FM RF amplifier (LNA) for Si4825 FM Radio - Page 1
Forum section: electronics / rf-microwave

author: Sudhir7
author_rank: Newbie
timestamp: June 17, 2025, 05:13:45 am
content: Hello,Working on FM Radio prototype using Si4825-A10.Changing FM section from BK1198 to Si4825-A10, re-designing PCB for this part.Issue with BK1198 FM receiver circuit: Receives only FM Stations with strong signal, when tuned to FM 94.3 Mhz, it shifts to FM 96Mhz or 107Mhz which has good signal reception.When going through BK1198 and other older FM receiver IC (LA1260, TDA2021T) datasheets, found some are using transistor LNA / RF Amplifier connected to antenna.I do not understand these Transistor RF amplifier circuits, I only worked on some IC based audio pre-amps and amplifiers,Purpose is add amplifier circuit to antenna of FM receiver module to improve reception if weaker signals (far away FM Station or lower power FM Station).used circuit from RF amplifier section of FM receivers which uses 5v to 9v DC, but Si4825-A10 designed to work on batteries, 3.3v vcc.Need help with selecting stable circuit which can work with 3.3v. Attached schematics.Images:FM Amplifier from LA1260, designed for vcc 5v, Filter - Murata SFELF10M7FM Amplifier from TDA7021FM Amplifier from BK1198, designed for vcc 5v (correction)Thank YouSudhir

author: radiolistener
author_rank: Super Contributor
timestamp: June 17, 2025, 06:03:07 am
content: it’s important to first verify whether the root cause is truly weak signal reception at 94.3 MHz. Sometimes, the problem isn't insufficient signal strength from the intended station, but rather the presence of a much stronger nearby transmitter that causes front-end overload or interferes with the PLL locking mechanism. This can result in spurious tuning behavior or false locking.Can you show the spectrum taken with RTLSDR around your station received on the same antenna?

author: Sudhir7
author_rank: Newbie
timestamp: June 17, 2025, 09:22:51 am
content: Quote from: radiolistener on June 17, 2025, 06:03:07 amI do not have RTL-SDR dongle, thank you for suggesting.., did not think about this.. I will order one today.Not just weak reception, was difficult to tune in (lock in) to 94.3 , tuning jumps to other stations at 94Mhz, 94.6Mhz. when I checked BK1198 datasheet, schematic was not as per the application design given in datasheet, therefore decided to design one using Si4825-A10 , design is based on application note and evaluation board schematic.Si4825-A10 Application not has RF amplifier for AM/MW/SW , not for FM.SI4825-A schematic:

author: radiolistener
author_rank: Super Contributor
timestamp: June 17, 2025, 09:32:58 pm
content: Based on your description, this issue is likely not due to weak signal reception, but rather poor selectivity of the receiver. What you’re observing - difficulty locking onto 94.3 MHz and tuning jumping to adjacent frequencies like 94.0 or 94.6 MHz, strongly suggests that the receiver is being influenced by a strong nearby station, and the internal PLL is being pulled off-frequency.This is a common limitation in many low-cost FM receiver ICs, especially those with minimal front-end filtering and no dedicated RF tracking circuitry. When selectivity is poor, a stronger adjacent-channel signal can effectively “capture” the tuner and cause it to lock onto the wrong frequency.However, this should be confirmed by analyzing the actual RF spectrum.If that's the case, adding an LNA is unlikely to help and may, in fact, worsen the situation by further amplifying interfering signals, potentially leading to even more severe effects due to dynamic range limitations such as front-end overload and increased intermodulation.

author: Sudhir7
author_rank: Newbie
timestamp: June 18, 2025, 04:50:15 am
content: Quote from: radiolistener on June 17, 2025, 09:32:58 pmYes I thought of same thing, this is the reason I am changing Beken BK1198 based circuit with Skyworks SI4825A10 , Before selecting Skyworks IC checked older circuits, older circuits have LNA/Amplifier.As SI4825A10 does not have amplifier for FM Band. I felt its better to add suitable LNA circuit,as I do not understand transistor based RF circuits. Need help in selecting compatible schematic for FM signal amplification (from Antenna) or use IC LNA such as BGB717L7ESD as per schematic from datasheet note.I will analyze RF signals after receiving RTL-SDR, may take around 15 to 20 days.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: June 18, 2025, 02:19:09 pm
content: C32 in your second schematic is very odd.

author: LM21
author_rank: Regular Contributor
timestamp: June 20, 2025, 05:45:44 am
content: Yes. Short circuiting antenna to ground seldom helps. C32 has 1.6 ohmsimpedance at100MHz

author: rf-fil
author_rank: Regular Contributor
timestamp: June 23, 2025, 12:59:00 am
content: These RF preamps don't look too good. Maybe have someone experienced review them? (which is us here, I guess, lol).In the first schematic, L2 is a DC short circuit to ground, which will prevent the circuit from being biased and it just won't work.In the second schematic, the preamp is an emitter follower, which does not have any voltage gain. Also, I would not expect L3 to be there, in an emitter follower. Also, what's O33? Is it a SAW filter for the FM band? I don't know if such a thing exists. Also, C32 doesn't belong there, as others have pointed out.The third preamp has some chance to work - but these radio ICs usually don't need one. Actually, adding a preamp to a good IC will only make it perform worse.If you do want to design your own preamps, you can use LTSpice to model them. It's free and works pretty well, as long as you include all the components and parasitics in the model.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: June 23, 2025, 01:31:00 am
content: PGA 103+ can be oprerated at 3V. Ultra low NF even @ frequenciey below 0.1GHz down to SW. For stability some mods at input are necessary - see M-C app notehttps://www.minicircuits.com/app/AN60-064.pdf.Fully assembled PCBs starting at ~ 20$. You can add a LT3042 ULN-Reg for eliminating psu noise

author: Sudhir7
author_rank: Newbie
timestamp: June 23, 2025, 09:22:39 am
content: Quote from: rf-fil on June 23, 2025, 12:59:00 amSorry, There was mistake in schematic, corrected it.Quote from: rf-fil on June 23, 2025, 12:59:00 amO33 is Murata SFELF10M7 ceramic filter for FM receivers (Old one, available in ebay).Quote from: rf-fil on June 23, 2025, 12:59:00 amThank you all for your inputs, suggestions, part values corrected, I will download LTSpice.

author: Sudhir7
author_rank: Newbie
timestamp: June 23, 2025, 09:28:41 am
content: Value was not correct or Should not be there ?Link for FM Radio circuit (Electronics For You Magazine).https://www.electronicsforu.com/electronics-projects/simple-fm-radio-using-la1260Thank You

author: LM21
author_rank: Regular Contributor
timestamp: June 23, 2025, 09:44:15 pm
content: The latest schematic diagram does not work.You should buy a preamp from Ebay as others suggested.Much more difficult way is to build a working preamp. Your current design looks like a some kind of self oscillating mixer. Or not working mess. C2 grounds the base and VC1 and L1 tune the oscillating frequency. If X1 is really a 10.7MHz filter, it wont let 100MHz through, so it should be IF filter. Q1 and X1 wont work.At least you must connect C3 to the collector. Take off the X1, Take off C2 and it from emitter to ground.And study how amplifiers are made..

author: Sudhir7
author_rank: Newbie
timestamp: June 24, 2025, 06:04:23 am
content: Quote from: LM21 on June 23, 2025, 09:44:15 pmThank you for guiding, I only worked on small op-amp and audio amp (IC based) projects, Transistoor circuits are complicated for me, lack of understanding. I will read more. or use LNA MMIC BGB717L7ESD (datasheet attached)