Build 2X RF amp with 5.5Vrms out into 50 ohms. - Page 1
Forum section: electronics / beginners

author: Qmavam
author_rank: Regular Contributor
timestamp: April 18, 2017, 11:35:42 pm
content: Hi guys,I have a signal generator that will put out 3Vrms into 50 ohms, I have a project where I need5.5 Vrms into 50 ohms. Frequency range is 500kHz to 10 MHz, but 50MHz would be a bonus.I have scoured the internet and not found anything I can build.I have parts and a 24 volt supply.Can you help,Thanks, Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2017, 01:00:49 am
content: Try searching "1W wideband amp".Not hard to do with a 2N3866 or such, or even just a handful of more ordinary transistors really. IRF510 will do that easily enough.Tim

author: pigrew
author_rank: Frequent Contributor
timestamp: April 19, 2017, 01:11:31 am
content: You could use some sort of op-amp, as the frequency isn't so high. The problem seems to be the signal level requirement. You may want to look at video amplifiers, such as the AD810, though your requirements are right around the limit of most of the integrated circuit op-amps.EDIT: Sorry, I'm off by a factor of two with respect to voltage. To output 5.5Vrms, you'll need to output double that with a 50 ohm impedance matching resistor. This would be a 29 Vpp output. So, the amplifier probably would need a +/-20 V supply.One could get around this supply requirement by using a transformer on the output.

author: Qmavam
author_rank: Regular Contributor
timestamp: April 19, 2017, 02:46:57 am
content: Quote from: T3sl4co1l on April 19, 2017, 01:00:49 amI just got an order that has 2N3866 transistors, I also have a bunch of mosfets, may have IRF510, not sure.I see pigrew responded, and he is right I will terminate this with 50 ohms so I need to double the output voltage.I will try your search term, but if anyone has a design I'm interested.I'm building a tube replacement circuit, so I have 258 Volts available.Thanks, Mikek

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 19, 2017, 07:57:32 am
content: buried so deep down in the net... consider yourself luckyhttp://www.soasystem.com/eng/goltek/amplifier.htm

author: Qmavam
author_rank: Regular Contributor
timestamp: April 19, 2017, 09:00:55 am
content: Quote from: Mechatrommer on April 19, 2017, 07:57:32 amThanks Meccha,I don't know how close to the rail that will go, the largest signal I see on the data sheet is 20Vpp.IF, it goes within 1V of each rail the I could get 28Vpp, and that's all I need.I'll think about that, I like no coils, with 2X gain, it should do 100MHz.No info except the picture and schematic :-($10 plus shipping, Digikey and Mouser, 5 for $23 with free shipping from Aliexpress.Mikek

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 19, 2017, 10:39:14 am
content: with ±15V (30Vpp) power supply, the THS3095's output can swing ±11.8V (23.6Vpp) at 100 ohm load and 70°C. you can use LM317 and LM337 as power supply feed by voltage doubler setup diodehttps://en.wikipedia.org/wiki/Voltage_doublerwhich in turn feed by normal 240Vac to 20Vac transformer. but that depends on your FG setup, you may reuse existing PSU and tap a boost or buck and inverting modules to get the power for the opamp. remember CFB opamp needs good heatsink, whether on top or from the bottom pad to the pcb.

author: Qmavam
author_rank: Regular Contributor
timestamp: April 19, 2017, 12:49:01 pm
content: Quote from: Mechatrommer on April 19, 2017, 10:39:14 amOne point, My voltage available is a 258VDC line, I'm not sure about it's current capability.I also have a 150VDC line. Both are shunt regulated. I may be able to adjust the series resistor for a little more current.If things go as planned I may use the 6.3Vac heater voltages to develop the voltages I need. I have two separate 6.3Vacheater supplies to work with.A little about the project.I have Boonton Q meter, I'm trying to develop a tube replacement for a 'hand picked' unobtainable tube.The tube has two functions, it has a very high input impedance, and it rectifies the RF voltage across a capacitor.I have the high input impedance amp, and a gain stage, now I'm testing diode detectors.In use the detector will see a maximum of 5Vrms, My generator will only put out 3Vrms, so to test the detectorsto their full use, I need an amp for my SigGen.Thanks, Mikek

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 19, 2017, 02:37:29 pm
content: Quote from: Qmavam on April 19, 2017, 12:49:01 pmman thats nasty. i mean what model is your signal generator? it must have low ± voltage rail inside you can tap for your diy gain opamp amplifier..

