Question about the output power of the Ramsey LPA1 RF amplifier kit - Page 1
Forum section: electronics / general

author: jfedison741
author_rank: Contributor
timestamp: November 17, 2012, 01:19:23 am
content: The Ramsey LPA1 looks like a solid kit and is designed to provide a gain of about 30dB at 10MHz and somewhat less at higher frequencies. I have built one and tested it using a 12VDC supply. With a 10MHz, 1mW sine wave input and a 50 Ohm resistive load, I get 8.50V peak-to-peak on the output. This corresponds to 180mW of output power and 22.5dB of gain. However, this falls short of the expected value of around 1W of output power and 30 dB of gain as claimed in the product description. Has anyone seen anything similar?Jeff

author: RCMR
author_rank: Frequent Contributor
timestamp: November 17, 2012, 08:08:04 pm
content: Unfortunately there don't see to be a lot of RF guys hanging out here -- it's more digital and lower frequency stuff.Although I do a lot of RF stuff, I'm not familiar with the RF amp you're talking about so can't help either.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 17, 2012, 09:05:48 pm
content: Yeah well, the way I see it is that 8.5 volts over 50 ohms is almost 1.5 watts, so that is like 31.5dB gain.

author: kg4arn
author_rank: Unknown
timestamp: November 17, 2012, 09:43:44 pm
content: The OP has the correct gain, he specified 8.5 Vp-p.Could you post the schematic so we can give you some advice?

author: poptones
author_rank: Frequent Contributor
timestamp: November 18, 2012, 12:00:20 am
content: Do you know "radio king?" He has oodles of great buys on parts and kits.http://www.kitsandparts.com/parts.phpOther than that I can't help. Never built one of those kits.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2012, 02:41:27 am
content: Quote from: jfedison741 on November 17, 2012, 01:19:23 amI'm not familiar with the amp,but your figures are "spot on" for a gain of 22.5dB.Two things:Is it possible there are some "select on test" tweaks you need to make to get the full gain?A lot of small RF amps drop a bit in output if they aren't run off 13.8Volts, although 7.5dBseems "a bridge too far" for that.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 18, 2012, 03:45:59 am
content: I Googled for "Ramsey LPA1".One ad called it a "lab amplifier"but ,(nudge,nudge,wink,wink),"some people in "other countries" use it as a linear amplifier to increase the power of small transmitters.The same ad quoted the supply required as 12 to 15 volts,so the specified gain may be at 15v.Apart from that,just about every mention was in the context ofillegaluse by "pirates" on the FM Broadcast band.One thing I turned up was a request for help,where the poster hadexactlythe same input & output power measurements as you,(funny,that,eh?),but they also add that " the output signal looked like 1/2 cycles".That sounds a lot like no bias on Q2 to me.I don't know if they gave you a schematic,but the guy on this website has one for download,along with some comments.http://www.diybanter.com/electronic-schematics/292277-ramsey-lpa1-rf-amp-schematic-manual-0-3-a.htmlI was going to suggest you ask this question on QRZ.com,but I'm not sure what reception you would get!

author: jfedison741
author_rank: Contributor
timestamp: November 18, 2012, 09:12:48 am
content: Here is the LPA1 schematic along with measured dc voltage (without any drive at the input). I have since measured an output power of 440mW at 10MHz with VDC=15V and Pinput=2.5mW (power gain of 22.4dB). I will try different Q2 collector currents by adjusting R1 and see what effect that has on the power gain.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 18, 2012, 11:49:15 am
content: Quote from: kg4arn on November 17, 2012, 09:43:44 pmYes, I should probably not post while already sleepingComparing your schematic with another one on the net I see a difference for R3: yours is 10K while the other one is 7.5K. No other differences spotted.

author: kg4arn
author_rank: Unknown
timestamp: November 18, 2012, 12:37:54 pm
content: I would call this amplifier "frugal" rather than "solid"Here's my opinion, others may vary:I wouldn't give Q2 any more bias current as it is dissipating 900mW already. I believe it is a MRF581. According to the data sheet from Microsemi, the thermal resistance (junction to case) is 100 C/W (reciprocal of 10 mW/C on the datasheet). That means the junction temperature is already at least above 100 C. A thermocouple temp on the case would allow a better estimation of the junction temperature.There is no thermal runaway protection for Q2. I am surprised that Q2 isn't over heating.My guess is that the input impedance at the base of Q2 is pretty low and that you may be able to increase the overall gain of the system by better matching to the MAR-3. You could try placing a matching transformer between C8 and the base of Q2.It would require cutting the PC board trace.You could make the modification by unsoldering one end of C8. I will include a schematic. If you are not in the experimenting mood, I would suggest you accept the gain that you have. This is a cheap amplifier design.If you don’t have the parts and you are in the USA, I will send you the transformer via mail for free. Just end me a PM with your address information.

author: kg4arn
author_rank: Unknown
timestamp: November 18, 2012, 12:43:59 pm
content: Quote from: PA0PBZ on November 18, 2012, 11:49:15 amI meant no offense my friend73

author: kg4arn
author_rank: Unknown
timestamp: November 18, 2012, 01:01:44 pm
content: Quote from: RCMR on November 17, 2012, 08:08:04 pmWe are here ... to learn about digital and lower frequency stuff

author: RCMR
author_rank: Frequent Contributor
timestamp: November 18, 2012, 09:16:03 pm
content: Quote from: kg4arn on November 18, 2012, 01:01:44 pmAren't we all -- the day *I* stop learning is the day they'll be scattering my ashes (hopefully)

author: jfedison741
author_rank: Contributor
timestamp: November 18, 2012, 11:47:46 pm
content: Quote from: kg4arn on November 18, 2012, 12:37:54 pmThank you kg4arn, I will give that a try and see how it goes. I have an FT 37-43 on hand but thanks for offering one.

author: jfedison741
author_rank: Contributor
timestamp: November 18, 2012, 11:52:21 pm
content: I am wondering does any one know of other forums that may also discuss experimenting with RF electronics?