selecting input amplifier transistor and topology - Page 1
Forum section: electronics / beginners

author: reimius
author_rank: Newbie
timestamp: May 19, 2021, 07:04:01 am
content: Hi all,I am trying to figure out what should be the best LNA input amplifier for an DCF77 receiver that I would like to build. However, even going any further I am struggling to decide two points:* The input stage from the active antenna (LC) should be first a inverting gain amplifier(common emitter) or an voltage buffer(emitter follower)? There is any Maths that can sustain the proper response?* What is the most suitable transistor for the input stage? As far as I know, BJTs have the highest transconductance, but not the highest input impedance. Say that, because a degenerate emitter resistor is encourage for decoupling the gain from variations of the manufacturing process for any type of transistor, I deduced (maybe wrongly?) that at the end all transistor will give me kind of same gain so I should aim for the highest input impedance, so then we have the FETs. Between the small signal NMOS and nJFETs that I've explored the all seem have similar input capacitance so I concluded that a nJFET should be the way to go since biasing looks easier. Someone could see if this explanation is sensible or it is wrong? by the way, the transistors checked for this are the2N34062N3904(BJT), 2N3819 (nJFET) and 2N7000(NMOS)Extra info: Ive been struggling for months to build the most simple receiver and after reading tons of books my scope always shows a depressing flat line from the collector/drain of my transistors. By the way, here in Germany we do not have AM radio that I could use for putting in practice my (lack of)knowledge.Thanks a lot!

author: ledtester
author_rank: Super Contributor
timestamp: May 19, 2021, 09:05:08 am
content: I don't think your choice of transistor will matter that much.Check this thread for two discrete receiver designs (for WWVB):https://www.eevblog.com/forum/rf-microwave/wwvb-receiver-advice/Both use jellybean BJTs -- e.g. 2n3904, BC549And here's another WWVB front end:https://twitter.com/codedweller/status/638824875340132352from this video:

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2021, 02:49:55 pm
content: The transistor choice depends on the antenna. The usualy antenna for DCF77 is a ferrite rod with a LC tuned coil. This antenna is rather high impedance and for this a FET amplifier is a better choice than a BJT (even at a lower impedance the noise figure of even the best BJTs is usually worse than a fet for a high impedance source in thís frequency band). The usual choice is a JFET, because is is realatively easy to bias.Quite often the reception is not limited by the amplifier noise, but more with background signals. So there is no need to get the absolute lowerst noise and there is a good enough. I would consider a MMBFJ201, MMBFJ309 or similar. With TO92 case the choice is usually limted. The first amplifier should be right at the antenna - the rest of the receiver could be separate, powering the 1st amplifier throung the same cable.WWVB is at much higher frequenies and the antenna there ususally lower impedance.

author: fourfathom
author_rank: Super Contributor
timestamp: May 19, 2021, 03:07:40 pm
content: Quote from: Kleinstein on May 19, 2021, 02:49:55 pmYou are probably thinking of WWV, which broadcasts on 2.5 MHz, 5, 10, 15, and 20 MHz.WWVB broadcasts at 60 KHz (and DCF77 at 77.5 KHz) so the receiver designs can be quite similar.