Hi Jim, Thanks for all the effort that you put into this explanation, but yes, I am aware of all the considerations that you mention. I am not confused about what a class C amp is or what it is used for, I just wondered about why RF power transistors were not speced for switching performance, and I now realize that it is because class C amps are usually not run completely into saturation. Yes, I planned on discussing the appropriate applications for each amplifier class. I don't expect my tutorial to make seasoned RF designers out of people, just to help beginners over some of the "humps" that they encounter in trying to understand RF work. Incidentally, I would appreciate your comments on the tutorial when it is finished. Thanks again, Sean On Tue, 10 Jul 2001, Jim wrote: > Hello again ... > > "First of all ... I am not designing for a specific > application ... . I am just trying to write a short > explanation of how class C amplifiers are designed. > I want to make sure I explain, in this writeup, *how > to select a transistor* for such an amplifier." > > A lot of competant men (and women) schooled in solid > state physics and semiconductor fabrication as well as > practical engineers versed in the practical experience of > RF amp design have spent countless hours putting > together those design guides and data manuals. The > 'selection' of device based on arbitrary parameters > described coldly in a text on design or in a short paper > will be pressed to do really do this topic justice (no > offense) without 'shorting' some aspect of this art. > > I wonder, do you plan on desribing what applications > demand what style/type of amplifier: class A, class AB, > class B and class C? > > Each class has it's *prime* application - but special > treatment of RF amplifiers (as opposed to audio amps) > and each classification demands special descriptions > that audio applications usually do not need. > > For instance: a class C amp is not sutable for low RF power > apps like RF front ends! This should make sense as the > *driving* signal is insufficient most all of the time to even > cause the device to conduct. > > A class C amp *is* suitable, however, as the final PA (power > amplifier) in an FM transmitter such as those used by hams > in the 2M (144 MHz) band. > > A class C 'amp' may also be plate, collector, or drain modulated > (called hi-level modulation) as the output stage in an AM > transmitter. > > A class C amp used as an outboard amp following either an AM > or SSB (both are considered linear modulation techniques) exciter > would prove to be the wrong move as it would either be full on > with just the carrier driving the device into conduction or RF > would only be generated on modulation peaks of the AM/SSB > signal envelope. > > A class A - or more likely, as is done in the actual practical case, > a class AB amp (a 'linear' amp) would be used following an AM > or SSB exciter/transmitter. > > Two devices operating in class B (working in push-pull) would > also be suitable following an AM or SSB transmitter. > > Did you have a particular environment that you were slating > this paper for: low power consumer unlicensed devices (milliwatts) > or ham class (2 to 3 watts on up to 1000 watts) or commercial > (1000 watts and over)? > > As an aside - there are many ways to also generate > "AM" signals besides the hi-level technique described > above - there is also "low-level" technique would require > all stages following the modulated/modulated stage (of > course) be 'linear' (not class C) in nature. There is also > the pulsed-modulated technique that is beyond the current > scope of this description. > > I guess you do also understand the important aspect that LC > networks (like plate or drain/collector 'tanks') play in 'completeing' > the portions of the waveform where the class C device is not > conducting (on) in an RF amplifier? > > This 'flywheel' effect is a very important aspect of insuring that > sinusoids (as opposed to square waves) with sufficient purity > are the ultimate result (besides the matching duties these LC > 'tanks' provide in performing impedance matching). > > > "If you are using a FET or tube, then, AFAIK, there is no > speed-reduction penalty for going to the completely on > (ohmic) region of operation, so the "saturated" model of > class C operation makes complete sense for FET or tube > circuits." > > There are other factors that come into play that serve to limit > performance with these devices. There are still no miracle devices, > but devices have improved in the last decade. > > "It also has the advantage that you can do AM modulation > by changing the supply voltage. I think this is why my > references use this model." > > This trick has been played with bipolar devices since > their introduction - and with good results (millions of > design/production cost effective CB radio designs use > this technique, aircraft transmitters