Switching Diode | Dirk Mittler's Blog

This posting attempts to resolve the inadequacies of the circuit that I had already designed, using the Open-Source package ‘NG-SPICE’, and that I had described in This Earlier Posting.

Essentially there was only one problem which I was able to resolve: The bad input and output impedances of the former circuit. I now have a circuit with an input impedance of approximately 100kΩ, and an output impedance of approximately 10kΩ. A source-follower resistor for the output can be substituted where R3 is, to halve the impedance further, at the cost of also attenuating the output amplitude slightly more…

These were the SPICE Model-Cards used:

http://dirkmittler.homeip.net/text/NMOS1.mod.txt

http://dirkmittler.homeip.net/text/BAS16W.mod.txt

Please do not attempt to build this circuit using discrete components. As already described in the previous, main posting, the ultra-low capacitances used in this circuit will get overpowered by stray capacitance which breadboards, wires, etc. will introduce. This circuit can only be implemented as part of a larger circuit, on the same IC. Therefore, it should only be considered hypothetical.

(Updated 7/4/2019, 8h20 … )

Continue reading NG-SPICE: Low-Powered Saw-Wave Generator, Revisited.

The goal of my latest exercise at using the Open-Source circuit simulation software named ‘NG-SPICE’ consisted of designing a low-powered saw-wave generator. Here were the premises of the project:

A train of pulses is to be taken as input, that are approximately of 1μS duration, 2V in amplitude, and that have a steady rate of recurrence of 100kHz.
They are to be converted into a saw-wave that has an attack as fast as the pulses are short, and which has approximately linear falloff after each input pulse.
One active component is a monolithic N-channel enhancement-mode MOSFET transistor with a gate size of approximately 100 microns squared – which therefore has poor qualities if compared to discrete components – but which is plausible as part of an IC with Medium Scale Integration (:2)
The other active component is a bipolar diode of unknown weaknesses, which has been approximated as a discrete 1N4148 switching diode.
The entire circuit is to operate off a 3V power supply.
The maximum output load is in the vicinity of 100kΩ – 40kΩ, and must not change the internal workings of this circuit block. (:1)
The output amplitude is to reach approximately +1V with respect to the circuit ground.

What was observed:

The diodes were difficult to get into a conductive state at the low pulse-voltage.
The chosen MOSFET makes a very poor output driver.

The experiment seems to have been successful.

(Updated 7/3/2019, 8h35 : )

Continue reading NG-SPICE: Low-Powered Saw-Wave Generator