Wave Wraper revisited #1

[Grant Richter]

The LM3900 is one of the most versatile chips ever made.

The design is pretty much targeted towards single supply
applications but can be used bi-polar. The output does not like
to go below 600 mv to ground, so for near ground work you add
a diode in series with the output.

The design works like an op-amp but with current flow into each
terminal. A couple of cute things you can do with that.

1. Neurons: Since each input is a current summing node, and the
output reflects which input has more current, you can build simple slow
"neurons" which are weighted summing networks with "inhibitory"
inputs. Marvin Minsky had an interesting book on artificial intelligence
with a chapter on (I think) "McCulloch-Pitts" neurons. In this chapter
they showed how complex logic can be realized on a very few
of these "neurons". For example, if two 100K resistors are brought
to the positive input (excitatory inputs), and a 51K resistor is brought
to the negative input (inhibitory input).
(Lets assume we are powering the chip off of +10 volts)
If 5 volts is applied to the inhibitory input, then the current from
either 100K resistor is enough to turn the output "on" and the logic
function is an OR gate. If 10 volts is applied to the inhibitory
input, then the current from both 100K resistors is required
to turn the output "on" and the logic function is an AND gate.
They had a table showing how to realize all the common
logic functions and even designs where the logic function could be
voltage controlled. Pretty cool, perfect for a LM3900.

2. Current Controlled Integrator. If you build and integrator around
a LM3900, the integrator will ramp positive for current into the
positive terminal, and negative for current into the negative terminal,
and stay at the last voltage level for no current into either terminal.
Thats pretty much the heart of a linear envelope generator.
CMOS switches can be used to switch the currents
into the various terminals for the ADSR functions and a
voltage comparator sets the peak value.
You can also make a sample and hold from the circuit (sort of).
The comparator controls a CMOS switch, the switch is turned on
and routes a strong current into to positive input until the voltages
are equal, then the switch is turned off and the integrator "holds"
that voltage until reset by a current into the negative terminal.
So you can build an envelope generator that will "fake" a crude
sample and hold (good enough for Jazz, anyway).

----------
> From: Don Tillman <don at till.com>
> To: tiar at canamcomputers.com
> Cc: harrybissell at prodigy.net; synth-diy at mailhost.bpa.nl
> Subject: Re: Wave Wraper revisted #1
> Date: Friday, June 25, 1999 9:53 AM
> 
>    Reply-To: "Thierry Rochebois" <tiar at canamcomputers.com>
>    From: "Thierry Rochebois" <tiar at canamcomputers.com>
> 
>    >This circuit produces waveforms that llok similar to some of those
>    >I saw in the discussion of the Serge wave multiplier. But I still
>    >don't understand the LM3900....
> 
>    I do not too. It was also used by ARP for their filters. It seems to
be
>    interesting for low noise designs...
>    Is there anybody knowing how to use these "Norton amplifiers?".
> 
> National's AN72 application note tells all.
> 
> http://www.national.com/an/AN/AN-72.pdf
> 
>   -- Don