Applications of an OTA Current Controlled Amplifier

A Practical OTA Current Controlled Amplifier

4.3K = Signal Gnd
1/2 LM13700
240R = 1/2 Power Supply
1K - 4.5Vdc
240R = Power supply,
+ 9Vdc
- 1uF Output
Input
0.1uF
5mV - 500mV + 10K
Dual Opamp 22K 10K 2.2M
2.2M 2.2M
Ictl

The schematic above shows a practical Current Controlled Amplifier (CCA) for musical effects use. It’s made from half a
dual opamp of garden variety as an input buffer, and half of an LM13700 or NJM13700 dual Operational Transconductance
Amplifier (OTA). The circuit has been designed to fit within the normal constraints of musical effects - single 9V battery
power supply, and an expected max input of something like 100mV to 500mV.
The signal is buffered by the opamp front end and then gain controlled by the OTA. The OTA circuit as shown has a gain of
about +3db (2 times) at a control current (Ictl) of 1ma down to about -80db from that, or 1/10,000th of the input signal,
almost full muting.
The 1K trimmer pot is important to the operation of the circuit. It balances the differential input so that changes in Ictl do not
feed through into the output signal as badly as they otherwise might. This is important whenever the control current
changes quickly and the changes might be heard as a click or thump in the output if the balance is not done.
CCA
For the purposes of the rest of this article, I will show that whole circuit above as a single block, illustrated like:

+1 is the same as this: - a non-inverting opamp circuit

+

is the same as this: - a non-inverting opamp circuit

-1
+

+ is the same as this:

+
-
- a non-inverting linear mixer circuit

The biasing details of the circuits above do matter, but the details will be clear from the context of the circuit diagram, and
will be included in any completed schematics.
 Application 1: A Volume Pedal
We can manually set the control current Ictl in the CCA. The simple way
to do this is with a foot operated rocker salvaged from a wah pedal, a
buffering opamp to provide the current needed to drive the CCA, and a
Input CCA Output series resistor to convert the control *voltage* at the opamp output to a
control *current* as expected by the CCA. The LM13700 current input
looks like two diodes in series to ground from the outside, so it will never
get more than about 1.4V above ground. That means that if our opamp
can provide 0-9V of control voltage, the current that will be pushed into
7.5K the CCA will be a maximum of( 9V-1.4V)/7.5K, or about 1ma.
100K +1
The LM13700 can actually live with Ictl up to 2ma, but many other OTAs

Application 2: A Tremolo Pedal Of course, once we’ve gone to the trouble of making the CCA have a
clean voltage controlled input of 0-9V corresponding to gains of
Input CCA Output essentially zero up to a gain of about two, we can go ahead and supply
any control voltage we like, as in the output from an LFO. If the control
voltage is zero to +9V, the gain is whaled from full on to full off. If we use
a square wave control, the sound is chopped instantly on and off.
7.5K If we bias the +1 buffer feeding the CCA at half the power supply voltage,
+1 4.5V and feed no LFO voltage in, the gain of the CCA is about unity, no
gain or loss. We can then AC couple in an LFO signal from a “depth” pot
LFO signal, 0 to 9V which feeds in a variable amount of the LFO, for a full featured tremolo
circuit.

4.3K

1/2 LM13700
240R
1K -
240R
+
- 1uF Output
Input
0.1uF
5mV - 500mV + 10K
Dual Opamp 22K 10K 2.2M
2.2M 2.2M
Ictl

4.3K

1/2 LM13700
240R
1K -
240R
+
- 1uF Output
Input
0.1uF
5mV - 500mV + 10K
Dual Opamp 22K 10K 2.2M
2.2M 2.2M
Ictl

As a practical matter, we will often use both halves of the two chips involved in one CCA circuit to make a dual CCA,
as shown above. The inputs, outputs and Ictl inputs can be interconnected in many interesting and useful ways.
 Application 3: A Pan Pedal
The two-CCA extension of the manual volume pedal is the pan pedal.
Input CCA Output Taking the same circuit as Application 1 and adding to it one more CCA
and a DC coupled inverting amplifier to invert the control signal, we get a
pan setup. With the rocker pedal centered (that is, the input control
voltage at 4.5V) the gain of both CCAs is about unity. Rocking the pedal
away from this center point rainses the gain on the higher channel to
Input CCA Output about 2 and lowers the gain on the lower channel to almost off.

