Help with 10MHz Distribution Amplifier - Page 1
Forum section: electronics / testgear

author: MadMax01
author_rank: Newbie
timestamp: September 12, 2024, 01:52:44 pm
content: Greetings, I'm trying to create a 7 channel distribution amplifier by modifying an existing 4 channel amp available on github. I had a prototype board and soldered it up but when I power it on, the op-amps get really hot instantly, so I know I messed something up (Be gentle, I'm pretty new to this and still learning). My circuit is shown below. I'm guessing there is current feeding back into the op-amp thru the RF circuit but I've seen single channel op-amps wired in other designs the same way so that doesn't make sense to me but that's the only thing I can think of. If you could examine the circuit and point out any obvious flaws, I'd greatly appreciate it.

author: RFDx
author_rank: Regular Contributor
timestamp: September 12, 2024, 07:19:42 pm
content: The schematic would be helpful. The non-inverting inputs of the buffers are not biased with VCC/2 and are floating because you put DC-blocking caps in front of the inputs. The inverting inputs need DC-blocking caps and shoudn't be tied directly to ground. The VCC/2 bias voltage generator doesn't work as intended. The 10MHz lowpass filter is shunted twice with 49.9 Ohm, at the input as well as at the ouput. The 49.9 Ohm isolation resistors should be placed close to the outputs of the buffers.

author: 807
author_rank: Frequent Contributor
timestamp: September 13, 2024, 11:31:34 am
content: I assume it's the amp from this post:-https://www.eevblog.com/forum/projects/my-10mhz-distribution-amplifier/As RFDx says, you have removed the bias on the non-inverting inputs of the chips by adding a 100n capacitor in series.Have you checked the supply on pin 4? As both chips are affected, it may be a PSU problem.Have you double checked that the chips haven't been connected the wrong way round?Was this board designed by yourself, or was it designed by someone else?

author: MadMax01
author_rank: Newbie
timestamp: September 13, 2024, 01:37:17 pm
content: Thanks for the great feedback... yes, that is the design I used initially and modified to double the output channels. I designed the board myself but yes, the circuit is derived from the one in your link. I think I got it exactly the same as their circuit other than splitting the RF signal into two paths (one for each op-amp), maintaining the 50 ohm impedance by going thru two 24 ohm resistors for each path, and splitting the supply voltage in parallel, one to each op-amp. Those are the only two changes from the original circuit. I will go back thru and double-check everything to be sure. I am sure the op-amps are connected the right way and I am also sure my supply is ok. I checked it and it's outputting 12v to the 9V LVR. When powered up, it was outputting 12V and drawing 0.35A which is too much for the design as intended. I didn't check the pin 4 output as I immediately disconnected power when I noticed how hot the op-amps were getting. They are $7US a pop and I really didn't want to fry them by taking any time to try to troubleshoot the circuit.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 13, 2024, 03:00:09 pm
content: can you post your "enhanced" version schematics ?? maybe you added an error while expanding the design ?

author: 807
author_rank: Frequent Contributor
timestamp: September 13, 2024, 03:49:09 pm
content: Quote from: MadMax01 on September 13, 2024, 01:37:17 pmAs mentioned above, you have also removed the bias from the non-inverting pins.To prevent any further damage to both chips, I would work on just one at a time. Remove the supply from one of them, either by cutting the track from the supply, or desoldering & lifting pin 4. Then short out the 0.1uF capacitor that connects pins 3,5,10 & 12 to the 24R resistor on the other chip to restore the bias. Assuming the non-inverting inputs draw almost zero current, then you should see 1.7v on those pins.Check the supply voltage by putting your multimeter probe on pin 4 before you switch on to minimise the time it's switched on in case it's still drawing too much current. According to the data sheet, the maximum supply voltage should be 11v. I'm wondering if there may be a problem with your 9v regulator circuit.EDIT...actually, on second thoughts, I would remove the supply tobothchips & check the regulator first.

author: MadMax01
author_rank: Newbie
timestamp: September 13, 2024, 04:12:46 pm
content: So I don't actually have schematics for the design, I just created the PCB layout directly in the PCB Editor of KiCAD using the schematic from GitHub. Looking at the schematic and where I split it up and reading the comments from you all, I realize that I probably need to duplicate the whole circuit for each op-amp, not tie in where I did in my first design so that I can maintain proper biasing. Since that's the case, there probably isn't much sense to try troubleshooting that design so I did a quick update to the PCB and duplicated everything but the LVR portion of the circuit. It's only a few extra cheap components so while this may not be the most efficient way to solve the problem, I think at least now, I should get the desired result. Thoughts?

author: MadMax01
author_rank: Newbie
timestamp: September 14, 2024, 03:18:12 am
content: You guys nailed it... it was the 100n series cap. It jumped the RF voltage into the op-amp from 1.5v to 5.2v. Thanks all for the assistance