silicon is used on standard panesl you might use for a field day or emergency activation.
When light strikes the material electrons are forced to move, creating current. Each cell only generates 0.5 volts, so many need to be places in series to generate useful voltages. Open circuit voltage is often significantly above the voltage of the battery to be charged.
The University of New South Wales in Sydney, Australia is one of the global leaders in solar, including ones under 10 cm in diameter which work by having light concentrated on them using many large mirrors.
Removed are photoconductive materials, which exhibit a considerable reduction in resistance when illuminated.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC.
Why is gallium arsenide (GaAs) useful for semiconductor devices operating at UHF and higher frequencies?
A. Higher noise figures
B. Higher electron mobility
C. Lower junction voltage drop
D. Lower transconductance
GaAs material has higher electron mobility, answer B.
Which of the following device packages is a through-hole type?
C. Ball grid array
DIP stands for Dual-In-Line Package (also called DIL), these have been used for ICs, resistor arrays, and Darlington arrays for many decades, and are through-hole components, answer A.
The remainder are surface-mount.
Which of the following materials is likely to provide the highest frequency of operation when used in MMICs?
B. Silicon nitride
C. Silicon dioxide
D. Gallium nitride
This is the relatively new material, Gallium Nitride, GaN, answer D.
Which is the most common input and output impedance of circuits that use MMICs?
A. 50 ohms
B. 300 ohms
C. 450 ohms
D. 10 ohms
The standard RF communications system impedance is 50 ohms, and this applies to MMICs, answer A.
Which of the following noise figure values is typical of a low-noise UHF preamplifier?
A. 2 dB
B. -10 dB
C. 44 dBm
D. -20 dBm
2dB, answer A.
What characteristics of the MMIC make it a popular choice for VHF through microwave circuits?
A. The ability to retrieve information from a single signal even in the presence of other strong signals
B. Plate current that is controlled by a control grid
C. Nearly infinite gain, very high input impedance, and very low output impedance
D. Controlled gain, low noise figure, and constant input and output impedance over the specified frequency range
Unlike simple RF transistors, MMICs simplify RF device design by having controlled gain, a low noise figure, and constant input and output impedance over the design frequency range, answer D.
What type of transmission line is used for connections to MMICs?
A. Miniature coax
B. Circular waveguide
C. Parallel wire
Microwave circuits, including those using MMICs use microstrip construction, answer D.
How is power supplied to the most common type of MMIC?
A. Through a resistor and/or RF choke connected to the amplifier output lead
B. MMICs require no operating bias
C. Through a capacitor and RF choke connected to the amplifier input lead
D. Directly to the bias voltage (VCC IN) lead
Power is supplied via a resistor and/or RF choke, connected to the amplifier output lead, answer A.
Which of the following component package types would be most suitable for use at frequencies above the HF range?
B. Axial lead
C. Radial lead
D. Surface mount
Surface mount is most suitable for VHF and UHF circuits, answer D.
What advantage does surface-mount technology offer at RF compared to using through-hole components?
A. Smaller circuit area
B. Shorter circuit-board traces
C. Components have less parasitic inductance and capacitance
D. All these choices are correct
Surface mount uses components without leads, soldered directly to the PCB, answer D.
What is a characteristic of DIP packaging used for integrated circuits?
A. Package mounts in a direct inverted position
B. Low leakage doubly insulated package
C. Two chips in each package (Dual In Package)
D. A total of two rows of connecting pins placed on opposite sides of the package (Dual In-line Package)
These have 2 rows of pins, one down each side of the package, answer D.
Why are DIP through-hole package ICs not typically used at UHF and higher frequencies?
A. Too many pins
B. Epoxy coating is conductive above 300 MHz
C. Excessive lead length
D. Unsuitable for combining analog and digital signals
What absorbs the energy from light falling on a photovoltaic cell?
Light striking the photovoltaic material generate electrons, answer C.
What happens to the conductivity of a photoconductive material when light shines on it?
A. It increases
B. It decreases
C. It stays the same
D. It becomes unstable
Conductivity increases when the material is illuminated, answer A.
What is the most common configuration of an optoisolator or optocoupler?
A. A lens and a photomultiplier
B. A frequency modulated helium-neon laser
C. An amplitude modulated helium-neon laser
D. An LED and a phototransistor
This couples data or other information between circuits while providing electrical isolation. These typically consist of an LED and a phototransistor, answer D.
What is the photovoltaic effect?
A. The conversion of voltage to current when exposed to light
B. The conversion of light to electrical energy
C. The conversion of electrical energy to mechanical energy
D. The tendency of a battery to discharge when exposed to light
This is the conversion of light energy to electrical energy, answer B.
Which describes an optical shaft encoder?
A. A device which detects rotation of a control by interrupting a light source with a patterned wheel
B. A device which measures the strength of a beam of light using analog to digital conversion
C. A digital encryption device often used to encrypt spacecraft control signals
D. A device for generating RTTY signals by means of a rotating light source
These detect rotation by having a patterned disc interrupt an optical path answer A.
While they can be used for sensing in industrial applications, in Amateur Radio they are used for the tuning knob on transceivers. They were also used ball-based PC mice.
Which of these materials is affected the most by photoconductivity?
A. A crystalline semiconductor
B. An ordinary metal
C. A heavy metal
D. A liquid semiconductor
This is a crystalline semiconductor, answer A.
What is a solid state relay?
A. A relay using transistors to drive the relay coil
B. A device that uses semiconductors to implement the functions of an electromechanical relay
C. A mechanical relay that latches in the on or off state each time it is pulsed
D. A semiconductor passive delay line
An SSR uses an optocoupler (LED controlling a phototransistor or photodiode) connected to a high current semiconductor to emulate a regular relay, answer B.
Why are optoisolators often used in conjunction with solid state circuits when switching 120VAC?
A. Optoisolators provide a low impedance link between a control circuit and a power circuit
B. Optoisolators provide impedance matching between the control circuit and power circuit
C. Optoisolators provide a very high degree of electrical isolation between a control circuit and the circuit being switched
D. Optoisolators eliminate the effects of reflected light in the control circuit
These provide good electrical isolation between the low voltage circuit, and the mains voltage circuit, answer C.
What is the efficiency of a photovoltaic cell?
A. The output RF power divided by the input DC power
B. The effective payback period
C. The open-circuit voltage divided by the short-circuit current under full illumination
D. The relative fraction of light that is converted to current
This is the fraction (percentage) of light energy converted to electrical energy (current), answer D
What is the most common type of photovoltaic cell used for electrical power generation?
C. Cadmium Sulfide
D. Copper oxide
Ordinary roof-top solar panels are silicon, answer B.
What is the approximate open-circuit voltage produced by a fully-illuminated silicon photovoltaic cell?
A. 0.1 V
B. 0.5 V
C. 1.5 V
D. 12 V
These generate half a volt per cell, answer B, so many are placed in series to generate a useful voltage.
On to: Practical Circuits 1 - Digital circuits & Amplifiers
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, June 2022.
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