Where are losing current?

[wivern]

Hello,

I have a circuit into which the current 100pA, the circuit has two points at which the current from the circuit output.
Why is the current I1 > than Iprobe1 + Iprobe2.

Where to lose 32pA?

[johnsold]

This looks like an academic exercise.  No one would design and build a circuit like that.

I suspect that the current difference is due to round off errors in the calculations.

The operating point (if you can call it that) of this circuit is so far below the nominal limits of the devices that the accuracy of the device models probably renders the simulation completely meaningless.

For the BF421 I_CBO <= 10 nA and I_CBO <= 50 nA.  These currents are 2 orders of magnitude larger than your source.  I could not find a datasheet for the J404 but the J111, J112, J113 devices have I_GSS <= 1 nA and J304 has I_GSS <= 100 pA. Assuming that the J404 is similar, its gate reverse current is likely much smaller than the BJT leakage currents which explains the low current at Probe 1.

Note also that probe 2 shows a frequency of ~23 kHz, which seems out of place for a DC circuit!

Consider the current path if all the transistors were ideal (no leakage). The current would flow from I1 through Q5 emitter to base through Q8 emitter to base then through R3, R4, R5, R6, D1 and R1. The voltage across R1 would be 1 V.The voltage drops across the base resistors would be 15 uV each.  With such small drops none of the other transistors would be forward biased so they remain essentially non-conducting. The voltage at the drain and source of Q10A would be about 1.6 V and the 2 pA leakage current at Probe 2 seems reasonable.

Simulators apply Kirchoff's laws at each node but generally do not look at a whole circuit such as this one as if it were a single node with three paths to ground.  If you make a subcircuit out of all the transistors, the diode, and all resistors except R1, do you get the same result?  What happens if you only use one BF421?

Lynn

[wivern]

Thank you for your response.

You are right, the current 100pA for this circuit is ridiculously small, I used it to illustrate.

Circuit at Real works for current 500nA with PNP MMBT6520L and FET BF4117A.

The problem with "missing" current on simulation is also apparent in the 500nA just not as exemplify, "missing tens of pA".

What is this circuit?

Surge Protection min 1000V.

If is on cathode D1 +1000V is circutit  protected D1

If is on cathode D1 - 1000V spread the voltage on Q2-Q5.

In the simulation, I wanted to test the behavior of the limit states or verify what the current is able to function.

[johnsold]

To study that kind of behavior via simulation certainly avoids lots of smoke, sparks, and foul odors! However, you must be very careful with the device models to make sure that the models are accurate at the boundaries of normal operation. Many models are not.  Models often do very nicely at simulating typical behavior but very poorly at simulating the secondary effects or out-of-nominal-range effects.

I suggest that you study the model of each component separately, both by examining the code and by testing with simple circuits, to see how the behavior of the model compares with the datasheets and with the real devices.  For example the Vishay datasheet for a GP10G shows it rated for 400 V (GP10M for 1000 V) and that the expected DC reverse current at that voltage can be up to 5 uA with the typical value at 25 C about 1% of that.  Try a simple simulation with just a diode and source and check the reverse current as the voltage reaches and then slightly exceeds the rated voltage. Does it behave like the typical datasheet vales, like the worst case values, or completely differently?  You need to do similar things for the transistors.

Lynn