Testing the TK Tar Baby

[poynt99]

MH,

I do not believe it is acceptable to ignore the current draw from whatever is used to bias on Q2.

Regardless of whether an FG or a 555 circuit, etc is used, something must provide the current necessary to bias Q2 on and if ignored, represents an error term in the final power calculations.

This seems to be getting confusing. Q2 does not require any significant "bias" to turn it on. It's a MOSFET remember guys? What IS needed is a relatively low impedance AC path for the MOSFET SOURCE and GATE legs to ground.

The real DC bias and AC paths can be completely separated, as I have done with my design. This circuit is not that complex. You give the MOSFET a lot of stray inductance on its leads, a little forward DC bias, and off she goes.....oscillation.

[TinselKoala]

This seems to be getting confusing. Q2 does not require any significant "bias" to turn it on. It's a MOSFET remember guys? What IS needed is a relatively low impedance AC path for the MOSFET SOURCE and GATE legs to ground.

The real DC bias and AC paths can be completely separated, as I have done with my design. This circuit is not that complex. You give the MOSFET a lot of stray inductance on its leads, a little forward DC bias, and off she goes.....oscillation.

No, perhaps we aren't being clear. We are not talking about turning Q2 on. We are talking about supplying a negative potential to the Q2 _source_ pins, with sufficient oomph to put 200 mA in there, to induce and maintain the oscillations.

As Stefan has inquired and as I have just confirmed with live circuitry, this can be done with a 9V battery and a pot, no switching just continuous oscillations. This has to be "negative" though: the positive from the pot/battery has to go to where the "negative" FG lead is connected to the NERD circuit: the gate of Q1 and the sources of Q2. And the negative from the pot/battery goes to where the "positive" FG lead connects to the NERD circuit: the gates of Q2 and the source of Q1. Thus, no mosfet sees a positive gate potential at all, but the source bias causes the oscillations and allows some current through the oscillating mosfets... and the source bias _source_ is contributing significant power to the system, at anywhere from 60 to 200 plus mA.

I've got to hit the road, so I won't be able to post again until later this evening. Thanks for your comments, and there is a new video up with a bit of a puzzler that I could use some help with.

[picowatt]

This seems to be getting confusing. Q2 does not require any significant "bias" to turn it on. It's a MOSFET remember guys? What IS needed is a relatively low impedance AC path for the MOSFET SOURCE and GATE legs to ground.

The real DC bias and AC paths can be completely separated, as I have done with my design. This circuit is not that complex. You give the MOSFET a lot of stray inductance on its leads, a little forward DC bias, and off she goes.....oscillation.

.99,

Are you sure about this?  In the common gate configuration, I agree there will be very little gate current once biased on (as with all MOSFETS), but by holding the gate at ground and having to supply a negative voltage to the source, whatever is used to bias on Q2 must be able to provide the bias voltage "for" Q2 at the bias current "of" Q2.

I too would consider placing the gate of Q2 above ground as you do in your burst osc schematic, as switching the gate voltage would be a very low current point to do the switchng (and eliminates the need for a negative voltage source).  Try placing your gate at ground in your sim circuit and providing -10V to the source thru 50R.  What would the sim say regarding the current on the -10V?

If we are to adhere to RA's original circuit, and if the source is to be brought to say -10V relative to the gate via 50R, and if Q2 passes 150ma. at that applied voltage, then I would think that a constant supply of -10V at 150ma. would be required to keep Q2 biased on.

PW

[picowatt]

.99,

I have suggested a few times that a 50R be placed in the source leg of Q2 and that the gate be biased at a positive voltage to keep Q2 operating (similar to your circuit).  A 1meg pot across the first Batt+ with the wiper tied to the gate circuit (AC decoupled) would allow the operating bias of Q2 to be set/varied.  This would only draw 12-14ua. from the first battery (depending on its voltage).  Alternately, the gate coud be tied to the first Batt+ directly through a series resistor/cap to ground (for decoupling) and only the decoupling cap and gate leakage current would be drawn from the first battery.  The value of the 50R at the source could be modified to set the Q2 operating (bias) current.

PW

[TinselKoala]

Hmmm... I think I'm understanding this a bit more now.

The negative potential on the Q2 source pins effectively increases the potential difference between the gate and the source, with the gate being more positive... even though it's held at "zero" volts by the gate drive pulsation baseline. When the negative potential on Q2 source becomes great enough-- 4 or so volts negative wrt the gate at "zero" volts.... the mosfets begin to conduct a little and the oscillations start. Once the oscs start they provide a rising and falling potential difference between the Q2 gates and sources.... in other words, feedback oscillations.

In other words, PW and .99 seem to be describing the same thing, from different sides of the coin, if I am understanding correctly.

Is the power from the negative bias source being mixed in with the main battery power being partially switched by the mosfets during the oscillations? How does this happen if it does?  Is it coupled capacitatively through the gate-drain capacitance?