The Photo to the left is of the
LM13600/13700 STEREO Transconductance
Amplifier. { DATASHEET.PDF }
The LM13600 has designed by Bill Gross
and myself in less than 5 minutes.
At the time the Consumer Linear IC
design group was training a new mask
designer. We needed something with
a few transistors for her to learn
how to arrange them in a optimum
circuit arrangement while using
minimum silicon area.
At the time, electronic Organs were being
done using analog circuitry. I had just
made a trip to one such company and
of course they gave me a wish list of what
kind of circuits they would like to see.
The RCA 3080 has just come out and it
almost gave the Analog Organ folks
everything they wanted. They needed
something for analog variable gain.
The application was to shape the
attack and decay of various waveforms.
But the 3080 was a true Operational Transconductance Amplifier in that it had a voltage
input and a current source output. For most applications, an external buffer was needed.
At the time we were considering second sourcing the 3080 anyway.
Also at this very same time, the 16pin plastic dip package had just been developed.
So the development spec for the LM13600 was that it needed a schematic to train someone
in IC layout. It needed to have 16 pins. And we were going to layout the 3080 anyway.
The schematic part was easy. Just used the 3080 exactly. There were 16 pins, so we could
just mirror the layout to do a stereo. That left 3 extra pins per channel. The simplest
buffer was a darlington which needed two pins per channel. There were complaints about
high levels in input signal generated too much distortion in the 3080. That could be
addressed by connecting the left over pin to predistortion diodes.
After layout,design management decided to just go ahead and make it a product.
First silicon work even with very unusual beta. This last fact explains why both
the LM13600 and LM13700 came out together. Control of beta was not tight at the time
and all transistors needed to be biased up with currents well above the leakage
current levels in the process. One way to do that was to use area scaling to force
the beta of the output transistor in the darlington to 50. That what was used to
the LM13700.The other method involve feeding some of the control current back to
the darlington. At the time, this enabled the LM13600 to have usable dynamic
range well above 96dB.
The development of the datasheet borrowed heavily from the quad Opamp datasheets.
Now that you were giving the customer something they never had before, what new things
could now be done? All the application circuits were developed and debugged in
the lab. I was thinking about publishing an article in Popular Electronics. But I did
not need to. About ten years later Popular Electronics changed their name to
Radio Electronics, and copied my data sheets into two of their articles almost
verbatim.
Don Sauer... DSauerSanJose@Aol.com