Millivolt in millivolt out not very impressive?
Sunday, September 20, 2020
Final thoughts on the PCT
Millivolt in millivolt out not very impressive?
Friday, September 18, 2020
discrete build of PCT - proof of concept
I connected an emitter to each end terminal and clipped a test lead to a base - collector pair then connected the far end of one pair to the base connection on the test jig.
The meter reads the base - emitter junction resistance.
I set the supply to 6 volts and the meter displays a negative resistance. I have PCT transistor action. There you go proof of concept.
revisting the point contact with discrete equivalent
We can make a Lambda Diode equivalent with two JFETs and it function much like a Tunnel Diode.
We can make a SCR equivalent with two transistors and it will function much like a SCR.
We can use complementary transistors and make a remote base transistor which acts much like a Point Contact Transistor.
The PCT collector is formed after fabrication. This forming causes doping which produces the PN hook. A PCT will not function without the PN hook. If we simply make a point contact diode with two points we have to parallel diodes but no transistor action. forming one point creates the hook and transistor action. Clear as mud? Lets look at the PCT structure.
The two diodes are just two PN junction sharing a common wafer until the collector is formed. The phosphorous in the wire is diffused into the P cathode forming an N collector. Fig 309 is the equivalent circuit made by junction rather than points. The center P is the remote base. in a SCS it would have a pin connected to it. The SCS is basically a SCR with a cathode and anode gate. The two gate connections allow the SCS to become a SCR or PUT. Fig 309 was a common device in the early days of semiconductors. Anywho the secret of the PCT is the junction formed when the collector is pulsed with current and the remote base transistor hidden in the device. Once you understand that you can use complementary transistors to make the equivalent circuit and you should be able to understand there is a great difference between the PCT and the junction transistor. Anywho lets take a look at the equivalent. I show the voltage and current in the analysis.
The forever dispute about transistors is do they amplify current, voltage, power etc.
The PCT has an alpha greater than 1. It amplifies current. Yes?
Micro amps in mill amps out. The current flowing threw the load resistor drops voltage. The current being higher the load voltage is higher. Careful don't start that back and forth again!
You can work it out in your own mind. It could be Q2 is a common collector amp and what I marked Emitter is actually the remote base?
Wednesday, September 16, 2020
comparing the Ge to the Si transistor amp
Tuesday, September 15, 2020
Loop stick antenna NOS and replica
Friday, September 11, 2020
Adjustable antenna coil from scrapsI wound a coil on the pen barrel and inserted the core.
This is good for what it is but what if you want this instead?
This one offers a starting point.
We need a screw, nut (insert) and tube (pen barrel).
|A core , screw and nut|
|I drilled a hole in the screw and inserted the core lead. A little solder should keep it in place.|
|I would a coil on the pen barrel and assembled them.|
I soldered the first one.soldered and glued glued the second. I think the glue helped.
The first adjust from 30uh - 100 uh.
The second adjusts from 40uh - 200uh.
The core is 3/4" and the screw is 40 tpi so the coil requires 30 turns for a full range adjustment.
If your core doesn't fit a tube you have available you can make one from craft paper or paper tape.
Check the junk box and think about it.
Enjoy the hobby.
The connector is a
strip of sheet metal about 1" long. I wrapped it around a rod and crimped it with needle nose pliers to form a loop. Then holding the loop split the ends to form mounting ears. They had a drop of glue applied then masking tape to hold them in place. a more traditional treatment would be to cut a strip of cardboard such a s a cereal box and split it to allow the tabs to be inserted. then gluing it in place. I think I may make a couple more using that idea but for now the proof of concept is looking good. Well? Not looking good but functional. The store bought ones are prettier but the price is right on these.
Tuesday, September 8, 2020
Earbud amp using transistors from GE computer boards - 2
Earbud amp using transistors from GE computer boards
Sunday, September 6, 2020
Using the RC filter to form a differentiator
Something to think about the next time you see a circuit with all those extra unless parts.
Saturday, September 5, 2020
putting a filter on an amp
I placed the filter across the FB resistor and ran a frequency response. As you can see it will virtually kill any short wave signal. The filter I used earlier was set to 1600Khz. You can adjust the values and pass a higher or lower frequency.
This series was in response to a question about the LM386 output filter. I think a review of the datasheet may shed more light on that. I have hear more than once the "cheap" LM386 amps are trash and prone to oscillations. I have also heard the "expert" on a radio forum say the filter is not needed. This makes me wonder if they left the did not use the filter and created the stability issues? The number one issue we have as home builders is keeping the output from the input especially when higher frequencies are concerned.
Consider a short wave build operating at 4 Mhz with an AF amp that will amplify 4 Mhz. Now let the AF amp oscillate. You just "built a regenerative receiver".
Use a ground plane.
Keep lead short and run them close to the ground plane.
Separate (shield) input and output.
Consider using filters to control the band pass for your amps.
Most of all enjoy the hobby!
Friday, September 4, 2020
Filter the noise out
In order to function we would set R bypass less than Z gen. The filter is less effective when Z gen is 50 ohms.
Z bypass also needs to be less than Z load. Simply stated the bypass must be lower than the source and load.
You should be able to see a reaction to the input frequency.
That reaction is defined as 1 / ( 2 PI * F * C).
The last amp I built was very stable with the input shorted but would pick up strays when the input was open. Putting a low value resistor across the input helped but it would then load the tuner so a filter was called for. I used a simple L filter and it worked but this filter is used on LM386 amps so it could be a good alternate.
Wednesday, September 2, 2020
low power speaker driver
I'll have to take time to build this one. I filed it here so I don't lose it.
I built it and it sings. With my generator on minimum it is loud. With the attenuator on 20db it is just barely there. I can not crank it up much it hurts my ears.
With a 6 volt supply it draws 4.5ma.
with a 9 volt supply it draws 7.25ma.
It passes the finger buzz test too.
Tuesday, September 1, 2020
TUP - TUN complimentary amp
I used the universal design for the preamp and added 2 diodes to keep bias in working range on the output the 2 4Ohm resistors help prevent cross over distortion.