author: Qmavam
author_rank: Regular Contributor
timestamp: April 19, 2017, 03:58:57 pm
content: Quote from: Mechatrommer on April 19, 2017, 02:37:29 pmYa, sorry I have confused things.You are right, I can tap ± 30V lines in my HP651B SigGen for the larger output.I can use the THS3095 OPamp for that.With that solved, I'll move on to the tube replacement.Second, for the Boonton tube replacement, I do need an amp that will drive a diode detectorwith 5 Vrms input. The diode detector has 50 ohm input impedance.For that I have the High voltage supplies available.With a later option of building lower voltage supplies.So as new starting point, let's say I have a high voltage to start with.I see the 2N3866 has a Collector/Base Voltage limit of 55 Volts.So lets say I can regulate my hgh Voltage down to 40 or 50 volts.For this amp I need a 3X or 4X gain.The driving circuit is an emitter follower, This:https://www.dropbox.com/s/x74jxix821oqdyx/Dagmar%27s%20Fast%20high%20Imp%20amp.png?dl=0Can I just add another emitter follower with a 50 ohm emitter resistor, to get my 10vrms into 50 ohms.Thanks, Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2017, 10:25:32 pm
content: 10V peak (which is a bit extra, say for linearity's sake) into 50 ohms is 0.2A.You need a transformer, or preferably a DC-DC converter...(A tube amp could do this directly, with peaking, or at worst, with a distributed amp, but it will need quite specialized tubes with minimal capacitance, and still consume quite a lot of supply current.)Perhaps you should reconsider some of the assumptions being made here...- Is it really 50 ohms input?- Does it really need 5Vrms?- How much current is available from those supplies? (Actually check the schematic.)Tim

author: Zero999
author_rank: Super Contributor
timestamp: April 19, 2017, 10:39:36 pm
content: Perhaps this is a crazy idea: use a transformer to drive an emitter follower class AB amplifier stage?Avoiding the need for any voltage gain at all in the amplifier should make it easier to stabilise.

author: Qmavam
author_rank: Regular Contributor
timestamp: April 19, 2017, 11:02:53 pm
content: Quote from: T3sl4co1l on April 19, 2017, 10:25:32 pmJust want to make sure you understand, I'm replacing the tube with transistors. The tube does two things, it provides a very high input impedance and rectifies the RF.QuoteI'll try to explain in as few words as I can.The machine resonates a series LC. The voltage on the capacitor reaches a maximum of 5Vrms.That 5Vrms is applied the a FET gate thru a 3 to 1 capacitive divider, making it 1.67Vrms.Then there is a 3X amplifier that brings it back up to 5 Vrms, Then a diode detector that dives a meter.I want 5Vrms because that drives my diode detector up into the linear region.I'm doing tests right now to see if I can raise the impedance of the driver to 500 ohms and keep thediode detector linear.QuoteBest I can figure is 37milliamps.Here's the schematic, lower right has power supplies, upper right has the tube I'm replacing.Last page of the manual.http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdfThanks, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: April 20, 2017, 01:34:12 am
content: I have attached the tube replacement circuit as far as I have got.I need suggested values for R7 and R8.I start out with a maximum of 5Vrms into the circuit.I have a capacitive divider, just to limit the swing on the FET gate.Q2 output drives a gain stage to bring the amplitude back up to avalue (5Vrms) that will drive the Diode Detector in it's linear region.Thanks, Mikek

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2017, 07:19:38 am
content: Oh, the diode load is some kohms, so it'll have an equivalent AC input resistance in the 500 ohm range.10Vpk is a lot more achievable!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2017, 07:53:30 am
content: You should probably go for something like this:Realize that, since you've got so damn much extra supply voltage, you might as well make use of it! Any resistors dropping from +HV to transistory-voltages looks like a not-half-bad current source.You still need some way to bias the stage, so, you might get feedback from the collector to the base... and you get that.Emitter resistor isn't needed (it can have a small impedance to ground, as shown), but a base voltage divider is needed. If you have a negative ("C-") supply handy, you can use that. Or elevate the emitter voltage for the same idea.Nice thing about shunt feedback (R3) is it reduces output impedance and distortion.I don't see that a 2N4416 is all that useful in a circuit like this. Its gain will be fairly low (i.e., not very close to 1, but lower, like 0.9 or 0., so the 20M gate bias resistor won't be bootstrapped very well (i.e., only a ~200M equivalent). A slightly more powerful transistor (like a 2N3819 or 2N5486) will lead to higher gain (better bootstrapping of both C and R), and its higher capacitance can be bootstrapped out better (Cgd can be bootstrapped by coupling the follower output to the drain circuit). Also, use a current sink instead of a resistor, for source load.Tim

author: Qmavam
author_rank: Regular Contributor
timestamp: April 20, 2017, 10:31:15 am
content: Quote from: T3sl4co1l on April 20, 2017, 07:53:30 amI had a bootstrapped circuit with lower capacitance, but then found the tube has about 1.7 pf of input capacitance,so the guy that was helping me, came up with this one which has slightly higher input capacitance but a higher input R.I want to try to match the tube capacitance to keep my calibrated capacitor aligned. I can work on that as a final change.I do have a question about the biasing on your circuit, does that have increased changes with temperature?this pcb will be put into a machine that still has 4 tubes running with lots of heat.How do you calculate the gain of that circuit?QuoteNot sure what you mean here.Will I gain anything by just swapping the 2n4416 with a 2n3819. I have those in my stock.Thanks for your time, Mikek

author: Qmavam
author_rank: Regular Contributor
timestamp: April 20, 2017, 10:41:25 am
content: Quote from: T3sl4co1l on April 20, 2017, 07:19:38 amI started with this paper showing a temerature compensated linear diode detector.https://www.rf-microwave.com/uploads/diodes/a_temperature_compensated_linear_diode_detector.pdfwhich was designed with 50 ohm input. It took me a few iterations before I figured out I could drive it witha 500 ohm source and not use a termination resistor. That eased up the output needed from my gain section.Quote