An Insight into the Functioning of SSPA Solid State Power Amplifier

A radio frequency power amplifier (RF power amplifier) is a type of electronic amplifier that converts a low-power radio-frequency signal into a higher power signal. Typically, RF power amplifiers drive the antenna of a transmitter. Design goals often include gain, power output, bandwidth, power efficiency, linearity (low signal compression at rated output), input and output impedance matching, and heat dissipation.

Many modern RF amplifiers operate in different modes, called classes, to help achieve their design goals. Some classes are class A, class B, class C and class E. Class D amplifiers are rarely used for RF purposes because they need even higher frequency devices.

Modern RF power amplifiers use solid-state devices such as bipolar junction transistors and MOSFETs.

Applications for RF Amplifiers:

Amplifier applications include electromagnetic compatibility (EMC) testing, defense components, communications testing and medical diagnostics. RF power amplifiers can be used in driving to another high power source and microwave heating. They can also be used driving a transmitting antenna, where the transmitter–receivers are used for voice and data communications as well as for weather sensing. Microwave or RF heating is used in industrial applications as well as in microwave ovens. Also, particle accelerators use RF sources.

Choosing the Right RF Amplifier:

When looking for the right RF amplifier, you can filter the results by the required category with the following types of RF Amplifiers:


• Gain Blocks

• RF Low Noise

• RF Low Power

• RF Power Amplifiers

Once you choose the RF amplifier category, you can narrow them down by various attributes: by frequency range, supply current, supply voltage and packaging type to name a few. You will be able to find the right RF amplifier for your RF amplifier circuit or IC, cable TV amplifier, linear radio frequency amplifier, MOSFET RF amplifier, solid state RF amplifier, video amplifier or TV RF amplifier using these filters.

Directivity (active directivity) is defined as the difference between isolation and forward gain in dB. It is an indication of the isolationof the source from the load, or how much the load impedance affects the input impedance and the source impedance affects the output impedance. The higher the active directivity (in dB), the better the isolation.

Dynamic range is the power range over which an amplifier provides useful linear operation, with the lower limit dependent on the noise figure and the upper level a function of the 1 dB compression point.

Gain flatness indicates the variation of an amplifier’s gain characteristic over the full frequency response range at a given temperature expressed in ±dB. The value is obtained by taking the difference between maximum and minimum gain, and dividing it.

Gain (forward gain, G) for RF amplifiers is the ratio of output power to input power, specified in the small-signal linear gain region, with a signal applied at the input. Gain in dB is defined as G (dB) = 10 log10G.

Harmonic distortion is produced by non-linearity in the amplifier, and appears in the form of output signal frequencies at integral multiples of the input signal frequency. Because harmonic distortion is influenced by input power level it is generally specified in terms of the relative level for the harmonics to the fundamental signal power.

Isolation is the ratio of the power applied to the output of the amplifier to the resulting power measured at the input of the amplifier.

Linearity of an amplifier signifies how well its output power can be represented by a linear function of the input power. A linear amplifier produces at its output an amplified replica of the input signal with negligible generation of harmonic or intermodulation distortion.

Maximum signal level refers to the largest CW or pulse RF signal that can be safely applied to an amplifier’s input. Exceeding the specified limit can result in permanent noise figure degradation, increased distortion, gain reduction, and/or amplifier burnout. Noise factor is the ratio of signal-to-noise power ratio at an amplifier’s input to the signal-to-noise power ratio at the output.

Noise figure NF in dB is related to noise factor F by NF = 10 log10F in dB.

Return loss (RL) is the ratio of reflected power to incident power at an RF port of an amplifier, expressed in dB as RL = -20 log |ρ|, where ρ= voltage reflection coefficient Stability of an amplifier is an indication of how immune it is to self-oscillation, so that it does not generate a signal at its output without an applied input. A commonly used indicator of stability is the k-factor. A k-factor of 1.0 is the boundary condition for unconditional stability. If it is greater than zero but less than 1.0 the amplifier is only conditionally stable.