Amplify 0.5 to 5 Vpp for 0.1 - 2GHz with ~50 Ohm load - Page 1
Forum section: electronics / projects

author: migi
author_rank: Newbie
timestamp: February 14, 2017, 12:19:45 pm
content: Dear all,I'm looking for a suitable solution to amplify 500mVpp signal up to 5Vpp under ~50 Ohm load from 0.1-2GHz. I've looked at various current opamps and RF amplifiers but I can't really find any suitable solutions. It seems that either the gain is good but the power output too low or the other way around.I thought there ought to be an obvious solution for this that I'm missing, any advice on how to approach this problem would be much appreciated.

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 14, 2017, 12:31:58 pm
content: First place I would check is Mini-Circuits. More specifically, this page:http://www.minicircuits.com/products/Amplifiers.shtmlYou can plug in the specifications you gave above into the search engine on that page: 20dB of gain (0.5V -> 5V), 5V operation, frequency range of at least 0.1-1GHz, and assuming a surface mount device, results in 8 devices.

author: migi
author_rank: Newbie
timestamp: February 14, 2017, 12:36:38 pm
content: This is perfect, thank you!

author: Fraser
author_rank: Super Contributor
timestamp: February 14, 2017, 12:41:26 pm
content: Be aware that anything less than an expensive lab grade amplifier will not have a flat gain response over such a large frequency range. Power amplifiers are often built to provide the required gain and flat frequency response over a limited frequency range. Watch out for any VSWR pitfalls as well.I tend to use Minicircuits gain blocks as they are reasonably priced and well documented. Just make sure you do not drive the stage into compression.Fraser

author: ConKbot
author_rank: Super Contributor
timestamp: February 14, 2017, 12:47:43 pm
content: So 20db of gain, about 18dbm out. I'd consider cascading 2 amps to ease the gain requirements, since you want something broadband. A TRF37A75 from TI has wideband matching internally. Park a digital step attenuator between the gain stages, put a detector on the output to measure your output and a micro to fiddle with the attenuator to keep your output power just right.Alternatively, make an attenuator with a gain slope opposite of the amplifier gain slope to get a flat response.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 14, 2017, 12:48:26 pm
content: Some kind of solution would be to use 2 or 4 of the commercially avaliable 3 Pin gain blocks and operate them in parallel. The input / output impedance must be matched to 50 Ohms using Power spltters / combiners. Some time ago I found this app note:http://cb.wunderkis.de/wk-nonpub/marex_mmic.pdf

author: mmagin
author_rank: Frequent Contributor
timestamp: February 14, 2017, 06:16:49 pm
content: Quote from: MagicSmoker on February 14, 2017, 12:31:58 pmI would like to point out that the subtle thing OP should be aware of in the above is that most of the RF amplifiers are specified for power gain in dB with 50 ohm input/output - Thus 10x voltage gain is 100x (20 dB) power gain.

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2017, 08:10:48 pm
content: Quote from: mmagin on February 14, 2017, 06:16:49 pmThe clever thing about the dB system is it's correct whether you're talking about power or voltage gain.Voltage gainAV= 100Gain = 10log(100) = 20dBPower gainA = 10Gain = 20log(10) = 20dB

author: mmagin
author_rank: Frequent Contributor
timestamp: February 14, 2017, 08:22:11 pm
content: Quote from: Hero999 on February 14, 2017, 08:10:48 pmOh, yes, that's the simple part, I just wanted to make it clear to the original poster that they're not looking for 10 dB of power gain when specifying an RF amplifier to go from 0.5V to 5V

author: dacman
author_rank: Frequent Contributor
timestamp: February 14, 2017, 11:24:27 pm
content: I suppose you're not interested in a used HP 8347A (from eBay or the like).

