Tuesday, October 29, 2019

TUN Amps with characteristics

These amps are designed to use 'universal' parts at AF. You can use adjust the capacitors and use them at higher frequencies. The post linked below has charts of useful transistors. Whatever you have on hand will be OK as long as they are rated for the frequency you want to amplify.

TUN TUP universal components

Time for some simulations using different transistors? A breadboard to test the circuit?

Monday, October 28, 2019

DC Receiver complete

I finished the sim on Ryan's receiver and added an audio amp.  15uw would be more than enough to rattle my earbud. The output could be adjusted for more output to drive a speaker.

For the sim I used 50mv LO. the output would be more with higher LO drive. I adjusted the preamp bias to conserve battery life.

The preamp matches the antenna Z. The second stage matches the detector input.
I'm using my Gerbert Cell. The hookup would be the same for a NE602.
I tacked the AF amp on to add a little drive.

It would be a good time to run a sim!

Thursday, October 24, 2019

DC receiver preamp

The antenna is 200 ohm so the common base is a good Z match.

I set the generator at 200 ohms with 1 mv out. The input is 500uv. The generator internal Z is dropping half the input.

The bias is eating up 2ma. With the collector current so low the bias circuit current could be lowered.

I increased the bias resistor X100.

The amp output is close to the same. 

The question is, "How much current should the bias network draw?".

Next post we can add the second stage.

Wednesday, October 23, 2019

DC Receiver part 1 the ne602. supplemental



With the reference set to one output measuring the difference between them.

The circuit on my previous post produces similar results so I will use it for the circuit testing. We can look inside the circuit not at a black box.

DC Receiver part 1 the ne602.

In order to look into Ryan's receiver we need a model for the ne602. I have one but it is a screen full DC biasing and such. For our purpose a simplified Gilbert Cell will do. So first a look at the Gilbert Cell and how it functions.
This circuit has the modulated signal and Local Oscillator so we can look inside and see how it works.
I'll be using an AM signal (the brave at heart and SPICE experts can use SSB). I probed the AF and output.
Here we see the modulated signal and the Local Oscillator.
Out A and Out B. Take a look at the 4 capacitors on the output lines. It could be an interesting exercise to remove and or change their values.
Here I show Vout and the current through the cap going to ground.

SPICE schematic
EDIT: If you run the sim try adjusting the signal levels. I have both set to 1mv. Try 100mv for the LO. The benefit of a Gilbert Cell is the conversion gain. The output will vary with signal and LO levels. If it tends to overload attenuating the LO signal may help?

*** I adjusted LO to 100mv and the signal went from 700uv pk-pk to 40mv pk-pk. ***

Monday, October 21, 2019

40 meter DC radio - designed and built by Ryan

My friend designed and built it. He was kind enough to allow me to post it.
He shows his work so it is a good project to study. It could be a good exercise in SPICE too.

Tuesday, October 8, 2019

Another look inside a transistor voltage and current relationship

I set the generator to produce a 10mv signal riding a 1700mv bias. The 1k emitter resistor will 'self adjust' to about 1 volt producing the bias required to allow a 1 ma collector current. The emitter resistor would drop AC signal and produce degeneration C1 provides a path for AC. C2 provides a path for AC to the load (R3). The .55ua pk-pk signal provided by V2 when set to 10 mv produces 10mv across R3. It appears the amp produces unity gain? Yes 10 mv in produces 10 mv output but look at the currents. I(V2) is .5ua pk-pk, I(C1) is 100ua pk-pk, and I(C2) is 100 ua pk-pk. So the current gain is 200. Next let's increase the input and see what happens.
I simply increases the AC to 100mv. I(V2) is about 5ua pk-pk. The increase created an increase in base current as would be expected. V(out) is 100mv with 100mv input. We have unity voltage gain again. The current gain is more than 200? Why?
Hint: When a semiconductor current increases its resistance decreases. re is 26 ohms at 1ma. re decreases as current increases. gain is the ratio of Rc to re.
It is a good time for a simulation using this circuit. You could try removing C1, increasing and decreasing C1 and C2 and changing the frequency. 

Saturday, October 5, 2019

voltage controlled resistor, voltage controlled current source, or CCCS?

Years ago the transistor was thought of as a voltage controlled resistor thus the name TRANsfer reSISTOR. Then someone decided it was a current controlled current source. The voltage controlled current source is popular today. The debate is clouded with the fact that ß is not a fixed factor it varies with current and voltage and temperature. In this series I'm not going to define the actions within the transistor but will run SPICE programs to demonstrate them. Hopefully the sims will speak for themselves. It would be a good time for you to run some too. You can vary the component values and probe the circuits to see what is happening.

5 ua in the base produces 1ma collector current. I put the 1 ohm resistors in the circuit to make measurements and later they can be changed to see the effect.
Using 656 mv bias gives simuliar results. Ohm's law says voltage , current and resistance go together. Trying to have current without voltage or visa versa just doesn't work. Therefore the debate is it voltage controlled or current controlled. More on that later. 

Stepping the load resistor from 100 to 5.2k ohm gives basically the same collector current. Injecting current into the base controls the collector current BUT there are other factors involved.

Probing the collector voltage displays one of these factor. As we change the load resistor the collector voltage varies and this has some effect on the collector current.

Probing the base current shows it is effected by collector voltage too.

Fixing the bias and allowing the collector voltage to swing displays the current variation. (changing Vce)

Adding a signal shows the current swing responding to it.

adding R1 and probing its current shows the carriers in the collector will produce some current when given a path.

I moved the ground reference to the emitter. In the previous shot it could appear the signal was driving the collector too.

Applying 1 volt to the collector encourages collector current.

With 2 volts it is stronger.

With 6 volts the amp is looking much better.

With a current source a transformer can be used to couple the signal. This transformer is 100mh primary and 50mh secondary. It produces good results as you can see.

100uv in producing 15mv out with PNP germanium general purpose transistor. Time for a build!

10 uv will drive an earbud.

My old ears don't hear well below 800Hz so for me it is ok. You could adjust the bandpass by changing the capacitors.

The gain drops well below my local AM station so that should not be a problem.

EDIT: I assembled this one and it drives my earbud with the AF generator set to minimum. The signal drops out after insert a 20db attenuator.