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Old 12th April 2006, 11:51 AM   #1
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Default RF Amplifier Problem

Hello everyone,


Well i guess this is the right forum for my topic.

I have attached ckt diag of an FM oscillator coupled to something which i tend to call as an UNKNOWN TYPE of RF Amplifier..

I/p is provided to the collector, and o/p taken from the base.. Sounds bad, atleast to me.. I have used this part of the circuit in constructing my first FM Transmitter. The complete ckt consists of a final class C power amp (2n3866) with a stated o/p of around 1WATT, and is driven by the o/p available with the circuit that have attached with this post.


The device works, but not as completely as it should. I certainly get very little power o/p with a range of around 10-15 meters indoor, using a 100cm aerial. Clarity and freq stability is good.

I havent designed this circuit. Just tried to implement it for fun. Since it works below par, i m now trying to analyze it.

If anyone can help me to understand that "alien" class C dirve amp., then pls reply back.

Thanks in advance.
Regards, Prasanna Prithviraj.
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Old 12th April 2006, 12:10 PM   #2
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I can't see as it can work?, and from what you say it doesn't work! - which isn't suprising!.

The range of just the oscillator part should be around 100m, basically it's probably just working with radiation directly from the oscillator section.
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Old 12th April 2006, 12:16 PM   #3
jez is offline jez  United Kingdom
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Agreed! I once built a 1W FM band 2 transmitter about 20 years ago, It could be received up to 4 miles away on a good day and in the direction that favoured good propagation!
The only thing I have seen that could be remotely like that circuit could be a parametric (varactor) power frequency doubler.....but then again....
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Old 14th April 2006, 11:51 AM   #4
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Default COMPLETE TRANSMITTER CIRCUIT POSTED HERE.

Thanks for the reply. So, i have modified the amplification section be simply using a Class A emiiter follower. The AC power dissipated by the load RE is about 200 mW at 12 Volts Vcc. I have posted the complete circuit here.

However, the range has not improved at all. Is there a problem in the antenna coupling ckt. (i havent used much of it eighter)..?


Waiting for a response..

Regards Prasanna Prithviraj.
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Old 14th April 2006, 11:54 AM   #5
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Default COMPLETE TRANSMITTER CIRCUIT POSTED HERE.

Ok here's the complete schematic..
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Old 14th April 2006, 12:25 PM   #6
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Default Re: COMPLETE TRANSMITTER CIRCUIT POSTED HERE.

Quote:
Originally posted by xitronics
Thanks for the reply. So, i have modified the amplification section be simply using a Class A emiiter follower. The AC power dissipated by the load RE is about 200 mW at 12 Volts Vcc. I have posted the complete circuit here.

However, the range has not improved at all. Is there a problem in the antenna coupling ckt. (i havent used much of it eighter)..?
An emitter follower gives no voltage gain, so you're not improving anything (apart from warming a resistor). It's usual to use a class C amplifier with a tuned collector loading coil, probably with a tap to match the impedance of the aerial.

Have a look here for an example.
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Old 14th April 2006, 12:55 PM   #7
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A few random thoughts:
Is 75cm a quarter wavelength antenna at your operating frequency? If so I take it you’re operating at approximately 400MHz. I hope your not doing this on a breadboard… For an FM system I think a Class-C amplifier will be the way to go [75% eff. Max]. When you design the input stage of your amplifier make sure it has an input impedance of at least 50-Ohms otherwise you could kill your oscillator by loading it down too hard. A 50-Ohm impedance matching band-pass filter on the output would also be wise.

Regards,
-=Randy
Mankato MN
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Old 14th April 2006, 05:43 PM   #8
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Default HOW IS THE APPROX. EVALUATION OF RF POWER GENERATED DONE..?

Quote:
Originally posted by Nigel Goodwin


It's usual to use a class C amplifier with a tuned collector loading coil, probably with a tap to match the impedance of the aerial.


Dear Sir,

Firstly I would like to thank you all for the patience and the worthy ideas given to me.


Alright, I would use an LC tuned ckt for the final stage. But how can i approximately calculate the RF power that the final stage would provide, as i may require that during designing.

As one final question sir.. My receiver detects the transmitted information at about 3-4 different frequencies (even though the reception at all; expect my actual transmitting frequency would completely vanish if i move 4-5 mtrs away from the Tx). Is it due to my cheap BC547 having slightly less Ro, which destroys the Q factor of the LC tank..?

Hoping for some suggesstions here.

_____________________

Thanks in advance,
Regds., Prasanna Prithviraj.
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Old 14th April 2006, 07:11 PM   #9
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Default Re: HOW IS THE APPROX. EVALUATION OF RF POWER GENERATED DONE..?

Quote:
Originally posted by xitronics



Dear Sir,

Firstly I would like to thank you all for the patience and the worthy ideas given to me.


Alright, I would use an LC tuned ckt for the final stage. But how can i approximately calculate the RF power that the final stage would provide, as i may require that during designing.



It's usual to specify the DC input to the output stage, you can measure the current and voltage simply with an ammeter and voltmeter.

Quote:

As one final question sir.. My receiver detects the transmitted information at about 3-4 different frequencies (even though the reception at all; expect my actual transmitting frequency would completely vanish if i move 4-5 mtrs away from the Tx). Is it due to my cheap BC547 having slightly less Ro, which destroys the Q factor of the LC tank..?
If it's transmitting harmonics it may be over driven, but a second tuned RF stage will help to reduce that - but you should also try lowering the first stage, try adjusting the bias conditions.
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Old 15th April 2006, 05:48 PM   #10
Rahul is offline Rahul  India
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Hi!

There are too many problems surprising it does not work. the coupling cap between stages is missing secondly its an emitter follower no gain .

make the o/p class C do a google on veronica and other FM tx design they are proven and good.

Need to transform the O/P impedence to 50 ohms
O/P impedence V*V/2Po

Use LC ratio to match.

This is the finest design that I have made till date as a free running OSC config:

http://www.geocities.com/Area51/Nebu...6/veronica.htm

Hope it helps

good luck
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