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Old 10th Oct 2019, 6:37 pm   #1
G1RAO keith
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Default Wide band RF amp

Hi all
Bought this wide band amp SPF 5189z recently to improve the UHF sensitivity on my frequency counter it works but the problem is checking the spec on line for this device states the max supply volts is 5.5 I am running it on 5.0 v,at 5v the current stated should be 90 ma my device takes 120 ma right at the limit for it ,it doesn't get hot or even warm,there may be some kind of regulation under the screening can I don't know ,the other scary thing is the DC resistance at the supply points its 4.6 ohm if you devide that into 5v it's a lot more than the current it takes. So has any one used one of these and can shed some light on it
The supplier is sending me a replacement,I am a bit concerned using it with that amount of current drawn.regards Keith
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Old 10th Oct 2019, 8:04 pm   #2
Bazz4CQJ
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Default Re: Wide band RF amp

When I looked at the picture, it reminded me of some LNA's I bought a while ago and are discussed here https://www.vintage-radio.net/forum/...d.php?t=150649, but the physical similarity may be totally misleading. My units were rated up to 12V, though I seem to recall limiting them to no more than 10V. I'll see if I can find my notes about the current they took.

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Old 10th Oct 2019, 8:21 pm   #3
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Default Re: Wide band RF amp

There are hundreds of MMIC amplifier devices and values of bias resistors that someone can put on a board in a box. Without knowing specifics, meaningful advice isn't possible.

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Old 10th Oct 2019, 11:37 pm   #4
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Default Re: Wide band RF amp

Assuming the MMIC inside the can is as stated on the label, there won't be a voltage reg inside the can.

The SPF5189Z is rated to run directly off 5V.

It definitely sounds like some kind of fault.

Once you get the replacement, I would remove the can and see if you can find anything faulty inside (assuming you don't have to return the faulty one).
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Old 11th Oct 2019, 3:22 pm   #5
G1RAO keith
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Default Re: Wide band RF amp

Hi
Thanks for the comments ,I am waiting for the replacement if they don't want the old one back I will lift the lid on the other,something is not right when the DC resistance measured across the supply input is 4.6 ohms.regards Keith.
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Old 11th Oct 2019, 4:42 pm   #6
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Default Re: Wide band RF amp

It will have active bias control of some sort, so it is very unlikrly to appear as a simple resistive load to the power supply, so the reading on an ohmmeter may not be any sort of guide to current consumption.

Amps with depletion devices need bias voltage to turn their current down. Unbiased low voltage ohms tests look almost like a short.


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Old 11th Oct 2019, 5:48 pm   #7
Jon_G4MDC
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Default Re: Wide band RF amp

On the increased current observed when operating - can we be sure it is not oscillating?
Measuring the current again with well screened SMA Loads fitted at In and Out ports might be worthwhile.
These things have "go" to 4GHz and could be sensitive to termination.

Last edited by Jon_G4MDC; 11th Oct 2019 at 6:03 pm.
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Old 11th Oct 2019, 5:59 pm   #8
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Default Re: Wide band RF amp

I'd definitely consider a 'wideband' termination on the load: in professional applications of these sorts of devices it's quite common to see a pi-section 3dB resistive 'pad' included on the output-port so the MMIC does not see a significantly-reactive output-load at any point across the frequency-range at which it has gain.

Remember that every amplifier is an oscillator that hasn't -yet- worked out how to achieve its destiny!
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