Testing the TK Tar Baby

[picowatt]

It is the NERD circuit.

Without any added capacitance, Fo=1.59MHz.
With 2800pF C_GS for each MOSFET (2 total), Fo=1.47MHz.

.99,

From this, am I correct in assuming that you are only using a single MOSFET in each position (Q1, Q2) in the NERD config?

If so, have you considered using all four MOSFETs in the Q2 position?

PW

[picowatt]

With Cgd=140pF, Cds=110pF, Cgs=2800pF per MOSFET, Fo=1.295MHz

.99,

Possibly the pspice model already has some amount of Coss?  Is there any way to tell?

PW

[picowatt]

With Cgd=140pF, Cds=110pF, Cgs=2800pF per MOSFET, Fo=1.295MHz

.99,

You might consider trying the sim with all four Q2 MOSFETS (and the single Q1) and then adjusting the added Cgs cap values until you get the correct Fo.  There may already be a bit of Cgs in the models

Maybe lose the added Coss (Cds/Cdg), as again, surely the models have some C already there as well.

If your time allows and you are so inclined, of course...

PW

[poynt99]

PW,

I think the most critical component is Cgs for that hidden AC path. The Fo is not that critical as long as we are in the ball park.

[poynt99]

Part 3.

First we look at an example of an open circuit FG setting of +/- 12V with 20% duty cycle. Once the FG is connected to the NERD circuit as shown, we see that not only is there a negative limit of -4V being created by R_OUT, but there is a positive limit of 10V as well, again because of a voltage drop across R_OUT. In this latter case, the current path is through Q2's forward-biased body diode. Here we see a current of 37mA.

During the negative portion of the FG output, the current is 160mA.

Next, we look at the current path while the FG output is positive. This is shown by the RED path. Note that without Q2's body diode, there would be no path. This is a closed-loop secondary path not including the battery. Once the FG output setting is below 10V, there is almost no current through this path.

Following this, we see the current path for a negative FG output. This is shown by the BLUE path. This path through the FG includes the battery and is part of the main current through the load. There is only -4V at the FG terminal due to the 160mA flowing through its 50 Ohm R_OUT, which constitutes an 8V voltage drop.

Next we take a look at the dynamics from end-to-end by imposing a DC sweep through the NERD circuit as shown in the simulation schematic. We are sweeping from -16V through to +16V to see where the limits are at each end. Also shown in red is the current through the 50 Ohm as a function of the sweep voltage.

.99