Testing the TK Tar Baby

[Groundloop]

.

[Groundloop]

Groundloop,

Your very nicely done drawings will hopefully enlighten.

I do have a couple of conditions to add.  When the FG is -12volts, the voltage at the source of the MOSFET will only pull to -3 to -4 volts (the threshold/turn on voltage of the MOSFET), so the drop across the 50R will be closer to 8 to 9 volts and not the full 12 volts.  The current/wattage dissipation then needs to be recalculated accordingly.

Also, during the oscillation, the MOSFET will be cycling between an increase and decrease current, so the bias current will modulate as well.

I greatly appreciate your time in drawing and posting this.  It is an excellent and concise representation of how the function generator is being utilized, and of the current flow through the function generator and the MOSFET.


Thanks!

PW

PW,

Yes you are right, but I want to keep it simple.

GL.

[TinselKoala]

TK,

OK, now we're talking.  Timebase, zero ref, and V/div included. 

Except for the jokes, don't I always mention these in the videos and the scope shots I post? As I said before I don't always mention the timebase, since RA's are all over the place and the pulsation frequency doesn't seem to matter and we know that my oscs are around 2 MHz. But the other parameters.... well, I do try. If you point out one where I forgot to do so,  but the information is pertinent, please let me know and I'l "fill in the blanks".

Were these shots taken with the 10R and pot for a bias source?

Technically yes, but the pot is fried and needs to be all the way against the stop to get out of the burned spot, so the PS voltage setting is doing the regulation. And these are also including the 50R in series as well. As I indicated earlier, I think that the Elenco has started putting some fuzz on the power it's putting out, or perhaps a mosfet is starting to go bad, because the waveforms I'm getting since yesterday have seemed different, with more HF components.

As you can see, there are little nuances on Rosemary's CH3 traces (FIG4) that are absent.  Her circuit demonstrates several plateaus that yours does not. 

Uh-huh. And her trace is presumably coming from the function generator signalling the circuit to switch, whereas my trace is coming from a DC bias supply held constantly on; and there is also the fact that the bandwidth of the HP180 is under 100 MHz, especially with the probes I've got.

I would again suggest using the FG or pwr supply and 50R to see what if any differences show up.  Also, alternate scope shots with the bias supply ground (or FG gnd) tied to either end of the CSR may help weed out the differences between the CH3 trace in FIG4 and FIG5 of the first paper.  Keep in mind that some of the FIG4 plateaus may be related to the Q1 issue indicated by FIG3.

PW

Your second-to-last sentence confuses me. Are you saying that "my" traces are different than the RA CH3 traces Fig4 and Fig5, or are you saying that Fig4 and Fig5 are different from each other?
Of course they are different from each other.... one is at 1 microsecond per division horizontally showing the oscillation region only (note the trigger time offset at the top: the yellow triangle pointing off screen left and the -5.0000 MilliSecond figure at the top) and the other is showing the pulsations at a much longer timebase setting of 500 microseconds per division with no trigger time offset. In the FIG 4 we are looking at only that part of the oscillation that is represented by the blue fuzz at the bottom of the pulsing rectangular waveform shown 500 times more slowly in FIG5.

I must be confused about something again. It looks like you are asking me to make the same oscillations as hers in fine detail,  but with different circuitry and device settings.

[picowatt]

PW,

Yes you are right, but I want to keep it simple.

GL.

GL,

Understood.

Again, excellent depictions!

Thanks,

PW

[TinselKoala]

. (DIagram showing doubled mosfets)

And also some of the FG's power is also dissipated in the mosfets themselves, isn't it? Or rather, there is a total power made up of the total resistance subjected to a total voltage so a total current flows. Both the battery and the FG or other source contribute to the total voltage, and thus to the total current. This current passes through the mosfet(s) both by the normal drain-source conductance but also by the various capacitances as well as the body diode, depending on the current's phase. So the mosfets are dissipating power, and the load is dissipating power. But how to assign the proportion of the contribution of each source to the dissipation?  If the mosfets dissipate 30 Watts, how much is coming from the batteries and how much from the bias source? And... ditto question for the load.