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AM broadcast amplifier

 
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AM broadcast amplifier
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AmRadiant
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Post AM broadcast amplifier Reply with quote
Can you point me in the right direction? I need a (simple) schema for a not so big power linear amplifier for AM Broadcast... for example... 100mW in 2-5-10 W out.

A class A would be very nice also, maybe with cb final transistor (2SC1969, 2SC2078, or BD135, BD329, ecc..).

I've tried only one work for 160 meters HAM (with two IRF510), but I need more ideas...

Any hope?

Thanks!

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** AM is a choice of life **
** FM is only a choice of fashion **
Thu Feb 09, 2006 10:56 am View user's profile Send private message
pcs
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Post Reply with quote
It is much more energy efficient to have a class-c or similar final stage
at say 40W peak and than modulate that directly with PWM modulator.

This generates a better sounding audio and uses a lot less energy
and thus generates far less heat.

CB tranzistors?
A 10W AM transmitter needs to be able of supplying 40W in mod. peaks.
Which CB transistor will handle this?

Amplifying 100mW to get to 5-10W will probably require a couple stages.
Each stage contributes a bit of distortion to audio since this is AM and you
will probably not run in complete linear A-stage. If you do, your transistor
will heat like crazy and you'll have 25% efficiency tops.
Its just a bad way to go about it. Its much better to modulate the output
stage directly.

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Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Feb 09, 2006 12:50 pm View user's profile Send private message Visit poster's website
AmRadiant
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Post Reply with quote
pcs wrote:

(CUT)
Its just a bad way to go about it. Its much better to modulate the output
stage directly.


Yes, you are true. But sometimes linear amplifier can be useful, for example with small PART 15 transmitters (100 mW max output) that works good and stable (pll or quartz controlled).

I think that a pair of 2SC1969 with AB class can work fine up to 8-10 watt... so I think that a 2SC2078 can work up to 2 - 2.5 watt class A.

I try to build some ugly and drifting ampli class A as above... and a PC supply fan to cooling a bit. I hope to have some sucess.

Naturally the real difficult part could be impedance matching

_________________
** AM is a choice of life **
** FM is only a choice of fashion **
Thu Feb 09, 2006 5:34 pm View user's profile Send private message
pcs
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Post Reply with quote
2sc1969 is rated at 16W max.

With a couple you may get about 25, maybe 30W, including losses.

With AM you need 4x the carrier power in modulation peaks,
which means you will get 30W/4, which is 7W carrier at best.

Now this is all with class C. I doubt you'll get more than 4W carrier in
practice in linear mode, if you want good modulation.

100mW is probably not enough to drive this amp. You need an extra
stage also. Lots of trouble for a result that will likely dissapoint you.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Feb 09, 2006 5:51 pm View user's profile Send private message Visit poster's website
AmRadiant
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Location: Italy

Post Reply with quote
I've make some test with a 2SC2078 with a very ugly and simple mine schema (680 ohm 1W resistor plus 1 KOhm trimmer in series from base to collector, an "rf" choke positive Vcc-collector, 10nF capacitor from cold side of choke and earth, in/out capacitors and a BIG fan)... with 70 mW in I've obtained 750 mW out at 1.2 Ampere (I think wasn't advisable go up further!) and 13.8V.

Naturally I must work heavly to impedance matching (my 70 mW do-it-yourself tx has only a 18dB lowpass filter, but impedence almost 2-3 ohm out, I rate...), but I haven't noticed big BF distortion (well... I use military dummy load however) and results are quite good for this crap amplifier.

Maybe a driver stage (BD135 ? 2N3866 ? BD329 ? I prefer this last one) and better impedance matching can make 1.5 Watt...

_________________
** AM is a choice of life **
** FM is only a choice of fashion **
Mon Feb 13, 2006 6:03 pm View user's profile Send private message
AmRadiant
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Post Reply with quote
pcs wrote:


With AM you need 4x the carrier power in modulation peaks,
which means you will get 30W/4, which is 7W carrier at best.

Lots of trouble for a result that will likely dissapoint you.


Oh yes, now I've understand what do you mean (transistor total failure!)

what do you think about AB class? It's more safe?

thanks!

_________________
** AM is a choice of life **
** FM is only a choice of fashion **
Tue Mar 07, 2006 12:10 pm View user's profile Send private message
pcs
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Post Reply with quote
This is what I'm talking about. Now you're in trouble and want to move more
into non-linear region to prevent overheating?

Yes, you can do that, but modulation quality suffers.
This is why my approach is better.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Tue Mar 07, 2006 12:17 pm View user's profile Send private message Visit poster's website
AmRadiant
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Joined: 02 Feb 2006
Posts: 11
Location: Italy

Post Reply with quote
pcs wrote:
This is what I'm talking about. Now you're in trouble and want to move more
into non-linear region to prevent overheating?

Yes, you can do that, but modulation quality suffers.
This is why my approach is better.


Eh eh, ok... you are true of course.

I think that for now this is the way (I think that I must use a very big lowpass filter... say 54 dB... for broadband 5 watt push-pull AB amplifier...).

Then I'll go to a completely new transmitter with C class with more power(maybe with a classic IRF510 or some old BUZ...)... and a bit less frequency drifting!

Many thanks, have a good work!

_________________
** AM is a choice of life **
** FM is only a choice of fashion **
Thu Mar 09, 2006 10:06 am View user's profile Send private message
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