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I'm simulating this circuit in LTSpice:

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The input is a 1 Mhz square wave with 10 ns rise and fall times. Plotted are the input, output as well as transistor base voltage.

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Now here is what happens if we replace the Schottky by a 1N4148 silicon diode model:

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It is substantially nicer! The rise time is much more steep, leading to a nice flat top. Furthermore, the off voltage is closer to zero. This niceness is also more robust against component changes.

If the simulation were realistic, why would we ever use Schottky clamps, so it can't be right.

What is wrong in the simulation?

Is it that this type of Schottky is not appropriate for the circuit (being a large-ish rectifier diode?) Or something else?

Update: Indeed, the issue with the rise time is simply that the Schottky model is a heavyweight. Scrolling through LTSpice's list of available Schottky diode models, I found one BAT48WJ. The results with this diode look a lot more reasonable in terms of rise times:

enter image description here

Still, the question remains is the result with the 1N4148 realistic?

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  • \$\begingroup\$ I have never worked with this circuit before, but nobody else is chiming in, so here are my thoughts: 1. I think the (a?) reason to use a schottky is that you want a lower Vf so that the diode becomes active before the bjt's bc junction, preventing full saturation of the bjt. You can see that with the 1n4148 Vout does drop all the way to 0, indicating saturation, whereas with bat48wj Vout is clamped around 0.5 V. \$\endgroup\$
    – The Photon
    Nov 7, 2013 at 19:01
  • \$\begingroup\$ 2. C1 is also there to help with turn-on/off times, and its effect may be masking the clamp effect. So, what happens if you compare the results for the 1n4148 and bat48wj with c1 removed? \$\endgroup\$
    – The Photon
    Nov 7, 2013 at 19:02
  • \$\begingroup\$ @ThePhoton That's the thing. If there is indeed saturation, then that 2N3904 should require over 200 ns to get out of it. \$\endgroup\$
    – Kaz
    Nov 7, 2013 at 19:36
  • \$\begingroup\$ @ThePhoton I found another Schottky: BAT54. With this one, the turn off (output rise) is about on par with the 1N4148 with C1 in place. With C1 removed, there is about a 100 nS lag with the 4148, but with the BAT54, the output starts ramping immediately. This shows that the cap plays a role in removing the storage charge, and that the Schottky is improving the situation also by preventing saturation. \$\endgroup\$
    – Kaz
    Nov 7, 2013 at 19:39
  • \$\begingroup\$ @ThePhoton Note, though, that if we have just the cap there, and no diode, it's basically DOA. The transistor will not turn off, which is expected. So there is no question that the 1N4148 helps. It seems that C1 + 1N4148 does the job; but is it realistic or just simulation. \$\endgroup\$
    – Kaz
    Nov 7, 2013 at 19:46

1 Answer 1

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Here is the real behavior of almost the same circuit, installed in a real device.

The differences are that the driving source impedance is unknown (V1 is ideal in the simulation), and the load impedance corresponding to R3 in the diagram is unknown.

The behavior is nowhere near the rosy picture painted by the simulation. (On the other hand it is good enough that the everything works in the DUT).

2N3904, diode, speedup cap

The horizontal divisions on the graticule represent 500 nS intervals. The upper trace is the transistor's collector: one division is 5V. The bottom trace is from the base: one division is 2V. As can be seen, the rise time is quite slow, taking over 500 nS to rise to more than 90%. What might be considered a solid high logic level is probably not established for some 250 to 300 nS.

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