wideband TV amplifiers - possible solutions ?

[Antey]

Welcome Everybody

I'd like to ask for a piece of advice regarding to wideband (40-860/890 MHz) TV signal amplifiers. So far I have found just one - which seems quite promising **broken link removed** . I'm going to buy it, as soon as I get my pay but I have to present ("for dicussion") also two other possible solutions; and here's my problem - I need them, that is, their schematics. Although I already have other three (from univ. textbook), they were all judged as "too academic"(!) and thus rejected.

As far as I understand, such projects are far too profitable, to publish them openly... however, I hope that someone has at least one such design to share. Primary requirements are bandwidth and I/O impedances (75 Ohm), rest is really secondary choice.

The device/schematics shall primary serve for simulation & demonstration purposes, later to be assebled for my personal satisfaction. Any help shall be deeply appreciated.


Best Regards,
Peter

BTW : Does anyone have an expirence with DRAs (Dielectric Resonant Antennas) ? I'm just curious...

 

[Russlk]

That is an interesting schematic. L1 is a peaking coil to extend the high frequency response. L2 probably resonates near mid-band with C5, C4 and the diode capacitance. The input impedance is certainly not 75 ohms across the band, but it could be close. For optimum performance, R4 or TR or both should be selected but I doubt this is done for the kit.

You might try a common base input stage for better impedance control but would need two transistors for power gain.

 

[Nigel Goodwin]

As far as I understand, such projects are far too profitable, to publish them openly... however, I hope that someone has at least one such design to share.

You understand wrong, as you can see from the circuit you gave the link to (which again gives an explanation of how it works), it's a very simple circuit. The reason you don't see many projects is due to constructional difficulties - although it's a very simple circuit, construction is critical because of the high frequencies and high bandwidth. There's a good chance that it will oscillate, and interfere with other users over a number of miles!.

There have been numerous projects over the years, for a while you could buy special wideband amplifier IC's, these were simple to use, and the manufacturers provided a suitable PCB layout.

 

[Antey]

Thank You very much for your answers

There's a good chance that it will oscillate, and interfere with other users over a number of miles!.

:shock: Very, very interesting... the energy of these oscillations will leak to the environment not through the circuit itself but through the antenna it is connected, so the part will act as a transmitter, right ?

There have been numerous projects over the years, for a while you could buy special wideband amplifier IC's

I have requirement for discrete parts, but for solutions overview it might do the job.

You might try a common base input stage for better impedance control but would need two transistors for power gain.

Do you mean a darlington configuration ?

 

[Nigel Goodwin]

You might try a common base input stage for better impedance control but would need two transistors for power gain.

Do you mean a darlington configuration ?

No, he probably means a 'cascode' configuration, commonly used in RF stages.

 

[Antey]

No, he probably means a 'cascode' configuration, commonly used in RF stages.

OK - I know that one; one gives bandwidth, the other - gain.
Anyway, I'm going back to work - and try to report back when it's done.

 

[audioguru]

I bought a cheap TV RF amplifier with that simple circuit. All the local TV stations severely overloaded it so that they were intermodulating all over the band. It is probably made to receive only a couple of weak distant stations.
Then I tried it as a cable-TV amplifier and it was even worse. A look in catalogs at "real" cable-TV amplifiers showed very complicated circuits with very high prices. They were rated for the quantity of TV signals they could amplify without intermodulation.

I modified my TV RF amplifier by using much more current for the UHF transistor and I got it working for cable-TV pretty well. The original circuit had the little transistor cutoff most of the time. :lol: