Microwave Devices

Devices that generate, amplify and control microwaves. Learn vacuum tubes (magnetron, klystron, TWT), solid-state devices (Gunn, IMPATT, PIN, varactor diodes; GaAs/GaN HEMTs, SiGe HBTs), MMICs, power-amplifier efficiency (PAE), noise figure cascades (Friis), and passive components (circulators, isolators, couplers).

Microwave ovens use magnetrons; satellites use TWTs; phones and 5G base stations use GaAs and GaN transistor amplifiers. Choosing the right device balances frequency, power, efficiency, noise and cost.

An analogy

Microwave devices are like different engines: a magnetron is a cheap powerful motor, a TWT is a precision jet engine, and a GaN transistor is a compact electric motor for everyday use.

Vacuum devices

Magnetron: efficient, cheap high-power oscillator (ovens, older radar). Klystron: high-power linear amplifier (broadcast, accelerators). Travelling-wave tube (TWT): wideband high-power amplifier (satellite transponders, EW).

Solid-state devices

Gunn and IMPATT diodes (oscillators), PIN diodes (switches, attenuators), varactors (tuning), Schottky mixers/detectors. Transistors: GaAs pHEMT (LNAs, low noise), GaN HEMT (high power, high voltage), SiGe HBT and CMOS (integrated transceivers up to mmWave).

Receiver noise

The first stage dominates system noise (Friis). A low-noise, high-gain LNA makes later noisy stages (mixers) almost irrelevant.

Passive and integrated components

Ferrite circulators and isolators route signals one way (radar T/R). Couplers, power dividers, filters and MMICs integrate many functions on one GaAs or GaN chip. Thermal management limits power density.

Formulas

• Power-added efficiency: PAE = (P_out − P_in)/P_DC
• Friis noise formula: F_total = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁G₂) + …
• Noise figure: NF = 10 log₁₀ F

Worked example: PAE

Given: P_out 10 W, P_in 1 W, P_DC 20 W

Find: PAE.

• PAE = (10 − 1)/20 = 45 %

Answer: 45 %

Worked example: Cascaded noise figure

Given: LNA NF 1 dB, G 20 dB; Mixer NF 8 dB

Find: System NF.

• F₁ = 1.259, F₂ = 6.31, G₁ = 100
• F = 1.259 + 5.31/100 = 1.312 → 1.18 dB

Answer: 1.18 dB — the LNA dominates

Common mistakes

• Wrong: Putting a lossy filter before the LNA carelessly. — Right: Pre-LNA loss adds directly to NF.
• Wrong: Ignoring heat in PAs. — Right: P_DC − RF out becomes heat.

Applications

• Radar & EW: GaN MMICs and TWTs.
• 5G base stations: GaN Doherty amplifiers.
• Satellite: TWTAs and low-noise block converters.

Key takeaways

• Tubes for extreme power; semiconductors elsewhere.
• PAE = (P_out − P_in)/P_DC.
• First stage dominates NF (Friis).
• Circulators separate Tx/Rx.

Going further

• Doherty amplifiers: High efficiency at back-off.
• Load-pull: Optimising PA matching.
• MMIC design: GaAs/GaN foundry processes.