#MACSPICE EXAMPLES CODE#
The actual noise calculations had to be performed explicitly, using the corrected SPICE equations this required some non-trivial code implemented in the MacSpice front-end. And because it had turned out that this was not the case, I could use SPICE here merely for the underlying operating point analyses. For this, I had to take the given noise data, together with the specified measurement conditions, for granted, no matter whether SPICE was able to treat the corresponding operating conditions correctly in its built-in noise simulations. The first, obvious aspect is that I wanted to derive reliable general-purpose KF values for all transistor models. There are two different aspects, namely model creation and model usage: With such KF values, however, the majority of ‘arbitrary’ noise simulations with SPICE would have overestimated the 1/f noise by one or two orders of magnitude. In particular, the KF values of my final SPICE models (i.e., 3.5f to 5.7f for the AC transistors) would have been significantly higher (850f for the AC127, 15f to 40f otherwise) when relying exclusively on genuine SPICE noise simulations. This may just have been a coincidence, but it would have prevented me from deriving correct KF values for all these transistors. In fact, the operating points specified for noise measurements of all AC transistors (except the AC128, for which no noise figure is given) happened to be not quite far from the zero-crossing of the external base currents. This is perhaps not as clear-cut as it might seem at first glance.
(Unless you have no base resistor at all, which is done, but not in low-hiss amplifiers.) Quote -You also have to be real unlucky to hit the base current null. But it will take me a while, too, to go through all these publications.Īnother Master Thesis (by Thomas Frederick Brennan, 1967), which I had found useful for my purposes, is available at:
Particularly Hatton's Master Thesis of 1951 might contain some valuable background information on this whole subject. Quote -Check out these for Noise Figure, For the older OC44 and OC45 transistors, I assumed a somewhat higher noise figure of 10 dB at 1 kHz, borrowed from an OC70 datasheet. Incidentally, these operating points were identical for all AC transistors for which noise data were explicitly given. I chose the common-base configuration because it was a bit easier to trim these circuits towards the specified operating points. Some further Internet research confirmed this basic relationship, too. Therefore, I simply tried both variants, common-base and common-emitter, but either set-up yielded almost exactly the same results.
I have to admit that I had also been a bit irritated by the fact that the Valvo datasheets didn't specify the exact noise measurement set-up. Quote -One thing that stands out is your noise figure set-up. (Unless you have no base resistor at all, which is done, but not in low-hiss amplifiers.)Īs a cave-man, if I discovered this null in the math but not reliably in real life, I would (as you suggest) plot two points away from the null and sketch a curve between. You also have to be real unlucky to hit the base current null. And since Silicon was now readily available and clearly better, why muck around with the math of wheezy old devices? The apparent simplicity (at normal operating points) of good clean Silicon devices inspired development of calculators. The casual cynic is tempted to say: "the *designers* of SPICE did not want to care about such old germanium transistors". > Some SPICE users may be tempted to say: “Who cares about such old germanium transistors?” SPICE primarily fails in the vicinity of the zero-crossing of the external base current. SPICE parameters for Germanium transistors?ĭIY Stompboxes > Building your own stompbox
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