lower frequency limits for hf transistors?

Can you post a data sheet where they say that? I have used RF transistors from DC to wherever I needed. The BJTs just don't have much beta and you need to maintain RF layout practices. If, for example, you use a BFS17A as a relay driver and run long lines then it might "sing".

Also, I have used UHF hotrods (AFAIK a BF998) as audio phase shifter devices. IOW as rather mundane controlled resistors. But don't tell anyone...

Can you give a specific example? Can you determine if it's a hard limit, or just a recommendation? The parts I've come across that are actually limited to some frequency range are more complicated that just a simple transistor. (I'm thinking, for example, of some integrated medium power amplifiers from MiniCircuits that are not rated to go below 10MHz. They really don't, too.)

One problem with using a UHF-rated part at HF is that you will have to pay very close attention to keeping it from oscillating at UHF. That is, if it has gain well beyond the frequencies of interest, and you don't pay attention to what it's doing over the whole range of frequencies, you can get stung. Clearly, if you are building a wideband amplifier, you'll need a part you can use over that whole bandwidth. Some folk build power amplifiers that cover four or more decades of frequency, up to hundreds of MHz.

Cheers, Tom

One example would be the input amp for lab gear such as network analyzers or even something as mundane as a frequency counter. They often need to go from DC to several GHz.

The input and output impedances of RF power transistors tend to be quite low. Some manufacturers put matching elements inside the package to help this problem, and of course they work only over a limited frequency range.

Cheers,

Phil Hobbs

One possibility: If you hook up a 450MHz transistor to a 7MHz circuit, the circuit is likely to have multiple spurious resonances at several places in the VHF range. Since the transistor has lots of gain at those frequencies, it's likely to break out into oscillation at the VHF frequencies.

That, and the beta is low. With the exception of MOSFETs, sometimes called UHF tetrodes for some reason. The BF998 one example:

Back in the aerospace daze, I had a broadband 50 MHz amplifier using

Output power meter would get "stuck" at 30 milliwatts with no input.

Using the fanciest test equipment then available, we verified a 950 MHZ oscillation.

Cured by putting a ferrite bead under one base lead.

Well, yes, although those don't generally have to put out several watts, which is what I inferred that the OP is looking at.

Cheers, Tom

On the other hand, the reactances of anything useful at UHF will generally be insignificant at 7MHz, and designing the appropriate matching at 7MHz should not be difficult. Or maybe I should say, "there are well-known techniques..." Now if the combination of internal reactances and the external circuit makes the part unstable, that would be worth considering!

Cheers, Tom

True, and the classic VFETs of the 80's are now history. Was great while it lasted :-(

Supposedly one of the German ham radio operators (DL9AH?) has published an article about how to build a huge shortwave power amp using a gazillion cheap RF transistors.

OK, I think I've got it. If you use a uhf transistor down at the lower frequencies you may run into parasitic oscillations at very high frequencies that could ruin your day (ie burn it up). And you may not even have test equipment on hand that you would let you observe the problem if you are way down there on the mhz range.

Makes sense.

tks Wa5ngp

Or have the neighbor banging on your door because the football game on TV was blurred out by it...