appropriate inductor type for inductively loaded CE amplifer - Page 1
Forum section: electronics / beginners

author: MrRadiotron
author_rank: Contributor
timestamp: October 14, 2015, 09:54:25 am
content: Hi,I'm trying to source parts for the TRF AM radio circuit attached.I'm having trouble picking the inductors L2 and L3.I think the following axial RF choke from JayCar will be appropriate for L2PN:LF1546L3 at 10mH seems to be harder to choose, jaycar doesn't have a fixed 10mH inductor.I tried winding my own with one of the following cores:PN:LO1234better info here: material L8Electus pdf:FERRITES.PDFfrom 15 turns I got 110uH, so for 10mH I'd need 1363 turns!(is that correct? 110/15 = 7.33/ -> 10000/7.3[uH/turns] = 1363.63)The other concern is the frequency response of the material only seems to go to ~700Khz so I don't know if that would work.The Inductive loads is for increasing the collector voltage to increase the gain of the CE amplifier for low voltage operation,so I think the inductance can't drop off at AM frequencies, or it would stop blocking RF at those frequencies.I could get a High Frequency Inductor(axialorradial) or a Power Inductor(axialorradial) from FarnellAlthough I would prefer to get something from JayCar.Any tips are greatly appreciated!

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 14, 2015, 11:01:17 am
content: MrRadiotronThe inductance of a coil is proportional to the square of the number of turns.So in your case if 15 turns gave you 110uH then you first calculate the Al (inductance index) value using the formula L = n^2 Al. This yields an Al value of (110 x 10^-6)/15^2 = 488nH/T^2.Then calculate how many turns will be required to produce 10mH. n = SQRT (L/Al) = SQRT (10x10^-3/488x10^-9) = 45 turns. You would also need to check that the inductor isn't self-resonant below 1.6Mhz.Having said that I think that you should choose another circuit as these simple circuits rarely produce decent results.

author: MrRadiotron
author_rank: Contributor
timestamp: October 14, 2015, 01:08:52 pm
content: Thank you for reeducating meI don't know why I got it stuck in my head that it was a linear relationship ... :SI'm aware the circuit won't give the best results, I'm working my way through a few different radios ending with a super-het, purely for the experience.Thanks again!

author: vk6zgo
author_rank: Super Contributor
timestamp: October 14, 2015, 01:47:07 pm
content: Quote from: MrRadiotron on October 14, 2015, 01:08:52 pmThat circuit isn't what is normally regarded as a Tuned Radio Frequency(TRF),radio.What you have there is a tuned preselector LC network,followed by two broadbanduntunedamplifiers.A TRF uses amplifier stages interconnected by tuned circuits.Each such circuit is isolated from the previous one by the amplifier stage.The variable tuning components(normally the C),were normally "ganged" so they varied at the same rate when adjusted.As nothing in life is perfect,"trimmer capacitors" are used to "trim" the LC networks to allow them to "track" correctly.But wait!--there's more! Amplifiers with tuned circuits in both the input & output rather like to oscillate,so methods of preventing this happening were devised.Google for "Neutralisation"Even with such problems,realTRF receivers offered very much better performance than the circuit you are building ever will!Annoyingly,many sites on the Internet present things like the circuit you supplied as TRF radios.The whole idea of TRFs is to cascade resonant circuits in order to increase selectivity.What you have will have about the same selectivity as a "crystal set".

author: MrRadiotron
author_rank: Contributor
timestamp: October 15, 2015, 05:30:49 am
content: ohhhh, aNeutrodyne!the Wikipedia article onTRFreceivers gives a great example of a valve radio with a "Right Vernier" and a ganged "Left Vernier".Were the intermediate tuners normally user tuned like that?Or was it more common to be trimmed once at the factory with trimmer caps?This is the first of five "TRF" receivers I'll be making, I haven't looked at the subsequent circuits too closely, but hopefully one then will have more than one tuned circuit.That being said, ganged tuner capacitors are hard to come by where I live, and bringing them in from the US is a pain.So any real TRF receiver I make will have to be for the trimmer variety rather that a ganged tuner!thanks for the info!

author: vk6zgo
author_rank: Super Contributor
timestamp: October 15, 2015, 06:53:44 am
content: The "Neutrodyne" was one particular way of neutralising such circuits,but there are others.Three terminal devices such as triode valves (vacuum tubes) have a problem with inter-electrode capacitance causing feedback between the output & input elements of the device.Without any applied operating voltages,the capacitance between,say,Collector & Base is quite small,but when the device is amplifying,the effective capacitance is multiplied by the voltage gain of the stage.---Look up "Miller Effect",or "Miller Capacitance".The possibility of mis-tuning the Base & Collector LC networks causing the fed back signal via"Miller Capacitance" to no longer be 180 degrees out of phase,with the Base signal but instead to be"in phase",makes oscillation likely.A particular kind of Oscillator uses just such a setup----Look up "Tuned Plate-Tuned Grid (TPTG) Oscillator".Early TRFs (& Superhets) used triode valves,so they all needed to provide some form of Neutralisation.(Intentionally feeding back a small amount of the output signal ,specially adjusted to be antiphase to the unintentional feedback signal via Miller Capacitance.)Tetrode & Pentode tubes reduce the inter-electrode capacitance to such a low value that even allowing for Miller Effect,the resulting value of "Miller Capacitance"is still too low to cause instability.This allowed the use of Un-Neutralised stages in receiver RF & IF stages.Transmitters usually still used some form of NeutralisationAlong came Solid State----BJTs are triodes,in effect,so Neutralisation was back!To add insult to injury,early transisators were prone to oscillation at low frequencies as well,so a combined correction was used to cover both LF,& the MF operating frequencies.This was dubbed "Unilateralisation".FETS also can exhibit "Miller Effect".