Materials by frequency
Why is gallium arsenide (GaAs) useful for semiconductor devices operating at UHF and higher frequencies?
Higher electron mobility
Which of the following materials supports the highest frequency of operation when used in MMICs?
Gallium nitride
The ladder is silicon → gallium arsenide → gallium nitride, and each step buys frequency. Electrons move faster, so the device can respond faster.
GaN additionally handles high voltage and power density, which is why modern solid-state HF and VHF amplifiers use GaN devices where they once used bipolar or LDMOS silicon.
MMICs
Which is the most common input and output impedance of MMICs?
50 ohms.
What characteristics of MMICs make them a popular choice for VHF through microwave circuits?
Controlled gain, low noise figure, and constant input and output impedance over the specified frequency range.
That constant 50 ohms is the practical point. A discrete transistor amplifier needs matching networks designed for it; an MMIC is 50 ohms in and 50 ohms out across its whole range, so it drops into a 50-ohm system with no matching at all.
What type of transmission line is often used for connections to MMICs?
Microstrip.
At these frequencies every track is a transmission line, so the board is designed as controlled-impedance microstrip.
How is power supplied to the most common type of MMIC?
Through a resistor and/or RF choke connected to the amplifier output lead.
There is no separate supply pin. DC is fed through the output, via a choke or resistor that passes DC while blocking RF — which is why an MMIC has so few connections.
Which of the following noise figure values is typical of a low-noise UHF preamplifier?
0.5 dB.
Half a decibel. Worth having as a reference point: it tells you what “good” means, and it tells you that a preamplifier claiming 0.1 dB is either exotic or optimistic.
Packaging
Which of the following device packages is a through-hole type?
DIP.
What is a characteristic of DIP packaging used for integrated circuits?
Two rows of connecting pins on opposite sides of package (dual in-line package)
Dual In-line Package — the classic rectangular chip with legs down both sides.
Why are DIP through-hole package ICs not typically used at UHF and higher frequencies?
Excessive lead length.
The lead-length problem from the electrical principles lesson, in packaging form. A DIP’s leads are perhaps 10 mm of wire, which at 450 MHz is a meaningful reactance between the die and the circuit.
Which of the following component package types have the least parasitic effects at frequencies above the HF range?
Surface mount
What advantage does surface-mount technology offer at RF compared to using through-hole components?
All these choices are correct — smaller circuit area, shorter circuit board traces, and less parasitic inductance and capacitance.
Surface mount is not merely a manufacturing convenience. At RF it is an electrical improvement: no leads means almost no parasitic inductance, which is why microwave circuits are entirely surface mount and why a through-hole prototype of a UHF circuit often will not work at all.
Check yourself
- You need a low-noise preamplifier for 1296 MHz. What semiconductor material, and what noise figure would be good?
- Why does an MMIC need no matching network?
- Why is a DIP-packaged op-amp a poor choice at 450 MHz?
Answers
- Gallium arsenide (or gallium nitride for power), and around 0.5 dB is typical of a good low-noise UHF preamplifier.
- Its input and output impedances are constant at 50 ohms across its specified range, which is what the surrounding system already is.
- Excessive lead length — the package’s leads present significant parasitic inductance at UHF.