Picking low noise amplifiers for my project - Page 1
Forum section: electronics / projects

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: May 13, 2025, 09:04:51 am
content: Hello everyone. I am looking for a low noise amplifier for two purposes. I first of all need one for a low frequency antenna(DC-to-500KHz spectrum) that I can use to give my antenna output some gain before the diode ring mixer. I am using an FG-200 function generator as my LO, so I need an amp that will give the antenna gain but won't output a stronger signal than the function generator is. Can anyone recommend specific mixers that would be suitable for this?The other LNA I need is for the transmitter. I'm taking the same signal from the FG-200 and also sending it to the transmitter, and I need an amp for that purpose as well. I really don't know yet how much gain this amp will need, but it can be much greater maximum output.It would be excellent if the LNA is inductorless. I'm trying to keep inductors out of my design except for the tx/rx antennas. Also, BNC connectors are a plus. Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2025, 12:41:12 pm
content: Atmospheric and ambient noise below the HF band is usually so high that a low noise preamplifier is not necessary and gain before the first mixer only lowers dynamic range.At 500 kHz and below, I would implement some variation of a feedback amplifier using a current feedback operational amplifier for both cases, and noise could be low anyway. AnLT1210orLT1206could be used for a higher output power.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: May 13, 2025, 05:07:14 pm
content: Thank you! I will look in to those! I should note that I'll be filtering out high frequencies that aren't within audio range of the target frequency. So if I'm aiming for 1KHz, then the filter will remove anything above 1.5Khz or so.

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2025, 05:33:50 pm
content: Nothing prevents designing and building a class-a or class-ab discrete feedback amplifier operating into the 500 kHz range, but I assume you want something simpler to work with hence my recommendation of the LT1210 and LT1206.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: May 16, 2025, 07:13:01 am
content: The current strategy I have is to get a board made with the ADA2050(a class AB part) for the transmitter amplifier, but I worry that 5W minimum output already might be overkill to my TX antenna. I think it will be less work to get a premade board and just replace the RC filter with different values.For RX I am still trying to figure out how much gain I will even need for the antenna because its output has to stay lower relative to the function generator. Given the very wide range this system will operate at: I probably will need more than one antenna to swap between.For the mixer I am leaning toward the AD831 mixer, but I can't figure out what sort of loss I'll have at those frequencies. I'm assuming the mixer loss will necessitate an active antenna rather than a passive one, but I'm still learning here.

author: qiongcao
author_rank: Newbie
timestamp: July 14, 2025, 01:33:54 pm
content: https://tejte.com/blog/bnc-connectors-overview-uses-and-adapters/Here is everything about bnc

author: jmelson
author_rank: Super Contributor
timestamp: July 14, 2025, 03:43:33 pm
content: I needed an ultra-low noise op amp in a precision regulator for a very sensitive circuit. I found the AD706 is tested down to one milli-Hertz, so the 1/f corner frequency is REALLY pushed down. I don't know if this will do what you are looking for, but you might take a look at their data sheet.Jon

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 04:20:40 pm
content: Your RX amplifier spec has the bandwidth going down to DC.What is the actual lowest frequency of interest in your signal?Low-frequency low-noise amplifiers will have "1/f" or "pink noise" that "outshouts" the white noise at low frequencies, characterized by a "corner frequency" below which the pink noise dominates.Typically, bipolar transistor amplifiers are chosen when low-frequency pink noise is important, and the data should be shown on an IC datasheet.An important detail about 1/fnoise is that once it is added to the signal, averaging the result does not reduce it in the same manner that averaging reduces white noise (e.g. thermal and shot noise).

author: Terry Bites
author_rank: Super Contributor
timestamp: July 14, 2025, 06:22:11 pm
content: Ultra low noise opamp specs can only be achieved with specific input impedances.You could just be chucking money down the drain if you don't analyse the circuit properly.The small print is a killer.I think going down to actual DC is not going to work out.Static charge buildup on the antenna might just kill the front end if you go down to DCBefore any gain, some serious filtering is needed. 50/ 60Hz will swamp everything, no?So bandpass and notch.

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2025, 07:04:16 pm
content: The standard reference for low-noise stuff, especially good for the "audio" and "video" spectral regions isC D Motchenbacher & J A ConnelyLow-Noise Electronic System DesignJohn Wiley (1993), the revised version of Motchenbacher'sLow-Noise Electronic Design(1973)Terry Bites' comment about source impedance is imperative.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 24, 2025, 10:20:21 pm
content: Will a nearby lighting strike kill the antenna's input ?

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2025, 01:37:03 pm
content: Quote from: GigaJoe on July 24, 2025, 10:20:21 pmYes, but so can the static discharge associated with thunderstorms without a nearby strike. There are various ways to protect against anything except a direct strike.

author: mawyatt
author_rank: Super Contributor
timestamp: July 25, 2025, 01:53:25 pm
content: Quote from: TimFox on July 14, 2025, 07:04:16 pmThose are a couple of the best references available, we were fortunate to have Dr Motchenbacher as another Honeywell Fellow back in the day!!BTW those books and a few discussions with Dr Motchenbacher guided us in the development of the Rate Compensated Preamp utilized in the MSP (MILES Signal Processor) which had to detect nanovolt signals in the presence of extremely high noise backgrounds down to 0.1Hz. The concept of "pushing the signal integration" right up on the input from the MILES cable outperformed any other signal processing techniques at the time, and handily won the fly off with a much larger, more power, complex and costly system (long story if interested on how we were supposed to lose the fly off, but didn't!!).Best