Friday, November 10, 2006

Should Class C Amps Saturate?

I have been trying to figure this out and have been posting the following questions on If anyone out there can help, please send me some comments or e-mail. Thanks, Bill

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? -------------------------------------------------------- SECOND POST
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!"
Thanks to all who responded. I think I'm starting to understand this. LTSpice helps a lot. I set up a class C amp and looked out power dissipated in the transistor vs. power dissipated in the load. The big efficiency gains that come with saturation were very apparent.
To better understand WHY this happens, I set up a spreadsheet that looked at power dissipated in a variable resistor as it swept from .1 ohms to 10 ohms. It had a fixed 10 ohm resistor in series and 10 volts DC across both of them. Yes indeed, the power dissipated in the variable resistor drops off dramatically when the resistance (and hence the voltage across it) gets very low. I guess is why this happens in the saturating Class C amp, Right?
But I still have some questions. When we design a Class A amp, the familiar formula Rload = (Vcc-Ve)^2/2Pout allows us to come up with a load value that will prevent the amplifier from saturating. A load of this value will cause the voltage across the transistor (collector to emitter) to vary from zero to twice Vcc. But it won't go into saturation.
Why then do so many of the books (EMRFD, SSDRA, the W1FB books) call for the use of essentially the same formula for the load when selecting a load for Class C amps? We're no longer worried about staying out of saturation, correct? In fact, we want to saturate. So why the same formula? In fact, it seems to me that if you have a Class C amplifier that is designed with this formula and is operating just below saturation, you can get it to saturate just by increasing the value of the load presented to the collector. Power out and efficiency immediately improves. Linearity, of course, does not.
Thanks, 73 Bill M0HBR