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

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

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

"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.

"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

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

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'."

"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

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

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.