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 14, 2017, 11:33:42 pm
content: Quote from: mmagin on February 14, 2017, 08:22:11 pmYes, I was a bit sloppy with my units. The parametric search applet on Mini-Circuits site asks for power gain in dB, but the actual parts are listed in dBm. The OP needs 5Vpp into 50R which is approximately 250mW(rms), or ~24dBm, whereas most of the parts I saw maxed out around 18dBm or so (though I by no means looked at all of them).

author: dacman
author_rank: Frequent Contributor
timestamp: February 14, 2017, 11:58:27 pm
content: I get 5 Vpp is 17.96 dBm at 50R.

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 15, 2017, 12:52:36 am
content: Yes, 18dBm. Power is usually specified at the 1dB gain compression point. So you probably want P1dB greater than about 20dBm to make sure you are not compressed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2017, 04:25:09 am
content: Do you need a flat 20dB gain, or how sloppy can it be?Most RF amplifiers are pretty crude, within 3dB or so in the passband.If you need a tighter tolerance, you may find it necessary to either:a. get a cheap amplifier, with excess gain (25-30dB), and equalize it with a filter network (optionally with a preamp in front / buffer after, to keep input or output impedance stable). You'll generally be applying Zobel networks, which are dissipative (= lost gain).b. Get an expensive amplifier, with considerable excess of gain (40dB, 60dB?) and negative feedback, so that the error in gain is reduced automatically. It may also be internally compensated for flat response, linear phase (good step response), etc.It's not practical to combine (a) and (b) yourself, because at >2GHz, the negative feedback path has to physically be less than ~10cm, and preferably under 1cm. So it will be internal to the amplifier module, or on the same circuit board anyway.You could of course build the amplifier yourself (MMICs and RF transistors are relatively cheap, very plentiful, and perform very nicely!), but then you need to measure and adjust its response to meet your requirements. If you don't have time (or equipment) for that, you should consider buying a ready-made module instead.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2017, 08:58:42 am
content: Quote from: Hero999 on February 14, 2017, 08:10:48 pmRight conclusion, wrong reasoning.Gain(dB) = 10log10(P1/P2), where P is power in Watts.Now P = V2, ignoring the resistance because that constant factor will cancel out in the next equation.So Gain(dB) = 10log10(V12/V22), or to recast it, Gain(dB) = 20log10(V1/V2)

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 15, 2017, 02:25:55 pm
content: Well, I'm thoroughly confused now (admittedly, that's not a grand achievement...)10x voltage gain is 20dB, yes? And 5Vpp is 3.54Vrms and across a 50R load that results in 250mW, yes?And 250mW is 250x the 1mW reference level of 0dBm, so that is +24dBm power, yes?So where do 18dBm and 1dB compression come from?

author: rfeecs
author_rank: Frequent Contributor
timestamp: February 15, 2017, 03:55:25 pm
content: It's 5V peak to peak. So the voltage peak value is 2.5V. That's where the 18dBm comes from.1dB compression just comes from the way that output power is often specified in data sheets. It is the output power where the gain drops by 1dB. So it is the point where the amplifier is just starting to saturate.

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2017, 04:46:14 pm
content: Quote from: MagicSmoker on February 15, 2017, 02:25:55 pm5Vpp is 1.77Vrms.1.77Vrms in 50ohms is 62.5mW62.5mW is +18dBm.The maximum amplifier output is specified in terms of the 1dB compression point, i.e. where the output power is 1dB less than you would expect it to be. In practice, at that level there will be significant (in RF terms) distortion and hence harmonics, so you usually leave an appropriate level of headroom by keeping the power somewhat below the 1dB compression level.

author: MagicSmoker
author_rank: Super Contributor
timestamp: February 15, 2017, 05:13:04 pm
content: All I can really say is:Kind of amazing I could actually spell out the erroneous calculation and still not see it...

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2017, 05:21:13 pm
content: Quote from: MagicSmoker on February 15, 2017, 05:13:04 pmWe all have brain failure sometimes. The trick is to recognise "something isn't right", and then to "makenewmistakes".