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