Saturday, October 4, 2014

The OTA, and why it's important

Many (in fact, most) analog synthesizers rely heavily on an integrated circuit called the operational transconductance amplifier, or OTA for short. The OTA is a variation on the common operational amplifier, or op-amp. From an electrical engineer's point of view, the main difference between an op-amp and an OTA is that, while an op-amp outputs a voltage that is proportional to the difference in voltage between its two inputs, the OTA outputs a current that is proportional to the difference in voltage between its two inputs.

This, however, is not what makes an OTA valuable from a synth designer's perspective. Analog synths rely on two behaviors in which the OTA differs from the op-amp:

1. The OTA has a third input, called the amplifier bias current or Iabc for short. (The letter I is the standard electrical engineering symbol for current.) With the conventional op-amp, the gain is determined by an external feedback resistor. The OTA's gain is set by the amount of current that is allowed to flow into the Iabc input.

2. The OTA's input impedence is proportional to the current flowed into the Iabc input.

What does this mean, in practical terms? Consider the first: It's a simple matter to convert a voltage to a current, or vice versa. So if you take an external voltage input, convert it to a current, and feed that to the Iabc input to control the OTA's gain -- well, that's what a VCA is. Another way of looking at it is that the OTA is an analog multiplier; the difference between the two input signals is multiplied by the Iabc input. That opens up all kinds of possibilities, such as ring modulators and wave shaping circuits.

As for the second characteristic, if a voltage is converted to make the Iabc input, then the OTA becomes, in effect, a voltage-controlled resistor. As it happens, a voltage-controlled resistor is a very handy thing to have when designing VCF circuits.

Now the problem: Other than analog synth circuits, there are few uses for OTAs these days. There was a time when OTAs were widely used in radio and microwave systems, but digital signal processing has taken over in these applications. Consequently, OTA integrated circuits are disappearing from the market. A survey of the casualties:

  • Intersil CA3080: Originally designed by RCA and introduced in 1969. A good, basic OTA cicruit widely used in both commericial and DIY designs over the years. Because it was produced in huge numbers, there's some stock still around and it's still possible to find them. But of course, that stock is a finite number and it will run out eventually. (Rochester Electronics still has some, although they aren't cheap.) 

  • Intersil CA3280: Long regarded as the Cadillac of OTA designs (and often priced accordingly), this was also widely used particularly in top-end VCA designs. Blogger Don Tillman made a heroic effort to rescue the 3280, but was unsuccessful. Because of the higher price, it was probably not produced in as large a quantity as the 3080, and existing stocks have nearly dried up.

  • Intersil CA3060 and 3094: These were variants of the 3080. I believe the 3094 was a dual OTA with buffered outputs, and the 3060 was a triple OTA. They are both long out of production and there are no sources that I know of.

  • National LM13600: I think this part was used mainly in DIY designs. It can still be found, but except for repair of existing circuits, it isn't as widely sought out because it's not the easiest part to incorporate in a circuit, and it's specs aren't as good. The still-in-production LM13700 is better in both respects.  (Internet rumor has it that the original design was undertaken by an intern, as an exercise.) 

  • Rohm BA662: Heavily used in '70s and early '80s Roland gear. Long out of production. There is no source that I know of.

  • Rohm BA6110: Widely promoted as an "almost pin compatible" substitute for the BA662 after the latter was discontinued. However, the 6110 is also now discontinued, and existing stocks have nearly dried up.

There are only two OTA ICs in production at this point, the above-mentioned National LM13700 and the On Semiconductor NE5517. The LM13700 is regarded as a decent design, but not as well spec'ed as the 3280 and not quite as versatile as the 3080. But by default, it is the part of choice for new designs. The NE5517 is a bit of a story; it was formerly manufactured by Phillips, but the fab in France that manufactured it burned down in 2003. On Semiconductor then purchased rights to the design and picked up manufacturing. I've seen some Web references that say that the On Semi parts aren't spec'ed as tightly as the former Phillips parts.

So where does all this leave us? We have no indication as to what National's future plans are, regarding how long the LM13700 will be in production. Probably for a while, since there is little competition now, but one never knows. There are people in the synth design world who know how to design OTA circuits from discrete components, and to name one example, the Synth Tech MOTM-440 uses just such a circuit. But it's not as DIY-friendly an approach; there are a lot of subtleties to designing a good OTA circuit, and it's more expensive and more trouble then using an IC. But as far as technological progress goes, it's definitely a step backwards.

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