Can someone help me find sutable rf transistor for my TV transmitter circuit

[sm1346]

Hi, i need help because i can't find suitable transistors for the rf preamp/buffer/final stage for my homemade analog tv transmitter (audio+video)
the signal will be received with a standard analog tv
it will be operating in VHF or UHF band, with output of ~5W
can a RF mosfet such as RD06HVF1 work as the output stage?(fT 175Mhz| Power output 6W with ~0.4w input | 13dB gain)
for the analog tv signal generation before the preamp i'll be using an standard rf modulator (because the output is already filtered)
i've heard that class C rf amp cannot be used for TV transmitting purposes, is it true or not? If so, can someone help me find an amplifier type that would be suitable for my project

Thanks

 

[Audioguru]

Do you have a license to operate 5W at the frequency you want and is the circuit certified to produce very low level harmonics (class-C produces harmonics) to avoid interference with other communications?

 

[G4BCH]

First take heed of Audioguru's comments above.
Making a TV transmitter amplifier is not as simple as choosing a few transistors. The device you mention is nowhere near adequate to amplify an analogue TV signal to 5W without significant distortion products being generated. You might get 0.5W perhaps 1W if well designed, and it won't work at UHF.
If by a standard rf modulator you mean something that is plugged into a TV set than that will not give you a signal that will be acceptable to amplify to 5W an put into an antenna. If it is a professional modulator with VSB filtering and up-conversion then the design of the power amplifier that follows it is not going to be trivial even at 5W O/P if the signal quality is to be maintained.
I would not want to discourage anyone from learning how to do it.
Look up linear power amplifier design, there are plenty of application notes for TV power amplifiers, NXP would be a good place to start. From there you should be able to find out about input, output and inter-stage impedance matching.
Then look at power distribution and bias supply design, this will be important to maintain signal quality.
Another important topic is filter design, you will need a good low-pass filter on the output of the amplifier.
Before you build it learn about EMI (electromagnetic interference) screening and filtering or you will build yourself a power oscillator.
Lastly, take heed of Audioguru's comments above.

>> i've heard that class C rf amp cannot be used for TV transmitting purposes,
They can, I've done it, it is very difficult; you need all the above skills and more to make it work.

 

[BigBoss]

-C-Class Power Amplifiers are not used in TV Broadcasting Transmitters due to AM Modulation.C-Class Amplifiers are used for "Constant Amplitude" signals.
-For Power Transistor, you have many possibilities such as Bipolar.LDMOS ,GaN etc.You have to choose one appropriate transistor from different manufacturers such as;
www.ampleon.com
www.nxp.com
www.wolfspeed.com

 

[sm1346]

What kind of rf amp type could I use then? Class A?thx

 

[betwixt]

Yes, class A is what you need. The amplification must be linear for AM and you may need the amplifier to be rated higher than you imagine. Almost all TV sound uses 'inter-carrier' modulation which requires that the modulation index is kept well below maximum so that dips in the video don't cut off the sound carrier.

Brian.

 

[sm1346]

Yes, class A is what you need. The amplification must be linear for AM and you may need the amplifier to be rated higher than you imagine. Almost all TV sound uses 'inter-carrier' modulation which requires that the modulation index is kept well below maximum so that dips in the video don't cut off the sound carrier.

Brian.

Can you provide me an example circuit?
Thx

 

[vfone]

You can use the Test Circuit schematic provided in the datasheet at page 5. There is all the information how to build the circuit:
**broken link removed**

As it is, the PA will work with gain higher than 15dB in the range 130MHz - 250MHz.
Mitsubishi recomend using PTFE substrate, but using PTFE at 175MHz is a non-sense. You may change the PCB traces width and lengths for a much cheaper FR4 substrate.
For linear class-A (needed for a TV AM signal) the Vgg voltage may be adjusted for an idle drain current of about 300mA to 500mA.

 

[sm1346]

You can use the Test Circuit schematic provided in the datasheet at page 5. There is all the information how to build the circuit:
**broken link removed**

As it is, the PA will work with gain higher than 15dB in the range 130MHz - 250MHz.
Mitsubishi recomend using PTFE substrate, but using PTFE at 175MHz is a non-sense. You may change the PCB traces width and lengths for a much cheaper FR4 substrate.
For linear class-A (needed for a TV AM signal) the Vgg voltage may be adjusted for an idle drain current of about 300mA to 500mA.

I dont really understand, the example circuit is class C I think(correct me if im wrong)
Thx

 

[vfone]

Is not class-C.
Set the transistor idle drain current as I mentioned above and you will get a linear PA suitable for AM modulation.