| Gain Compression |
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RELEVANCE Gain compression is relevant in any system with a wide dynamic range, audio or RF but is more common in Tube circuits than Transistor circuits possibly giving what has been called Valve Sound . The front end RF amps of Radio Receiver s are particularly susceptible to this phenomenon when overloaded by a strong unwanted signal. AUDIO EFFECTS A tube radio or tube amplifier will increase in volume to a point, and then as the input signal is further increased beyond the linear range, the effective gain is reduced. It exhibits soft limiting, but is said not to be acting as an Audio Level Compressor . Tube or transistor, gain compression means the same thing and is caused by clipping, soft or hard. It's relevant in any system with a wide Dynamic Range , audio or RF. DIFFERENCES BETWEEN CLIPPING AND COMPRESSION Gain is a linear operation. "Gain compression" is not distortion as such, but its effect is one of distortion, due to the nonlinearity of the transfer characteristic which also, causes a loss of 'slope' or 'differential' gain. So the output is less than expected using the ''small signal'' gain of the amplifier.
'Soft' clipping or limiting means there isn't a sharp "knee point" in the transfer characteristic. A Sine Wave going through soft clipping would become more like a Square Wave with rounded edges but will still have lots of extra harmonics.
"Limiting" can mean either of the above. Compression of gain is caused by non linear characteristics of the device when run at high levels. With whatever sort of signals, as the input level is increased, gain compression will occur at some point. A transistor's operating point may move with temperature, so higher power o/p may lead to compression due to collector dissipation. But it's not a change in gain; it's non-linear distortion. The output level stays relatively the same as the input level goes higher. Once the Non-linear portion of the transfer characteristic of any amplifier is reached, any increase in input will not be matched by a proprtional increase in output. Thus we have compression of gain. Also, at this time because the transfer function is no longer linear, Harmonic Distortion will result. RADIO FREQUENCY COMPRESSION "Gain compression" in RF amps is similar to soft clipping. However, in Narrowband systems, the effect "looks" more like gain compression simply because the harmonics are filtered out after amplification. Many Data Sheet s for RF amplifiers list gain compression rather than distortion figures simply because it's easier to measure In Wideband and low Frequency systems, the non-linear effects are readily visible, e.g. the output is Clipped . To see the same thing at one GHz , you would need an Oscilloscope with a Bandwidth of at least 10+ GHz. You could see it with a Spectrum Analyzer , but what you see is the fundamental compressed and the Harmonic s picking up. Examples of RF compression A low-noise RF Amplifier if fed by a Directional Antenna to a consumer 900 MHz receiver should improve the transmission range. It works, but the receiver may also pick up a couple of UHF stations around 700 MHz. For example, if channel 54 is transmitting 6 MW of AM , FM , and PM , the RF front end, expecting -80 dBm, would be grossly overloaded and generate mixing products. This is a typical effect of gain compression. HIGH POWER LOUDSPEAKERS A form of gain compression also takes place in Loudspeaker Voice Coil s when they heat up and increase their Resistance . This causes less Power to be drawn from the amplifier and a reduction in SPL . SEE ALSO |
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