There are three general purpose microwave tubes. The first is ordinary gridded tube, having electrodes like vacuum tube diode and triodes. The second type are those in which interaction between the electron beam and RF field takes place. The klystron is the example of the second type of microwave tubes. The third category of the device is one in which interaction between an RF field and electron beam is continuous. TWT (Travelling Wave Tube) is the example of this category.
For best low-level noise performance in the X-band of microwave region, an amplifier should use :
A step recovery diode
A step recovery diode — option 3.
What the question is really asking. “Best low-level noise performance” means the amplifier that adds the least noise of its own to a very weak signal — the front end of a radar or satellite receiver, where the noise figure of the first stage dominates the whole chain by Friis's formula:
\(F_{total}=F_{1}+\dfrac{F_{2}-1}{G_{1}}+\dfrac{F_{3}-1}{G_{1}G_{2}}+\dots\)
Only the first term is un-divided, which is why an X-band receiver's performance is decided almost entirely by the noise of its first amplifier.
Why the step recovery diode wins here. It belongs to the varactor family — a reverse-biased junction used as a voltage-variable reactance. A reactance is very nearly lossless, and it is loss that generates thermal noise. Amplifying by pumping a non-linear reactance is exactly the principle of the parametric amplifier, the classic low-noise microwave front end:
| Property | Consequence for noise |
|---|---|
| Gain comes from a reactance, not a resistance | Almost no thermal noise is generated in the amplifying element itself |
| No DC current flows through the amplifying mechanism | No shot noise from a bias current |
| Can be cooled cryogenically | Noise temperatures of a few tens of kelvin are achievable |
Why each alternative is worse.
| Option | What it is | Why not |
|---|---|---|
| 1. Bipolar transistor | Active three-terminal device | Perfectly good at lower frequencies, but at X-band (8–12 GHz) a silicon BJT is near its \(f_{T}\): gain has collapsed and the noise figure has risen sharply. Transit time and base resistance both work against it |
| 2. Gunn diode | Negative-resistance oscillator, using transferred-electron bulk effect | A source, not a low-noise amplifier. Its gain mechanism is a negative resistance, which is inherently noisy |
| 4. IMPATT diode | Negative-resistance device using avalanche multiplication | The noisiest of the whole set. Avalanche breakdown is a random multiplication process, so IMPATTs are notorious for high noise. They are chosen for power, never for low noise |
The ordering to remember runs by amplification mechanism, not by device glamour: a pumped reactance is quietest, a transistor is next, a bulk negative resistance is worse, and an avalanche device is worst of all. Options 2 and 4 are both negative-resistance oscillators and can be dismissed together.
The modern answer, for context: parametric amplifiers have largely given way to GaAs MESFETs and HEMTs, which reach sub-1 dB noise figures at X-band at room temperature without a pump source. The physics of the question is of its period, but the reasoning — loss generates noise, so amplify with a reactance — still holds.
Hence, the answer is A step recovery diode.
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