page_title: Valve RF amplifier
section_title: Distortion
hierarchical_section_title: Valve RF amplifier / Valve characteristics / Distortion
caption_attribution_description: English: Two Siemens transmitters 20 Kw PEP linear amplifiers, used at HCJB, Quito Ecuador.
context_page_description: A valve RF amplifier or tube amplifier, is a device for electrically amplifying the power of an electrical radio frequency signal.
  Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s, initially for receivers and low power stages of transmitters, transmitter output stages switching to transistors somewhat later. Specially constructed valves are still in use for very high power transmitters, although rarely in new designs.
context_section_description: The most efficient valve-based RF amplifiers operate Class C. If used with no tuned circuit in the output, this would distort the input signal, producing harmonics. However, class C amplifiers normally use a high Q output network which removes the harmonics, leaving an undistorted sine wave identical to the input waveform. Class C is suitable only for amplifying signals with a constant amplitude, such as FM, FSK, and some CW (Morse code) signals. Where the amplitude of the input signal to the amplifier varies as with single-sideband modulation, amplitude modulation, video and complex digital signals, the amplifier must operate class A or AB, to preserve the envelope of the driving signal in an undistorted form.  Such amplifiers are referred to as linear amplifiers.
  It is also common to modify the gain of an amplifier operating class C so as to produce amplitude modulation. If done in a linear manner, this modulated amplifier is capable of low distortion. The output signal can be viewed as a product of the input RF signal and the modulating signal.
  The development of FM broadcasting improved fidelity by using a greater bandwidth which was available in the VHF range, and where atmospheric noise was absent. FM also has an inherent ability to reject noise, which is mostly amplitude modulated. Valve technology suffers high-frequency limitations due to cathode-anode transit time. However, tetrodes are successfully used into the VHF range and triodes into the low GHz range. Modern FM broadcast transmitters use both valve and solid state devices, with valves tending to be more used at the highest power levels. FM transmitters operate class C with very low distortion.
  Today's “digital” radio that carries coded data over various phase modulations (such as GMSK, QPSK etc.) and also the increasing demand for spectrum have forced a dramatic change in the way radio is used, e.g. the cellular radio concept. Today's cellular radio and digital broadcast standards are extremely demanding in terms of the spectral envelope and out of band emissions that are acceptable (in the case of GSM for example, −70 dB or better just a few hundred kilohertz from center frequency). Digital transmitters must therefore operate in the linear modes, with much attention given to achieving low distortion.
caption: 20 kW PEP linear amplifiers used by short wave radio station HCJB in both SSB and DRM broadcasts
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