What's the use of an OTA?

[Douglas Beeson]

Hi all,

I am embarking on a project to "recreate" the sound of a 1990s Drumfire sound board. Somebody posted a plan on Yahoo. I noticed a couple of parts -- op amps with leading circles -- that Google displayed as OTA, operation transconductance amplifiers. I had never heard of that. The model in the circuit, the CA3080, is unobtanium but I found on Digikey a couple (very few) of replacement OTA parts that resembled the CA3080.

Was this OTA part a fad? What is its real-world use today?

thanks,

--
Douglas Beeson <c.difficile@gmail.com>

 

[Phil Allison]

Doug Beeson wrote:

I am embarking on a project to "recreate" the sound of a 1990s Drumfire sound board. Somebody posted a plan on Yahoo. I noticed a couple of parts -- op amps with leading circles -- that Google displayed as OTA, operation transconductance amplifiers. I had never heard of that. The model in the circuit, the CA3080, is unobtanium but I found on Digikey a couple (very few) of replacement OTA parts that resembled the CA3080.

Was this OTA part a fad? What is its real-world use today?

** The LM13700 dual OTA is still made:

https://en.wikipedia.org/wiki/LM13700

I've seen it used as a VCA in compressor /limiters and a few other voltage controlled jobs.

I still have a small quantity of RCA CA3280s ( improved dual 3080s) which are similar but also obsolete now.


..... Phil

 

[whit3rd]

On Sunday, March 20, 2016 at 6:32:56 PM UTC-7, Doug Beeson wrote:

I am embarking on a project to "recreate" the sound of a 1990s Drumfire sound board. Somebody posted a plan on Yahoo. I noticed a couple of parts -- op amps with leading circles -- that Google displayed as OTA, operation transconductance amplifiers. I had never heard of that. The model in the circuit, the CA3080, is unobtanium but I found on Digikey a couple (very few) of replacement OTA parts that resembled the CA3080.

Was this OTA part a fad? What is its real-world use today?

The OTA is an important amplifier type. CA3080 was retired due to technology changes,
but LM13700, NE5517, are similar; OPA861, MAX435 are upgrades.

Hans Camenzind's book has a chapter (Ch. 10) on these; you can download it from
<http://www.designinganalogchips.com>

Big benefits:
program current sets the gain
....and the power dissipation
near rail-to-rail output range
you can wire-add the outputs
differential inputs
output is a controlled current source/sink with high compliance

 

[Phil Hobbs]

Big benefits:
program current sets the gain
...and the power dissipation
near rail-to-rail output range
you can wire-add the outputs
differential inputs
output is a controlled current source/sink with high compliance

Big problems:
Current sets the bandwidth (from glacial to merely very slow)
Horrible nonlinearity unless input diodes are used (the Gilbert configuration)
Low and nonlinear input impedance when input diodes are used

OTAs in theory can be pretty useful, but the ones still made are either slow as molasses (LM13700) or not true OTAs (OPA860). About 25 years ago, you could get VT713s (from VTC) which were a good 100 times faster than the LM13700. I used to use those sometimes. I've considered designing in the LM13600/LM13700 several times, but it never seems to make the cut. The OPA860 is more of an improved BJT than an OTA, despite the title on the datasheet.

Also sometimes important: the current path goes through cascaded current mirrors without emitter degeneration, so the output current has several times full shot noise.

Cheers

Phil Hobbs

 

[John Larkin]

On Sun, 20 Mar 2016 21:32:53 -0400, Douglas Beeson
<c.difficile@gmail.com> wrote:

Hi all,

I am embarking on a project to "recreate" the sound of a 1990s Drumfire sound board. Somebody posted a plan on Yahoo. I noticed a couple of parts -- op amps with leading circles -- that Google displayed as OTA, operation transconductance amplifiers. I had never heard of that. The model in the circuit, the CA3080, is unobtanium but I found on Digikey a couple (very few) of replacement OTA parts that resembled the CA3080.

Was this OTA part a fad? What is its real-world use today?

thanks,

They were a minor fad for a while. They weren't used much and are
rarely used now.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Bob Masta]

On Sun, 20 Mar 2016 21:32:53 -0400, Douglas Beeson
<c.difficile@gmail.com> wrote:

Hi all,

I am embarking on a project to "recreate" the sound of a 1990s Drumfire sound board. Somebody posted a plan on Yahoo. I noticed a couple of parts -- op amps with leading circles -- that Google displayed as OTA, operation transconductance amplifiers. I had never heard of that. The model in the circuit, the CA3080, is unobtanium but I found on Digikey a couple (very few) of replacement OTA parts that resembled the CA3080.

Was this OTA part a fad? What is its real-world use today?

As Phil notes, it was used in a lot of voltage-controlled
circuits. In particular, a drum synth uses a
rapidly-changing pitch to give that distinctive "drum-like"
sound. OTAs would be used for the frequency sweep with a
VCO, and also as a voltage-controlled filter (VCF) acting on
a white noise source for snare sounds. In addition, the
overall amplitude envelope of the sound would use an OTA as
a voltage-controlled amplifier (VCA). All of these would be
controlled by envelope generators and/or the strike
detector.

Since we're talking about the creation of sound and not
reproduction, some of the limitations of OTAs for other
audio applications don't really apply here.

If you don't care about creating an exact clone of the
Drumfire, note that (IMHO, anyway) you can get pretty good
drum sounds from pretty cheesy circuits. (PAIA used biased
diodes for voltage controls in a lot of their stuff... can't
get much cheesier than that, but they were really popular.)

Best regards,


Bob Masta

DAQARTA v9.00
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, Pitch Track, Pitch-to-MIDI
FREE 8-channel Signal Generator, DaqMusiq generator
Science with your sound card!

 

[whit3rd]

On Monday, March 21, 2016 at 5:38:05 AM UTC-7, Phil Hobbs wrote:

Big problems:
Current sets the bandwidth (from glacial to merely very slow)
Horrible nonlinearity unless input diodes are used (the Gilbert configuration)
Low and nonlinear input impedance when input diodes are used

and one other
Temperature-dependent gain

The 'nonlinearity' is large-signal distortion; that's avoidable.

Hanbury-Brown's correlator used the same input circuitry, potentially had the same nonlinearity.

 

[Phil Hobbs]

On 03/23/2016 03:51 PM, whit3rd wrote:

On Monday, March 21, 2016 at 5:38:05 AM UTC-7, Phil Hobbs wrote:

Big problems: Current sets the bandwidth (from glacial to merely
very slow) Horrible nonlinearity unless input diodes are used (the
Gilbert configuration) Low and nonlinear input impedance when input
diodes are used

and one other Temperature-dependent gain

That too. Didn't want to appear too negative all at once.

The 'nonlinearity' is large-signal distortion; that's avoidable.

Getting anywhere near the maximum slew rate of those very very slow
parts requires running large input error voltages, though.

Hanbury-Brown's correlator used the same input circuitry, potentially
had the same nonlinearity.

But his got rid of the input offset by commutating the output of the
multiplier, and his incoming SNR was so low that the extra noise caused
by running the transistors at a very low level wasn't a problem.

I really like OTAs in principle. For instance, back in the late '80s I
built a PLL with a 100:1 frequency range, whose damping was constant and
whose loop bandwidth was proportional to the input frequency. It used a
charge pump to supply the bias current to OTA active filters, and a VCO
with an exponential V->F characteristic. (I was going to build a 1 Hz
- 50-MHz analogue lock-in.)

If you could still get VT713s, I'd probably use some, but not LM13700s
or OPA860s.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net