Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Transistor amplifiers v. frequency in SPICE
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Date: Sun, 12 Jan 2003 09:36:59 -0000
NNTP-Posting-Date: Sun, 12 Jan 2003 09:37:06 GMT
Mike Monett wrote:
> Paul Burridge wrote:
> [... see original for CKT file]
> I have to admire your energy and persistence. SPICE is not easy to
Well, I would quibble with this. Compared to doing design by hand
calculations, its trivial. Just about any modern Spice is all GUI
driven. imo, most "spice" issues aren't. Its not knowing enough
electronics that is usually the problem, not how to set a few check
boxes in a spice simulation system.
> and you are doing a good job of asking the right questions.
> A couple of quick comments. Your SPICE should be able to plot the AC
> response curve instead of having to do each frequency manually. What
> version are you using?
I can't imagine any viable commercial spice, whatever version, not doing
basic AC, DC and TRANS runs.
> The emitter and collector resistors are way too high for the frequency
> response you are asking. MicroCap calculates the ft at 1.72MHz under
> these conditions. Here are some suggestions you might want to try:
> Change the resistors to the following values:
> R1 = 1k
> R8 = 1k
> R7 = 750
> R2 = 470
> Change C2 to 1uF.
These are reasonable suggestions, and indicate, as I noted that, that
issue is nothing to do with spice, but the posters lack of electronic
> Add a 1pf from the collector to the base. The frequency response falls
> faster still.
> These are simple ways to see how a circuit is working, and to get a
> feeling for the real-world effect of strays. It tells you for wideband
> circuits, you need low impedance which means high current.
I do have some papers on this
> CMOS designers can get away with much lower operating currents since
> they work with much lower internal capacitances, and rarely have to
> drive external pins with high frequency analog signals.
In fact, in i.c. design in general you can use lower currents, even with
bipolar. A minimum size bipolar also has low capacitances. At the lowest
of currents, bipolar is actually faster than cmos. To avoid subthreshold
operation at low currents in cmos, you need to use a long gate length.
This results is a high WL capacitance product, and dreadfull ft. Bipolar
can be the minimum size at any current below its useful maximum
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.