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#1
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HF amplifier with PNP
Hi,
I feel a little lazy today, and Mr. Google didn't return anything useful to my searches. Can anyone point me to a circuit diagram of an HF amplifier that uses a PNP transistor instead of NPN? I know that they behave the opposite, but unfortunately I haven't been designing BJT amplifiers in the past years and I'm now a bit lost. I have some spare PNP power transistors with a decent GBW (or ft), and I'd like to see what happens if I use them as the PA stage of my homebrew CW transmitters. Thanks, Paolo IK1ZYW |
#2
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I found the appended article on the Google record, which may assist you.....
"PaoloC" wrote in message ... Can anyone point me to a circuit diagram of an HF amplifier that uses a PNP transistor instead of NPN? I know that they behave the opposite, but unfortunately I haven't been designing BJT amplifiers in the past years and I'm now a bit lost. -----OOOOO----- In article , writes... Hi, Would you please tell me what transistor do and how to do so? What is the difference between PNP transistor and NPN transistor? What is the concept of using a transistor as a switch? Thanks a lot. SemiConductor Theory -------------------- (c) 1997 by Gareth Alun Evans A transistor is rather like the human alimentary canal, after the typical USA diet of burgers and chips; - it constipates, as do semiconductor diodes with no applied bias - the available holes get filled in and nothing can move. The base current is like a small application of laxative; some of the constipation passes through, until the effect of the laxative wears off. The total throughput depends upon the Mobility. By applying a continuous feed of laxative, then a continuous current passes through. Applying too much laxative results in saturation - ie, there is a limit to the maximum throughput, depending on the external circuit; in this case, the maximum rate at which you can feed in the burgers at one end. (If you are a customer of MacDonalds's, here in Chippenham, Wiltshire, UK then this rate is very low - I have been there twice, and both times, the service was *APPALLING*.) The difference between PNP and NPN is the direction. In the old days, PNP was used, whereby one injected from the rear end, using a sort of huge syringe - hence PNP - "Put-in Near Poo". More recently NPN is more common, where the laxative is entered via a carrier of some sort, usually chocolate and so we have NPN - "Now Pleasant Nutrient". Despite the adverse effects, the USA diet of burgers and chips carries on, and recourse has to be made once again to the chocolate. Now the ratio of the carriers of the constipation, the burgers and chips, is much higher than that of the chocolate. Thus they are referred to as the Majority Carriers and the Minority Carriers. If you indulge too much, you find that the vendor will provide you with a paper bag, known as an Excess Carrier. More recently, there are problems with impurities and you find that the opposite effect occurs. You have no time to reach home before diarrhoea takes over. You have no option but to stop the car and nip over a gate into a field. Hence the Field Effect Transistor. This time you have to inject something to STOP the flow. Now, assuming that there was a certain control over events; nothing happened until the Gate was encountered, you then became the Source of flow, and the field itself acted as the Drain. What was originally dirt, became grass, was consumed by the Cow, you ate it as a burger, and it has now returned to the topsoil, an effect known in the trade as Surface Recombination. (Incidentally, did you know that Diarrhoea is hereditary? Apparently it runs in the jeans.) Some of the impurities accumulate in your rear end, and no matter how valiently you try, you cannot rid yourself of them. Hence In-de-Bum is known as a Try-Valient Impurity. In the same way, Arse-nic, well known for its ill-effects and accumulation in the body tissues is known as a Pent-Up-Valient Impurity. -- 73's etc de Gareth G4SDW (nee G8DXY) GQRP 3339 |
#3
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Connect it exactly the same as you would an NPN, but reverse the
polarity of the power supply voltage. If there's a bias circuit using a diode, the diode (and bias supply polarity, if it's a separate supply) should also be reversed. If it's necessary for you to use the PNP with a positive supply voltage, it's still possible, but it complicates the circuit, and adequate bypassing becomes essential and sometimes difficult. In that case, a diagram (and preferably a layout) of a known working circuit is desirable. All other things being equal, a PNP generally has lower ft and higher capacitance than an NPN, which is probably why they're less commonly used. This difference is due to the basic physics of the device (lower mobility of the carriers, as I recall). Roy Lewallen, W7EL PaoloC wrote: Hi, I feel a little lazy today, and Mr. Google didn't return anything useful to my searches. Can anyone point me to a circuit diagram of an HF amplifier that uses a PNP transistor instead of NPN? I know that they behave the opposite, but unfortunately I haven't been designing BJT amplifiers in the past years and I'm now a bit lost. I have some spare PNP power transistors with a decent GBW (or ft), and I'd like to see what happens if I use them as the PA stage of my homebrew CW transmitters. Thanks, Paolo IK1ZYW |
