RF transistor - Page 1
Forum section: electronics / rf-microwave

author: yalect
author_rank: Regular Contributor
timestamp: January 09, 2018, 10:23:18 am
content: Hi,I would like to ask, is transistor BC547 suitable for using with 27Mhz and which cheap transistor properties and references are prefer to use with such range of 100Mhz or 200Mhz?.what factors or rules that we based on to select high frequency transistor?Thanks

author: bd139
author_rank: Super Contributor
timestamp: January 09, 2018, 11:07:42 am
content: I've only gone as far as 145MHz...BC547 will work at 27MHz in the role of an amplifier but wont have a ton of gain. Lucky if you get 20dB out of it. I'll work fine as an oscillator right into VHF range (on the basis this happens accidentally quite a lot to me). You also have to stick a lot of collector current in to get the max gain out. I tend to keep them under the 15MHz territory.2n2369 / BSX20 is a good goto transistor for up to 100MHz or so (low VHF). 2n3866 for small power amps under 100MHz or so.2n5109 is a winner for higher VHF, low UHF. I scored a bag of these for not much money.Really the trick I have found when designing RF stuff in this space is get your signal downconverted to a more manageable frequency quickly. So if you have a VHF receiver (2m for example), start low and use a frequency multiplier for the VFO, then mix this down to a much lower IF ASAP. This keeps the price and complexity down as well as the usual benefits!I'm sure there are other parts so I'm quite interested to see the replies.

author: xaxaxa
author_rank: Regular Contributor
timestamp: January 10, 2018, 06:59:00 am
content: These are some of the parts I use for communication applications:small signal transistors for VHF:* 2n3904, 2n2222 - about 10dB of gain @ 100MHz, if conjugate matchedsmall signal transistors for VHF/UHF:* bfg425w - low noise, amplifier up to 3GHz, for signal level ~0dBm* 2sc3355, 2sc3357 - low noise amplifier up to 1GHz (10dB gain), high linearity, for signal level up to 10dBm* bfg591, bfg135 - amplifier up to 1GHz (10dB gain), for higher signal levels (<20dBm), high linearity* bfp740 - amplifier up to 10GHz (15dB gain)These can all be used at lower frequencies (down to few MHz), but apply some feedback (resistor-capacitor between collector and base) to get good linearity. You can get >35dBm OIP3 at 100MHz using the 2sc3357 or bfg591 with this technique.These can be used unmatched; e.g. direct connection to 50ohm ports (dc blocked and biased of course). You can get slightly higher gain if you conjugate match them.Always test your circuit in rfsim99 or ltspice to ensure it has the right characteristics and is unconditionally stable. You can download S parameters for all of these transistors and import them into rfsim99. Some will also have spice models. Generally, use rfsim99 to check small signal gain, frequency response, and stability; use ltspice to check for large signal (transient) response and linearity.For low noise applications make sure you present the correct impedance (as specified by the datasheet for best noise performance) to the base of the transistor.More part numbers can be found on nxp/infineon website.medium power transistors:* rqa0009 - 4W, DC to 500MHz, requires matching* acpm-7868 - not a transistor, but a 2W mmic PA for 800/900MHz; internally matched to 50 ohms and internally dc blocked (!), very easy to use900MHz power transistors- usable for VHF and lower as well; requires matching; refurbished ones from aliexpress work fine* mrf184 - 60W* mrf9060m, mrf9060n, mrf6s9060n, mrf5s9070nr1 - 60W* mrfe6s9125n, mrf6s9125n - 120WS band transistors* mrf21085 - tested working at 2.45GHz, but not very high gain (10dB)* mrf6s27085h - tested working at 2.45GHz, 15dB gain, 20W outAll of the above can usually be found for reasonable prices on aliexpress, and they all work fine. I've personally measured (with a vna) some bfg425w transistor from aliexpress and compared them to mouser ones, and noticed no difference.None of these transistors have any lower frequency limit (other than the mmic) and can operate to DC.For the medium and high power transistors pay very close attention to the output impedance matching. The datasheet will specify for each frequency what load impedance the transistor must see. Generally for VHF you can get away with lumped element matching networks (make sure you use high Q high current capacitors! ATC100B is what's usually used); for UHF a very wide microstrip (fat line) impedance transformer is usually used; often the output matching network is no more than a rectangular piece of copper on the pcb.

author: G0HZU
author_rank: Super Contributor
timestamp: January 13, 2018, 08:56:55 pm
content: QuoteIt's not a device I would normally choose for an amplifier at 27MHz because its internal capacitance means it will suffer more from Miller Effect as a common emitter amplifier. Other devices will have better linearity and lower noise figure and can operate over a wider bandwidth with lower VSWR if you wanted to make a wideband 50R in/out amplifier for example. It could be used as a common base amplifier but I'd still prefer to use something else. Even cheapo 27MHz CB radios use better transistors than this in the receiver RF and IF sections.However, I did measure a typical BC547B on a VNA a while back to see how high it might be able to oscillate and the VNA model predicted it could oscillate at over 600MHz when operated at a reasonably high Vce with the right amount of feedback.

author: yalect
author_rank: Regular Contributor
timestamp: January 15, 2018, 10:07:24 am
content: Hi,thank youmaybe there are other suggestion and simple principles?

author: madires
author_rank: Super Contributor
timestamp: January 15, 2018, 11:27:45 am
content: BF199 or 256?