My first experience ever with an operational amplifier was with an LM3900 from a Philips experiments kit and these kits date from a long way back in time, when I was a young boy in primary school.
I rarely see designs with operational transconductance amplifiers. Are these type of devices (not necessarily the exact type mentioned above) still being produced, newly designed? Is it worth the effort to design a new circuit with them or are they soon to be remembered in melancholy?
Not quite dead, it seems. Though Digikey lists only 49 of them, out of a total of 30 300 operational amplifiers, that's not even 1 %. Several datasheets are at least 5 years old, but the TI OPA861 datasheet had a recent update a year ago.
The good old LM13600/13700 also seems to be still alive: still available from two different manufacturers: NJR (datasheet 2006), and TI (née National, datasheet 2004).
The OPA861 lists as typical applications:
• Video/Broadcast Equipment
• Communications Equipment
• High-Speed Data Acquisition
• Wideband LED Drivers
• Control Loop Amplifiers
• Wideband Active Filters
• Line Drivers
They still exist, and make sense for a number of applications, like the ones listed. The reason because they are not very common is simple: their principle of operation (taking a voltage and giving a current) is not obvious to the most, and many people don't even know of their existance. Moreover, it's part of the world of analog electronics, which will always be present, but it's being confined to a smaller range of applications than in the '70s and before.
So it makes sense to use them if you need them.
I remember of a question where the ratio of two voltages were used to set the gain of an amplifier: in that case, an OTA could be used to give the current that sets the gain of another amplifier.
As stevenvh mentions, the LM13700 is still in production and has a huge number of possible analog applications, such as:
And lots more. It's probably not going anywhere anytime soon. The only downside is that it's kind of sluggish (2 MHz GBP IIRC.)
The LT1228 is much faster (100MHz) than the LM13700, is a single OTA instead of dual and uses another op-amp instead of a darlington as buffer (already connected to the output of the OTA) for voltage output.
http://www.linear.com/product/LT1228
I've ordered some samples to try old CA2080 designs with this much faster chip.