#4
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"Airy R. Bean" wrote in message ... I found the appended article on the Google record, which may assist you..... SemiConductor Theory -------------------- (c) 1997 by Gareth Alun Evans Now be a good little boy Gareth and admit you and "Airy" are one and the same. You know it makes sense. |
#5
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"Roy Lewallen" wrote in message ... Connect it exactly the same as you would an NPN, but reverse the polarity of the power supply voltage. If there's a bias circuit using a diode, the diode (and bias supply polarity, if it's a separate supply) should also be reversed. If it's necessary for you to use the PNP with a positive supply voltage, it's still possible, but it complicates the circuit, and adequate bypassing becomes essential and sometimes difficult. In that case, a diagram (and preferably a layout) of a known working circuit is desirable. All other things being equal, a PNP generally has lower ft and higher capacitance than an NPN, which is probably why they're less commonly used. This difference is due to the basic physics of the device (lower mobility of the carriers, as I recall). Roy Lewallen, W7EL I always wondered why Motorola (of all peopled) used PNP RF transistors in the final and two driver stages in their Micor base station and mobile radios. Pete |
#6
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Uncle Peter wrote:
I always wondered why Motorola (of all peopled) used PNP RF transistors in the final and two driver stages in their Micor base station and mobile radios. Were those perhaps germanium transistors? The first Ge transistors were PNP, and the first (Ge) audio power transistors I recall seeing in wide use, in auto radios, were PNP. I'm not sure why PNP was favored in the early days, whether due to some basic physics reason or some more practical processing or production considerations. Roy Lewallen, W7EL |
#7
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Roy Lewallen wrote:
I'm not sure why PNP was favored in the early days, whether due to some basic physics reason or some more practical processing or production considerations. I always had the distinct impression it was processing/production considerations. -- Frank Abagnale on "Rathergate" (the CBS forgery scandal): "If my forgeries looked as bad as the CBS documents, [the movie "Catch Me If You Can"] would have been 'Catch Me In Two Days'." |
#8
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"Roy Lewallen" wrote in message ... Uncle Peter wrote: I always wondered why Motorola (of all peopled) used PNP RF transistors in the final and two driver stages in their Micor base station and mobile radios. Were those perhaps germanium transistors? The first Ge transistors were PNP, and the first (Ge) audio power transistors I recall seeing in wide use, in auto radios, were PNP. I'm not sure why PNP was favored in the early days, whether due to some basic physics reason or some more practical processing or production considerations. Roy Lewallen, W7EL Hello Roy These fairly modern devices, with 211-xx style flange mounts. Although they were marked with Motorola Communications inhouse numbers, they appear to have been part of the general Motorola RF transistor family, with complimentary NPN equivalents. pete |
#9
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Hm, y'got me, then.
Maybe they had a big stock of them left over from a canceled project, so decreed that the engineer had to design them in so they'd get used up. That sort of thing does happen. . . Roy Lewallen, W7EL Uncle Peter wrote: Hello Roy These fairly modern devices, with 211-xx style flange mounts. Although they were marked with Motorola Communications inhouse numbers, they appear to have been part of the general Motorola RF transistor family, with complimentary NPN equivalents. pete |
#10
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PaoloC wrote:
Hi, I feel a little lazy today, and Mr. Google didn't return anything useful to my searches. Can anyone point me to a circuit diagram of an HF amplifier that uses a PNP transistor instead of NPN? I know that they behave the opposite, but unfortunately I haven't been designing BJT amplifiers in the past years and I'm now a bit lost. I have some spare PNP power transistors with a decent GBW (or ft), and I'd like to see what happens if I use them as the PA stage of my homebrew CW transmitters. Thanks, Paolo IK1ZYW PNP transistors were more common in the days when germanium was the semicoductor material of choice, with silicon NPN is the more common type. The material used just happens to 'work better' with one polarity than the other. Even so, there are some good HF pnp power transistors, but as you go higher in power and frequency they become rarer and more expensive. For QRP hf the 2N2905/7 is a good PNP output transistor (1-5w), and you can use 2-4 in pp/p. |
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