---
title: "Short note: Multi-cavity klystron amplifier"
subject: "RF and Microwave Engineering"
chapter: 5
key: "semester_7/rf_and_microwave_engineering:multicavity-klystron-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/multicavity-klystron-note
site: BCT Engineering
---

# Short note: Multi-cavity klystron amplifier

RF and Microwave Engineering · Chapter 5: Microwave Generators

- Asked in: 2082 Baisakh, 4 marks; 2080 Chaitra, 5 marks (asked 2 times)
- Video: [Multi Cavity Klystron Amplifier | working of multi cavity klystron amplifier | manish talk](https://www.youtube.com/watch?v=7-lF_2UqEIo) — ManishTalk
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/multicavity-klystron-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-5

## Question

Write a short note on multi-cavity klystron amplifier.

## Short answer

A multi-cavity klystron is a high-power microwave amplifier using an electron beam velocity-modulated by a buncher cavity; intermediate idler cavities enhance bunching, and a catcher cavity extracts amplified RF. It provides gains of 40–60 dB with peak power up to tens of MW, used in radar and TV transmitters.

## Answer

A **multi-cavity klystron** is a linear-beam (O-type) microwave **power amplifier**. Three or more resonant cavities are placed along an electron beam to get high gain and high power.

```text
 cathode  input    idler     idler    output
  gun    cavity   cavity    cavity   cavity  collector
 |>==== [ C1 ] == [ C2 ] == [ C3 ] == [ C4 ] ====|
  beam ->  RF in                        RF out
       <---- drift spaces with focusing B ---->
```

**Working**
1. The electron gun produces a uniform beam accelerated by V₀ to v₀ = √(2eV₀/m).
2. The weak RF input at the **buncher (first) cavity** velocity-modulates the beam. Some electrons are sped up and some are slowed down.
3. In the drift space the fast electrons catch up with the slow ones, which forms **bunches** (density modulation).
4. Each **intermediate (idler) cavity** is excited by the bunched beam. It builds up a larger gap voltage and modulates the beam again, so the bunching becomes tighter. Each cavity adds about **15–20 dB** of gain.
5. The **catcher (output) cavity** is placed where the bunches are tightest. Its retarding field takes kinetic energy from the bunches, and amplified RF is coupled out.
6. The spent beam is collected by the collector.

**Features**
- Gain of about 40–60 dB with 3–5 cavities. Peak power up to tens of MW (pulsed). Efficiency of about 35–50%.
- **Synchronous tuning** (all cavities at one frequency) gives the highest gain but narrow bandwidth. **Stagger tuning** widens the bandwidth at the cost of gain.
- An axial magnetic field (solenoid or PPM) keeps the beam focused.

**Applications:** UHF TV transmitters, radar transmitters, particle accelerators (linacs), and satellite uplink stations.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How do bunches form in the klystron drift space?
   A. Electrons are stopped
   B. Velocity modulation makes faster electrons catch up with slower ones
   C. The magnetic field squeezes the beam
   D. The collector attracts them
   Answer: B. Velocity modulation at the buncher becomes density modulation in the drift region. (Key point: Bunching mechanism)

2. What do intermediate cavities do?
   A. Absorb the beam
   B. Are excited by bunches and re-modulate the beam, tightening the bunching and adding gain
   C. Collect electrons
   D. Supply DC
   Answer: B. Each adds about 15–20 dB of gain. (Key point: Role of idler cavities)

3. The catcher cavity extracts energy because:
   A. Bunches arrive in its retarding field and give up kinetic energy
   B. It accelerates bunches
   C. It is outside the beam
   D. The input is applied to it
   Answer: A. Bunches in the retarding field give energy to the RF output. (Key point: Output cavity)

4. Stagger tuning of the cavities compared with synchronous tuning gives:
   A. Higher gain, narrower bandwidth
   B. More bandwidth at the cost of gain
   C. No change
   D. Oscillation
   Answer: B. Synchronous tuning gives maximum gain with narrow bandwidth; stagger tuning trades gain for bandwidth. (Key point: Tuning)

## Related in RF and Microwave Engineering

- [Density modulation and multi-cavity klystron oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Adensity-modulation-klystron-oscillator) — RF and Microwave Engineering, chapter 5
- [Cross-field effect and multicavity klystron amplifier](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Acrossfield-multicavity-klystron) — RF and Microwave Engineering, chapter 5
- [Short note: Two-cavity klystron](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Atwo-cavity-klystron-note) — RF and Microwave Engineering, chapter 5
- [Cross-field cavity amplifier with 90° phase shift](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Acrossfield-device-90-degree) — RF and Microwave Engineering, chapter 5