utilize AM and have > use high-level style modulation as well). > > I don't want to pound this point to death, but there is little > substitute for consulting someone in the field who has the > background and knowledge to understand the intricacies > involved and the pitfalls lurking just around the corner > when if comes to designing things 'RF' ... > > Jim > > > ----- Original Message ----- > From: "Sean Breheny" > To: > Sent: Tuesday, July 10, 2001 11:17 AM > Subject: Re: [EE]: Class C amps > > > Hi all, > > Hi all, > > As usual, thanks for the quick replies. > > First of all, let me explain that I am not designing for a specific > application right now. I am just trying to write a short explanation of > how class C amplifiers are designed. I want to make sure I explain, in > this writeup, how to select a transistor for such an amplifier. > Therefore, I don't want to just say "pick one which says it will work for > this application", I want to explain what is required for a transistor to > work well in a class C amplifier. > > The whole point of my tutorial is to help make people "RF literate". It > does this, in part, by explaining how RF circuits work at the transistor > level. I think it is valuable to know how this works even if all you are > doing is just using prebuilt modules. In addition, the lessons learned in > trying to understand such RF circuits are useful in making sure that you > make intelligent design choices even in using off-the-shelf modules. > Besides, aren't you curious about how radio works at all levels? I > wouldn't be in the hobby or in the profession if I weren't. > > Secondly, thanks for the book recommendations, but I already have > Motorola's RF Device Data book and Mini-Circuits RF designer's guide. In > fact, it was looking through Motorola's book that raised this question in > my mind to begin with. > > After considering your responses, I think the problem is that "class C" > can mean several different things and the references I have give only one > meaning of it. They consider class C operation to be where the active > element (BJT,FET,tube,etc.) is only either completely on or completely > off. While in the on state, they consider it to act like a small > resistance. This gives you a behavior which can approach 100% efficiency > (looks like 85% max in their graph for a practical case) where the output > amplitude depends only on the conduction angle and supply voltage, not on > the input amplitude. > > Apparently class C can also refer to the case where the transistor or > tube is not fully on in the conducting portion of the cycle. In this > case, the output amplitude depends on the input amplitude. Because the > average DC current can still be much smaller than for class A, it is more > efficient than class A. So, the a similar efficiency analysis applies as > applied above, but the circuit's output amplitude is now much less > dependent on the supply voltage and totally dependent on the input > amplitude. > > If you are using a FET or tube, then, AFAIK, there is no speed-reduction > penalty for going to the completely on (ohmic) region of operation, so > the "saturated" model of class C operation makes complete sense for FET > or tube circuits. It also has the advantage that you can do AM modulation > by changing the supply voltage. I think this is why my references use > this model. > > For BJTs, though, internal charge storage effects create speed penalties > for going into saturation. In general, these cannot be modeled as > capacitances because they invovle some pure delays (called Td and Tsd, Td > being the pure delay to begin to turn on and Tsd being the pure delay to > begin to turn off after being completely on). These delays depend only on > the amount of base overdrive, and are pure delays (a sudden change in the > input causes no change in the output until Td or Tsd time has elapsed), > so they do not act like capacitances. Therefore, S parameters or other > small-signal models would not model them. Yes, real switching circuits > also have capactive delays which would be modeled by S paramters, but > those are not the whole story for BJTs. > > I think this now explains why the datasheets have only S parameters and > no timing parameters: they are for upper end HF,VHF, and UHF applications > where it is very difficult to make a BJT come out of saturation quickly > enough. So, they assume that you are thinking along the lines of the > "linear for part of the time" class-C model and give you the parameters > which help you to design such a circuit. > > I just wish that the books I have made this distinction. It's because of > all of these little frustrating, subtle points that I have wanted to > write this tutorial in the first place. It will contain a lot of > explanations of such things which can often stump beginners. > > Thanks again for your help, > > Sean > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu