Page 51 - The Art and Science of Analog Circuit Design

Analog Design Productivity and the Challenge of Creating Future Generations of Analog Engineers

1. Simulator could be very useful only for design engineers who
really understand how the circuit works.
2. It is very difficult to simulate such a circuit as having more than
two widely spread time constants, for instance PLL, AM/FM de-
tector, crystal oscillator.
3. It is also difficult to derive device parameters, and installed model
does not reflect many parasitic elements such as substrate current,
parasitic transistors, thermal coupling etc. Some of them can be
avoided by adding some appropriate circuits, however this is not
so easy for the average engineers,
4. It cannot cope with a variation of circuit topology. We need to
rewrite net lists and restart program whenever we change a circuit
topology.

These show that circuit simulators are indeed user dependent program
therefore it is very important to teach beginners how to use it.
Although the author mentioned about the shadow of circuit simulator,
it is still very powerful tool. Dr. Minoru Nagata, Director of Central
Research Laboratory Hitachi Ltd., showed the following evidence as an
example.
In the past 2 years, analog LSI has been developing, number of tran-
sistors per chip increases twice while available time for design de-
creases two thirds. But design engineers have 20% decreased in their
fail rate at the first cut. Dr. Nagata also said that layout productivity in-
creased 10 times and design correction decreased one tenth during this
period. He stressed that these result could not be got without circuit
simulators.
The author pointed out how Japanese engineers thinking about analog
circuit design productivity and circuit simulator. However analog circuit
design still strongly depends on the designer of talent. Comparing the
design of logic system to analog circuit, we would find that an one of
apparent difference between them is that analog circuits has usually more
than one complex function while one logic circuit element has only one
function. Most digital system designers think their design in logic ele-
ment or logic gate level, while analog designs are carried out in circuit
element level such as transistors, resistor etc. A resistor in collector circuit
works as a voltage dropper and same time it governs gain and frequency
characteristics of that circuit. Analog circuits design engineer should al-
ways pay his attention to trade-off between these complex Functions.
Professional analog circuit designer is a man who knows these trade-off
technology and who success to realize compact and high performance
circuits.
As demands for analog circuit rising, we should solve this design pro-
ductivity problem. How could we make beginner or computer designed
analog circuits? Professor Nobuo Fujii at Tokyo Institute of Technologies
and other members in the Technical Committee for Analog Circuit Design

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