Purists will note that we’re using a linear control voltage, not an
7.5K exponential one. There is a simple modification to the drive circuit for the
100K +1
CCA that gives exponential response, but in the case of an effects pan
pedal or crossfade pedal, the linear response is usually more useful.
7.5K
-1

Application 4: A Stereo Trem-Pan Pedal

Input CCA Output Just like with the single channel tremolo pedal, we can hook up an LFO to
the two channel CCA setup and have two sides that alternately fade
between one another depending on the LFO voltage. This is similar to the
action of the commercial Boss PN1 Trem-Pan pedal and the old Ibanez
Flying Pan pedal.
Input CCA Output

7.5K
+1
LFO signal, 0 to 9V
7.5K
-1

Application 4a: Ping Pong Panner

Input CCA Output

This is the stock hookup for the two channel Trem-Pan operation. The
same input is routed to both CCA’s and each CCA’s output goes to a
different place. This lets you pan a single guitar signal to two amps or do
CCA Output ping-pong jumps depending on the control signal you send them.

Application 4B: Fadeover Morpher

But you can also fade two separate inputsto one output if you put a mixer
on the output of the CCA’s. In this case, two different effects or effects
Fx 1 CCA chains are set up after a buffer. The CCA control fades or switches
between them. If the LFO control circuit is a digital signal (that is, full on or
Input
+1 + off) and one of the Fx loops is no effect at all, just the dry signal, this
Output circuit reduces to the Ibanez “bypass” system, where the bypass signal
merely selects the dry or effected signal.
Fx2 CCA
But it’s capable of much more. A slow fadeover LFO signal produces a
condinuous changing mixture of sounds.
 Circuit 2: A ramping control voltage generator

It’s very useful to be able to generate a

470 1/2 LM13700 ramp up or down signal to control CCA’s,
- as we’ve seen. One easy way to do this is
470 with another OTA. The OTA converts a
+ differential voltage at its input to a current
10K 9V at the output.
9V 0V The maximum current out or into the
0V +1 output of the OTA is equal to the Ictl at any
Input Signal Ictl Cr Output Signal moment. If we load the output of the OTA
with a capacitor, and bang the input fully to
one side or the other, the capacitor fills
with voltage at a rate of dV/dt=Ictl/C. By
changing Ictl, we can get any slope we
want from 0 (no Ictl, so C’s voltage never
It turns out that this little module is going changes) to 1ma/C. If C is 1uF and Ictl is
to be useful to us later, so let’s make a 1ma, then the change in capacitor voltage
shorthand symbol for the thing so we with time is 1000V/second, or more
don’t have to keep redrawing it. I call this UDRamp
usefully, 1V/mS, so we go from 0 to 9V or
an Up/Down Ramp (UDR) circuit. Ictl vice-versa in 9ms. If Ictl is 1uA, the ramp

Application 4C: Stompable Fadeover Morpher

Fx 1 CCA

Input
+1 + Output
Fx2 CCA

CMOS
FlipFlop UDRamp
TQ Ictl
-Q

UDRamp
Ramp Ictl
Speed
Application 5: Multichannel Fadeover Morpher
Now we’re getting complicated - here’s a four channel
fadover circuit. The pairs of jacks in the middle of the
mess are normalled insert jacks. The normally closed
switch inside the jack connects the signal between the
jacks when no plug is inserted, so if nothing is plugged
in, all the channels are just direct passthrough. If you
CCA plug an effect or effect chain into the loop, the loop’s
sound is only heard when its control voltage (A, B, C, or
A D) is more than 0. The outputs are all mixed in the output
mixer. If you control each channel manually, you have a
voltage controllable mixer - not all that interesting.

Input CCA
+1
B
+ Output
CCA

C
It gets more fun when you use a bit of logic to make the
thing switch from one channel to the other, and our UDR
CCA circuit to make that switchover happen in a variable time.
D Here’s one way to do the selection. A single CMOS octal
latch can work with a momentary footswitch to select one
and only one of up to eight output signal lines. Better, we
can use the ones we want and ignore the extra ones. So
a single CMOS chip does the selection of one channel of
the Morpher.
Footswitch Logic
Those of you who follow GEO will remember this from
+V the remote footswitching system.
CMOS interface circuits
+V 1/6 CD4584
A
10K +V -LE Vcc
74C373 OCTAL LATCH UDRamp
D1 Q1
Ictl
10K D2 Q2

0.01 D3 Q3
B
D4 Q4 UDRamp
Ditto... D5 Q5
Ictl
D6 Q6

Q7
C
D7
UDRamp
D8
Ditto... -OE Gnd
Ictl
Momentary Unused inputs 1 2 3 4
Stomp pulled to ground D
Switches LED indicators UDRamp
Ictl

Of course, by this time, the use of discrete logic gets a bit complicated, and you could just as easily use a $2.50 PIC
microcontroller to do the selection, and also tuck into it a lot more fancy stuff, like circulating patterns, random
channels, and so on.