Amplifiers

A Low Noise Amplifier often amplify weak signals to a useable level for further signal processing on the receiving side in radio communication. On the sender side a low level signal is amplified to a sufficient level on the output, it can be a few Watts to many kilo Watts. Amplifiers are also used in laboratories, radars, MRI (Magnetic Resonance Imaging), aerospace industry, space industry and EMC testing.

The task of a power amplifier is to amplify a given signal depending on the load. Amplifiers are specified mainly on frequency range and power level for continuous wave or pulsed power.

Gain shows the level of the output in relation to the input signal. Gain is often specified in dB and the value depends on the load and the impedance.

 

Semiconductor Based Amplifiers

Amplifiers based on semiconductors are fairly compact and can be integrated into equipments or be mounted directly to the antenna feeding in order to avoid losses in connections. When the amplitude increase, good linearity is important in order not to get a distortion of the input signal.

Semiconductor amplifiers are available from a few kHz to up to 100 GHz. Power ranges from a few mW to many kW. Power circuitry is often based on Galliumarsenide (GaAs) or GalliumNitride (GaN). GaAs typically offers better linearity while GaN offers higher efficiency. Semiconductor amplifiers are offered as board mounted components, as rack mounted equipment with coaxial connectors, as stand-alone units or as wave guide systems. Wave guide versions are used at higher frequencies and power levels when coaxial connectors cannot be used.

RF amplifies are often divided into categories such as CATV, Gain Blocks, LNA, Low Power, Medium Power and High Power, and Gain Block amplifiers are used for low power and are often surface mounted onto printed circuit boards. The frequencies reach up to 25 or 30 GHz depending on the package and the semiconductor type. Gain block amplifiers are internally matched for 50 Ohm on the input and output. Output power is normally 0 dBm at 1 or 2W or 33 dBm covering different frequency bands and gain.
 

Tube Based Amplifiers

High power tubes, such as Traveling Wave Tube Amplifier (TWTA), klystrons and magnetrons are used in high power applications such as particle accelerators, radar applications and microwave heating. TWTA is a tube amplifier of ”linear beam” type, where the radiowaves absorbs energy from an electron beam passing the tube. One advantage with TWT and tube amplifiers is the ability to amplify wide frequency bands. The bandwidth for a Helixtube can be up to 2 octaves, while cavity based versions have bandwiths of 10% to 20%. The frequency range from 300 MHz up to 50 GHz and the gain range from 40 to 70 dB, with output power from a few Watt up to Mega Watt.
 

Amplifier Parameters

Amplifiers are mainly chosen based on parameters such as frequency range, supply voltage, current consumption, power level and mechanical format. Some other important parameters in the specifications are listed below:

Directivity is defined as the difference between isolation and gain in dB. Higher directivity (dB) indicates better isolation.

Dynamic range is the power range in which the amplifier operates with linearly according to the specification, also taking noise level into account in the lower range and the upper level as a function of the 1dB compression point.
 
Gain Flatness indicates variation in the amplifier characteristics over the full bandwith at a given temperature. Gain flatness is given in dB by comparing maximum and minimum amplification over the frequency range. 
 
Gain factor for the RF amplifier is the relation between output power and input power.

Harmonic distortion, is non-linearity in the output frequency showed as multiples of the input signal frequency. The parameter varies with the input signal power level and is normally specified as a relative harmonic level of the fundamental signal level.

Isolation is measured as the relation between power applied on the output compared with power measured on the input.

Linearity of an amplifier is specified by how well the power level of the output is represented as a function of a linear variation of the input power. A good linearity amplifier delivers an amplified copy of the input signal with egligible generation of harmonic- or intermodulation distortion.
 
Maximum signal level specifies the highest continues wave or pulsed power level that can be applied on the input. If the maximum signal level is exceeded, it may cause a permanent increase in noise level, increased distortion, decreased gain  and in some cases the input stage is damaged.

Noise factor (NF) is the signal/noise level on the amplifier input compared with the signal/noise level on the amplifier output given in dB.

Stability of an amplifier is a measure of how immune it is for self-oscillation, so that it does not generate a signal on the output without a signal on the input.
 

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