More MOSFET data

This has some high math in it but I found the pictures interesting;).
Designing MOSFET amps

I made a model for the 2SK669 and tried it out. First the simulation  then the actual circuit build.

This is probably the most common circuit you will see. It is high current, high power and probably not to sensitive.I will be using ear buds or phones and don't need a lot of power and will be driving it with a DBM which is a small signal.  A non scientific simple test I ran was to pick up a cell phone and go to setup - about the phone. It displayed a signal strength meter. At the time I had 2 of 4 bars and the reading said I had a 2 uV signal. If we have a 7 db conversion lose with 2 uV in we will be dealing with 1uV AF input. Hopefully we will be receiving stronger signal but it was just something to consider.

The article says we need to be conscious of temperature changes and distortion. So I added the diode which may react to temperature changes and the 50 ohm resistor to provide a little degeneration.

This is what the sim says. 10mv in and 300mv out. A gain of 30 and no distortion would be a good place to start. I need to test the spice model against a circuit.

I did a quick and dirty breadboard build and this was the results. I did not search out the parts I just used what was in the box. The test indicated the design will work but I need to seek the proper parts and do it again. I used the 2SK669 but the resistor and diode were not as spec'ed. I did change the MOSFET in the sim using models provided with the program and found reasonably good results.

NOTE: My digital scope lowest scale is 20mV so I couldn't drive with 10mV  but it does show the amp to function.
NOTE: I reran the test an the results match the sim.

The .ASC file for the amp

the .ASC for a RF Amp using 2SK669

drawing the data together.

First let's take a look at a commercial kit. This is a converter to use with a car radio.

Now much to look at with the IC containing the oscillator and DBM. If you read the post ahead of this one you should have seen when using a level 7 DBM we will expect a 7db conversion loss. So this begs the question Are they loosing 7db  or is it more than meets the eye? Let's look at the circuit for the answer.

The signal goes in pin 1 and out pin 4. Where is the diode ring and transformers? They are using a Gilbert Cell.Interesting side story that I will not go into at this time but Gilbert did not design the cell? Anyhoo, the diode ring mixer produces a loss. The GC (Gilbert Cell) doesn't. In the transition from diode ring to the NE-602 there was a GC chip. I did a discrete build of one here.
Gilbert Cell from discretes
So the food for thought here is shall we build a GC from discretes, inset an RF amp to overcome the losses, or use an AF amp with some extra boost?

Some data on core material recommendations

I gathered a little data on the cores. Just excerpts from the manufacturer's catalog.

This could help with selecting a core material for the band we want to use. I ran the numbers of some.

Just looking at the broadcast band and beyond.

Here they tell the number and size of wire for different core size. The calculator is good but it helps to know you can actually get the necessary number of turns on the core.

They recommend the material for antenna coils. This could be good to know.

Settling on the LO for the DBM.

The datasheet shows the LO level at 7dbm for the DBM and it shows the effect if the Lo is weak.

As you can see higher LO signal is better.

So what does this ADE-2 or ADE-5 consist of?

They are ICs which have two transformer and a diode ring inside. So we need a pair of transformers. What are the requirements for the transformers? The coil must have an inductive reactance of 4 times the circuit impedance at the lowest operating frequency. We need to know the circuit impedance and the operating frequency to size the coil. Assume our Rf circuits are all 50 ohm and we will not be using the DBM below 500Khz and we will need Xl => 4 * 50 at 500Khz.

The calculator say 14 turns on a FT50-43 will be good . for 1-50Mhz. I will look at so more cores and see if I find one for 500Khz. So begins the search for a core. More on that later.

 I'm using a 10 mv feed and getting a fair drive. Andy's oscillator will produce 250 mv feed so it should be plenty.

The above spice program is linked here

If you click the link and save as .ASC file type you can play with the sim.
I'm going to look up some core data more later.

Some more DBM data to chew on.

So the question is What level is the DBM designed for and/or how does the design define that level?
So we go with  a level 7 design because it is most common?

Why use a MOSFET? or How much gain is in it?

I started to title this on 'the solid state transformer'.
Anyhoo, how much gain can we get from the MOSFET? The simulation show 1mV in and 2 mV out.  Sure doesn't sound like much does it? The text book says it has an infinite gain? So what is not so obvious?

 Look a little closer at what the book says. It offers infinite CURRENT gain. With no gate current any drain current would equal infinite current gain.So in theory the gate has no current. Look at my circuit I have a 10K resistor across the input. So If we calculate power drop across the 10K input resistor and power drop across the 50Ohm load resistor we get a more meaningful number than the voltage gain of 2.

You can do the math if you are into it but I just used a calculator. 1mV and 10KOhm gives .00005uW and 2mV at 50Ohm gives .04uW. .04uW/.00005Uw = 800. So we have a power gain of 800 and the MOSFET transformed the 10K input Z to a 50Ohm output Z.

Oscillator DB level - Square Law vs Linear?

I don't have a lab and my test are more trial and error so I can't confirm or deny the chatter I hear on the boards. The subject at the moment is local oscillator signal strength. A 7 DBM mixer or level 7 mixer would require a signal of about .5Vrms. This seems to be the most common one. Mr Tuggle (on TRB) using a one active device receiver has DXed stations 1000's of miles away. His single FET oscillator driving the mixer picking up worldwide signals begs the question, "Is he really getting .5 Vrms with one JFET?".  So with this in mind I'll offer some data from the manufacturer and try and make since of it. One other subject that is constantly declared on the board is Square Root vs Linear. This data should shed some light on that.

So Square law produces twice the output for the same change in input?

WOW! Now I'm truly shocked. Our world renown expert has declared it's all about  matching, coupling and Z many times, yet the manufacturer says otherwise in the last line.
As confused as I am I guess the next thing is to look at a small signal and large signal detector plot and try to determine which to use?

May need to chew on this a while?