Is it easy to amplify a crystal oscillator's output to 1/2Watts at 50 Ohms? - Page 1
Forum section: electronics / projects

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 11, 2021, 06:00:43 pm
content: Wondering is this is a fairly easy project to do.I need a clean and pure sinusoidal signal into 50Ohms that can be amplified up to 1/2 watts through some kind of DC bias.I can build the low pass filter that would follow the after the amplified signal. Can I just use some kind of simple transistor to amplify the signal? Would a Class A amplifier be the best approach in the instance, since the wattage is so low? A small heatsink should be sufficient. Was looking to amplify a 13.56MHz crystal oscillator.

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2021, 10:23:17 pm
content: 1/2 watt into 50 ohms is 100 milliamps and 5 volts so there are a few options. Simple transistor amplifiers can do it but several fast operational amplifiers in parallel are also a possibility. Note that if source termination is desired, then either the output voltage or current has to be doubled and half of the power is lost in the source termination.How much distortion is acceptable? What power supply voltages are available?

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 12, 2021, 01:33:22 am
content: Quote from: evb149 on September 11, 2021, 06:45:22 pmThanks for the reply! So I already have a class AB amplifier that can go up to 500 Watts, but right now I'm using my Siglent AWG to power it. It's not for amateur radio though, just making plasma.Right now I'm using a 0.5 Watt to 5 Watt amplifier, which is the LPF'd.Then 5 Watt to 500 Watt Class AB which again, is filtered.It's impossible to find an already made solution that will give me a simple 13.56MHz into 50 Ohms on the internet so I thought I would just have to build it myself. I would also like the ability to pulse it if possible, but that's for another day. I just want everything compact so I don't have to plug in my signal generator to make it work. Thanks for the suggestions!

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 12, 2021, 01:35:46 am
content: Quote from: David Hess on September 11, 2021, 10:23:17 pmIdeally I would like it as clean as what I'm able to produce with my Siglent ARB. I figured I could use a transistor amp, but wasn't sure about other components that might be needed, such as capacitors or inductors. I can throw whatever DC source on it to make it work, but would prefer 12Volts as that is also powering a fan.

author: langwadt
author_rank: Super Contributor
timestamp: September 12, 2021, 02:48:43 am
content: Quote from: evb149 on September 12, 2021, 02:28:46 amOPA2670 ?

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2021, 04:50:13 am
content: Quote from: Plasmateur on September 12, 2021, 01:35:46 amIf only a 12 volt supply is used, then an impedance matching circuit at the output is required because the peak-to-peak voltage for a 5 volt sine wave is 14.1 volts.Obviously RF amplifiers can put out a lot more than 1/2 watt with a 12 volt supply. Besides impedance matching, the tank coil on the output increase the output voltage swing. So the simplest design will be a traditional single transistor RF amplifier and RF power amplifier design procedures can be followed.A linear design like I suggested as a possibility is also feasible but will need a higher supply voltage, or more likely +/- 12 volts, or even higher if series source termination is used instead of parallel (1) unless something more exotic is done, like a 10 ohm series termination with positive feedback to make it look like 50 ohms; this is done to allow lower supply voltages. Or no termination can be used but then proper termination at the load becomes mandatory.There are class-AB amplifiers with the needed performance but they are awfully complicated compared to a single transistor RF power amplifier.(1) I almost never see parallel source termination used in this sort of application. I think it is because most designers are used to thinking in terms of voltages rather than currents.

author: Plasmateur
author_rank: Regular Contributor
timestamp: September 13, 2021, 06:07:21 am
content: Quote from: evb149 on September 12, 2021, 02:07:30 amHey evb149, thanks for taking the time to reply. All the information was very helpful, and I actually looked into some of your suggested methods in the past.I went back to the QRP route as I was able to find a solution for around $50.The source:https://www.ebay.com/itm/133686051489The Amp:https://www.ebay.com/itm/363147258357Now I just need to make or buy a LPF.Edit:....wait, maybe I only need that transmitter. lol. Oh well.

author: geggi1
author_rank: Frequent Contributor
timestamp: September 13, 2021, 04:02:51 pm
content: If you want to do this very simple just go on Ebay and search for "HF Power Amplifier" plenty of cheep kits are avalable.In case you need some extra drive to the linear other modules can be bought.These kits are usualy desigend with a IRF type FET or by repurposing mobile phone base transistors.For experiementing i would chose a kit with an IRF transistor for simple replacement and low cost when you see the white smoke.Most of these kits have a TX relay but this is simple to bypass with a short wire.You will have to some filtering, a power supply and the heatsink.In case you want to start form scratch look for 14Mhz / 20meter linear designs.Most of the designs for HAM-radio is class AB.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 13, 2021, 05:29:55 pm
content: as long as there is no modulation, but pure sine, it doesn't have to be a linear amplification.it's common practice to use output filters, which make from every distorted signal a sine signalas already mentioned a good quality BD139 or IRF510 will do the job, a 7th order chebycheff filter on the output and you're done; no need for any fancy stuff like video opampsthough it can be done with a simple RFC, I'd recommend to use a transformer

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 15, 2021, 10:45:25 pm
content: Would a chip like a BUF634 be useful here?It is just a buffer, and thus has no gain on itself, but can deliver 250mA and has a bandwidth of 180MHz