At the time when the LM13600/LM13700 datasheet was done, management
was beginning to feel that the number of applications was getting out of hand.
The LM13600/LM13700 data sheet was attempting to follow the LM3900 data
sheet in that it now allowed customers to do things which up until then were
not convenient. This application supplement attempts to add a few more examples
as to how this Operational Transconductive Amplifier can be used.


This IC was some what done at the request of an Analog Organ manufacturer.
Actually it was done as training lay out for a new mask designer. Some how it
managed to get turned into an actual IC, which was very unusual. Marketing was
not keen on supporting the creation of a new IC for a yet to be created market at
that time.

Of course the first application was as a Voltage Controlled Amplifier.  The Organ
people needed to be be able to adjust the attack and decay of a tone, and in
spice today that can be simulated mathematically.

The whole Operational Transconductance Amplifier can be defined in one line..

B_OTA1 OUT  0     I =   -1*v(VIABC)*tanh((v(INP)-v(INN))/.052)

                      0->1mA->0        RL = 100
                 VIABC  ^        _/\  /\  /\__
                       /_\      |   \/  \/   _|_
             ___   |\  _| _     |            ///
  VIN  ____ |INP|__|+\/ \/ \   _|_    ___  |\   ___
     _|_    |___|  | /  /\  \_|OUT|__|INB|_| \_|BUF|
    /_  \    ___   | \  \/  / |___|  |___| | / |___|
   // \  \  |INN|__|-/\_/\_/               |/
   \   \//  |___|  |/                        B_BUF1
    \___/    _|_     B_OTA1
     _|_     ///


Where a PWL source (VIABC) is set to track  IABC  over time...

V_Iabc  VIABC   0     PWL ( 0m   0m  3m  1m   10m  0m )

There is an extra feature in that the definition of the buffer need not include offset.

B_BUF1    BUF    0    V =    v(OUT)                  or  ...
B_BUF1    BUF    0    V =    v(OUT-1.2V

This can make the spice netlist of attack and decay simulations extremely simple.

The data sheet lists a voltage control lowpass filter as a typical application because of
a home Hi Fi project. The idea was to adjust the bandwidth of the audio channel
to always match the bandwidth of the music. A noticeable reduction in noise for
cassette tapes was the result. The Home project work so well that Delco decided
to use it instead of Dolby. More information on this can be found at Wikipedia.


Another application for this circuit could be as a Voltage Follower which has a
Voltage Controllable Time Constant. Perhaps one might want an envelope with
a slow or fast attack and a fast or slow decay.


In some cases the Four-Quadrant spice simulation might make the data sheet schematic
a little easier to understand.

For the Voltage Controlled Resistor application, being able to simulate using perfect
buffers instead of darlingtons has its advantages. This is especially true when it comes
to building
Voltage Controlled Capacitors and Voltage Controlled Inductors.

The Floating Voltage Controlled Resistor functions just fine with the darlington
1.2V offsets. The simulation of this circuit shows why. To see the floating
resistor in action, the example needs to be put into a resistor network.

   VIN      R1                                           R2
     _/\  /\  /\_ OUT0                         OUT1 __/\  /\  /\__
    |   \/  \/   |         IABC= 1mA->0            |    \/  \/   _|_
   _|_           |                                 |             \\\
  /_  \          |        INP0=INN1    INP1=INN0   |
 // \  \     /|  |      /|________    _____|\      |  |\  VOUT1
 \   \//  __/ |__|_/\/\/ |        |  |     | \/\/\_|__| \__
  \___/  |  \ |    \/\/\ |_______/|\_|   __| /\/\/    | /  |
   _|_   |   \|         \| |      |_____|  |/         |/   |
   ///   |B_BUF0  B_OTA0   |            | B_OTA1           |
         |           R3    |   R4       |       R5         |
         |_____/\  /\  /\__|_/\  /\  /\_|____/\  /\  /\____|
       VOUT0     \/  \/  1NN0  \/  \/  INN1    \/  \/

With IABC going from 1mA to zero, the floating resistor appears to be going
from 10K Ohms to Open. The wave forms below shows that.


But any type of Voltage Controlled Impedance can be made using OTAs.
A Voltage Controlled Negative Resistor can cancel out the current loading
of a Load Resistor. And a
Voltage Controlled Negative Capacitor can cancel
out stray capacitance to greatly improve speed.

Voltage Controlled Negative Resistor

The State Variable Filters are by far the most beautiful of all the filters.
Different filters can be made simply by changing voltage feedback. For
instance a Bessel filter has a set of feedbacks such that a low pass filter
signal is not shaped distorted. If maximally flat gain is required, then the
feedback can be adjusted to form a Butterworth filter.

Regardless of type of filter, the frequency response is independently controllable
over up to maybe 5 orders of magnitude for the LM13600 (not LM13700) and
may get as high a 10-12 orders of magnitude in
frequency if it were built in BiCMOS

The circuit above can come in very handy in a lab which tends to have
a shortage of equipment. The LM13600 output stage allows this circuit
to have almost a 1Hz to 1Mhz frequency range. This circuit can fill in
for a lab function generator in most applications. Of course to have a true
function generator, there should also be a sine wave output.

The wide dynamic  range of bipolar transistor maybe very under appreciated.
For a LM13600, almost 6 orders of magnitude in current range mean having
almost 6 orders of magnitude in control over timing. Given that transistors in
BiCMOS are much faster and perhaps have 10 to 12 orders of magnitude,
could there be some timing applications which could use this performance?
There are a whole class of nonlinear relationships that one encounters applying
electronics to the real world. The Schmitt Trigger application was an attempt to
investigate this area. The following are some of these nonlinear relationships listed
under their common system names.


The limited Saturation is of course the natural nonlinearity of OTAs.
This nonlinearity together with other types of nonlinearity may allow
good modeling of a saturating magnetic core.

Preloaded means something is under stress without an external load.

This one is called Spring Hardening.


This is called Dead Zone.

The Toggle which is also the schmitt trigger application has memory
of its past input signal built in. Combining this feature with some of the other
nonlinearities may allow close modeling of magnetic components.