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Class C amps and saturation (again)
Back in July I wrote:
I'm reading David Rutledge's excellent "The Electronics of Radio." In Chapter 10 -- Power Amplifiers, he discusses Class C amps and says, "In addition, if we drive the transistor clear to saturation, using the transistor as a switch, the dissipated power can be greatly reduced, because the saturation voltage is low. This is Class C amplification..." I'd always throught that in Class C, while you'd operate the device so that it was cutoff during most of the cycle, but not saturated. Is this just a different definition of Class C? I checked back with SSDRA and EMRFD, and didn't see anything about driving Class C amps into saturation? What says the group? Do we saturate in Class C or not? -------------------------------------------------------- I've been thinking about this some more. The 1980 ARRL handbook points out that "Solid State power amplifiers should be operated just below their saturation points for best efficiency and stability." Also, the formula that we use to determine load resistance (Rl=Vcc^2/2Po) implies that we are looking for a combination of Vcc, Load resistance and power out that will prevent saturation. And wouldn't we end up with far lower harmonic content if we only clip one side of the wave form (at cutoff) instead of both sides (cutoff and saturation)? I know there are more exotic modes beyond C, but for plain old ordinary ham radio applications, don't we normally avoid saturation in Class C amps? Also, what about this business of having the efficiency improve through saturation "because the saturation voltage is low" Could that be right? If you put a voltage across a conductor and generate a large current, you can't sit back and say "Great! Power consumption across the conductor is low because the voltage drop across it is now minimal!" 73 from London Bill M0HBR N2CQR CU2JL http://www.gadgeteer.us |
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