Has this regulated SMPS got too much ripple for an RF amp bias supply? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 02, 2013, 01:57:45 pm
content: I have a Skanti commercial solid state 250W RF amp, 1 to 30 MHz. It needs 50 volts and 5 volts supplied to it. The 5 volts is for the bias circuits. I am told they draw about 3 amps maximum. I have a good quality regulated 4 amp SMPS at 5 volts. Here is a `scope screen shot of the ripple, measured without ground lead tails on the probe, direct across the output to minimize pick up. The dots are the cursors, it's an elderly Tek 7854 analogue `scope. Is this OK to use? Or do I build a linear regulated supply? Thanks.

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2013, 04:24:42 pm
content: Looks good for me. You haven't specified which of the two power supplies you are viewing +5 or the other.If it the power supply is supplying the correct voltage, you are good to go.

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 02, 2013, 04:57:58 pm
content: Thanks for the reply Paul, and for the one re the safety resistor in the HV supply, in another thread. The scope capture is from the small 5 volt SMPS for the bias. The 50 volt supply is an HP server SMPS with additional filtering and a voltage trimmer adjuster added. I am confident that is fine, and at 60 plus amps more than up to the job. Thanks again, great forum!

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2013, 02:00:56 pm
content: I should of course have also measured it under load, and with it disconnected from the mains to show external noise, so for completeness I attach two scope shots. One of it disconnected from the mains and one under 3.7 amps load. The probe is connected exactly the same as above.

author: KJDS
author_rank: Super Contributor
timestamp: October 03, 2013, 02:05:22 pm
content: I'd not want that on the bias of any amplifier. Can you follow it with a linear regulator? Otherwise you'll need a lot of filtering assuming it is a linear amp.Do you know what the active devices are?

author: Chris Wilson
author_rank: Super Contributor
timestamp: October 03, 2013, 02:20:09 pm
content: I think I'll just build a small regulated linear bias supply, this is an attempt to build a very clean amp with lot of gain. Last thing I want is a splattering mess!The only schematic I have came in two parts and it's beyond my computer skills to join them better than below, apologies. Thanks for the support guys!Active devices are two MRF427 final pre amp, and two BLW95 PA devices.

author: Paul Price
author_rank: Super Contributor
timestamp: October 04, 2013, 01:35:30 pm
content: The noise you are looking at is probably just an artifact of your probes/ probes hook up and ground loops. You are good to go with the switcher! This amount of noise will not effect the operation of your RF amp.

author: KJDS
author_rank: Super Contributor
timestamp: October 04, 2013, 01:46:35 pm
content: Quote from: Paul Price on October 04, 2013, 01:35:30 pmWhilst it might not be a real issue, if that much crap get's onto the gate of a FET it's going to cause grief.

author: Paul Price
author_rank: Super Contributor
timestamp: October 04, 2013, 09:11:42 pm
content: Cause grief???The ripple waveform, even if it is not a probe artifact, is only a few ten's of millivolts.The 5V supply is a bias voltage that will be further bypassed at the RF amp.