In a three cavity klystron amplifier, the oscillations are excited in

An Impatt diode has n⁺ - p - i - p ⁺ structure and is used with reverse bias.

It exhibits negative resistance and operates on the principle of avalanche breakdown.

Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit.

The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode.

The features of Impatt diode oscillator are : frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%.

Its applications include police radar systems, low power microwave transmitter etc.

H_z = 0

E_x = ± Z_TM H_y

E_y = ± Z_TM H_x

where E₀ is the amplitude of the wave.

The expressions for ?, ?_c, f_c, ?_g, a are the same as for TE waves.

For TM wave the lowest cut off frequency in rectangular wave guide is for TM₁₁ mode.

If , the cutoff frequency for TM₁₁ mode is about 12% more than that for TE₂₀ mode.

In a klystron the resonant structure limits the bandwidth.

A TWT is a broadband device. Its main components are electron gun (to produce the electron beam) and a structure supporting the slow electromagnetic wave.

The velocity of wave propagation along the helix structure is less than velocity of light.

The beam and wave travel along the structure at the same speed.

Thus interaction occurs between beam and wave and the beam delivers energy to the RF wave.

Therefore the signal gets strengthened and amplified output is delivered at the other end of tube.

The main features of TWT are :

1. Frequency range - 0.5 GHz to 90 GHz

2. Power output - 5 mW at low frequencies(less than 20 GHz) 250 kW (continuous wave) at 3 GHz 10 MW (pulsed) at 3 GHz

3. Efficiency - about 5 to 20%

4. Noise - about 5 dB for low power TWT 25 dB for high power TWT

TWT is used as RF amplifier in broadband microwave receivers, repeater amplifier in broad band communication systems, communication satellites etc.

Shunt conductance can be neglected in most of calculations.

In a travelling wave both x and t increase simultaneously so that a constant phase point moves in the direction of positive (or negative) x.

Since fields are confirmed within a coaxial cable it is non-radiating.

Charging current and MVAR are proportional to frequency.

Reflection coefficient

and is independent of line length.

They are used in all the devices.