7 - Notes about inductors

I have a bit of a mental block about magnetic circuits, I found the site below very useful;
http://www.agder.net/la8ak/12345/n12.htm These are the notes of LA7MI, Stein Torp, translated by LA8AK (sadly sk).

Ferrite Losses;

Apply a highish power RF source to the ferrite core with, for example two turns, record the voltage across the core and increase the level until you get a 20 degree C temperature rise. This can then be stated as the core can take x Volts/turn, e.g 4 V/turn down to 1v/turn (e.g FB43-2401 on 80m)

For cores carrying DC: measuring DC saturation;

Apply a variable DC voltage through a small series resistor (to allow current calculation), and maybe an RFC to one side, Use an inductance meter on the other side, note DC current to reduce inductance by 10%

Testing Broadband Trafo by VSWR:

Load one side with a load and feed the other side thru a return bridge (See Solid Satte Design P154, fig 36)


Working out what XL you need:

working backwards from a assumed working PA you must have a load resistance of (Vce-Vsat)^2 / 2 * POUT, (or else you wouldn;t get the output power).

So, for example 8W and Vce-Vsat=10V implies Rload=6.25 Ohms.,

if two transisotrs are in pushpull the load is two times.  16W RF from two transistors on 12 volt supply must see 12.5 Ohms between the collectors, hence the turns ratio is 2:1 (voltage ratio i.e 4:1 impedance ratio). If the secondary is 50 Ohms then the inductance of the secondary should be >200 Ohms at the lowest frequency. (10uH for down to 3.5MHz or 20uH to 1.57MHz)

Note also that 50 Ohms receiving 16W must have 28.3Volts across it.
If we are restticted to 4 volts per turn then we must not exceed 7 turns.
If we use 2 cores then fewer turns gives the same inductance,

6 - Microphone ampliers - discrete

In keeping with not using any special parts, I would like to make my microphone amplifiers without using ICs (a bit daft I suppose since we wil always be able to get/buy opamps...)

A quick google on "mic amplifiers discrete" found the schematic below, I am unsure who to attribute it to as it pops up in 3 or 4 places on the web, google first found it at

http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm

And Andy Collinson says; amplifier for dynamic mic of 200 to 600 Ohms, gain of +39dB or x90. Q1 is run at low Ic to give better noise response, <5uV up to 5kHz.

I need to fiddle with the values to reduce the bandwidth to 5kHz, as it stands below it is flat to over 100kHz.

Andy also has an electret mic version by omitting the first stage above. Note if using a 2 terminal electret simply connect the series input capacitor to the bottom of the 1k bias resistor.
see http://www.zen22142.zen.co.uk/Circuits/Audio/ecmmic.html

Gain of this is 20dB to way over 100kHz, output Z very low.
I may run this circuit thru LTSPice to optimise frequency response and to test with feeding a 50 Ohm output load...