What are Temperature Variable Attenuators?

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- everything RF

Jan 22, 2021

A Temperature Variable Attenuator or TVA is a type of attenuator whose attenuation level varies with temperature. In most cases, the level of attenuation decreases with increasing temperature. TVAs are used to passively compensate the output of temperature sensitive components such as amplifiers, mixers, directional couplers, etc. without distorting the signal. More often than not, these attenuators are simply a cost-effective passive temperature compensation solution.

Though as the temperature rises, the attenuation factor decreases, but the attenuation factor changes at different rates above and below room temperature. Therefore, an output tweaked for temperatures above room temperature will perform poorly at temperatures below room temperature. Also, amplifier gain always has a temperature coefficient, and for solid-state amplifiers such as MMICs, that coefficient is always negative.

Let us take the case of an RF amplifier. The gain is usually inversely proportional to changes in temperature. So, if the temperature rises significantly over room temperature, the gain decreases and if the temperature is significantly lower than room temperature, then the gain might increase. So, if an amplifier has a gain of 30 dB at room temperature (25°C) and the temperature rises to 60°C, the gain could reduce to 27 dB. Also, the same gain could rise to 33 dB at -25°C.

TVAs are often used to compensate for amplifier gain changes over temperature. Temperature Variable Attenuators provide a linear response over the entire required temperature range allowing designers to use a single compensation solution without compromising performance.

The variation of attenuation with temperature is specified with the unit dB/dB/°C which is known as the Temperature Coefficient of Attenuation.

Temperature Variable Attenuators can be used in applications that require a known amount of attenuation change for a particular temperature shift. They can replace closed-loop temperature compensation circuits with a single chip device and produce no signal distortion. These devices are ideal for applications that involve multiple tones and complex modulation schemes such as cellular base station applications and radar systems. These attenuators can be used in series with power amplifiers that exhibit variations in gain due to temperature, either from environmental effects or from their heating effects at high power levels. By selecting a TVA with an attenuation/temperature response that’s close to the inverse of the amplifier’s gain/temperature response, the amplifier’s output power can be held fairly stable with temperature and without the need for more complex automatic-level-control (ALC) circuits.

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