Re: VHF transistor

I don't know what "chips" (be wary of people who speak in buzzwords) you are talking about, but.......

Transistors are simple to find for any specification. You charging $15 for a transistor and 3.85 to ship is a rip-off.

Estimating costs on the high side, I'd say $2 + $.37 and $.25 for the envelope, toss in a little for profit......hmmmm....$5 total might do it ok.

Replacement for what? (My server has expired the predecessor.)

I'd like to hear someone explain the use of VHF in conjunction with transistor. Never knew a transistor could be band specific.

More like max-frequency specific. There are plenty of transistors that won't work that high well, if at all.

Read some data books!

Most power transistors seem to be specified and sold for specific bands. At the lower VHF bands this seems to have more to do with the things getting more fragile as they get capable of higher frequencies, so they are generally sold for the as-designed bands. At higher frequencies (UHF and microwaves) the dice or packaging will contain matching networks, so e.g. a 900MHz power transistor's performance will drop off both above and below 900MHz.

Well, I've never been that much of a techie! But I get the impression that we're talking about something more than a simple NPN or PNP junction in a can with 3 leads hanging off it. I suspect this "chip" referred to is in fact a whole circuit in a chip designed for a specific band. And what I think of as a transistor is only a very small piece of it. Might even be more accurate to call the device a VHF transmitter as opposed to transistor. But that's a discussion of semantics and if nothing else, demonstrates how things change in the high tech world.

It is a single surface-mount (4-pads) transistor good for up to about 900 MHz. It's a good transistor, but there is nothing spectacular about it, either for use in a receiver or a transmitter.

It is not rated as a "power amplifier," but is simply for applications as a wide band amplifier. Given the very low power application in an RC transmitter, the radio manufacturers can call it a "power amplifier" given that it is it's function in the radio regardless of the low-power application. There is no technical parameter that would prohibit its use at much lower frequencies, including audio.

The low junction capacitances that make this usable in the VHF/UHF frequencies are not as necessary in the HF range.

Don

I wasn't paying attention to the original post, I was just distracted by your assertion.

For 72MHz and 1 watt you'll most likely want to use a transistor that'll be good anywhere from 0 to 100MHz, and you may end up with one that'll be good for up to 500MHz. This has been true for decades. And while there may be radios that use power amplifier modules, most will probably use transistors -- and I would hazard a guess that the transistors used will be 30 year part numbers, possibly in surface mount, for cost reduction.

Note that the data sheet says includes emitter resistors. I think this means that it is several transistors intragrated into one chip with balancing resistors in the emitter leads to a common emitter terminal.

ISTR that this is done to get more power at higher frequencies with out overloading any part of the die.

Hugh Not an RF designer but do know which end of a soldering iron to pickup.

:) mk

Hugh,

That was my point. These are not simple, plane Jane transistors. Rather they are in fact a circuit with multiple components of different kinds which is designed for a certain freq range. Not unlike a tuned RF tank of old!

Your note about the emitter resistors caused me to re-read the datasheet and I discovered I had the wrong one.

Having downloaded and read the correct :) datasheet, my view still is that it is a just transistor with provisions for spreading the load.

There is nothing limiting it to some range of frequencies; the specs have S parameter info for 100 to 200 MHz, and the RC equipment manufacturer is using it at 72 MHz.

Don