[quote author="bcarso"]Is it an opamp? Well, in the original meaning of the term, as amplifiers with a lot of inverting gain and a single input, Yes---differential inputs were not the norm for use in analog computers. See some of the very early Philbrick stuff for example, including their tube opamps.
Is it an Operational Transconductance Amplifier? Yes, within a sort of broadminded intent: Voltage in, Current out. The output impedance, sans load and feedback network, is relatively high for the amount of current coursing through or available at least. A voltage-to-voltage transfer function is obtained when you terminate with an impedance.
But when RCA started to tout the OTA---in the form of their CA3080 for example---which has differential inputs as well, they also emphasized that it could have its transconductance set externally via a programming current pin. Thus this feature became associated with many folks' notion of a transconductance amplifier. And you could do many more nonlinear processing tricks, like variable gain, given this feature. Clearly there is no such control pin on the Cello module.
In that sense, and in connection with NewYorkDave's simple opamp thread, a single common-emitter transistor is practically a transconductance amplifier. But it has a highly nonlinear and relatively low input impedance, which was one of the significant departures to deal with for engineers who had learned to use vacuum tubes. The single Q can also be thought of as a current amplifier, Ib in and Ic out.
On cost-strapped systems with modest performance needs I've used a single transistor as the only gain element in a given circuit section---in one case I got a full-wave rectification function with two outputs in opposed polarity for use with a compressor. I count that circuit as one of the most satisfying things I have ever done. Not only did it work well, but it was wonderfully obscure-looking on the schematic.[/quote]
Agreed I ASSumed OTA name was just a coincidence or marketing distortion since most OTAs I've ever encountered were configured as multipliers (i.e. input diff voltage x bias current) to effect sundry voltage or current controlled circuits.
Further, common emitter output stages are notorious for being difficult to stabilize in power applications thus their near total absence from power amp designs despite their benefit of pulling within a saturation voltage of the rails. Years ago I made a crude low power amp (for my sister) using common emitter outputs driven by an opamp’s PS draw (design approach from some electronics rag). It was far from Hi-fi due to needing way too much compensation, but marginally better than the circa ‘60s consumer amp it replaced in the former's gutted chassis.
If we want to reduce this simplest opamp to absurdity a basic CMOS gate can act like an inverting amplifier, but is little more than a bench curiosity.
While there is an elegance to simplicity, the complexity of modern integrated circuits is not without purpose or benefit. IMO it's a little like comparing an old model T to a modern automobile. Sure you could increase the displacement of a model T to make as fast as a modern car, but without the refinements that increase the complexity we have become accustomed to that new improved T probably wouldn't make it around the first turn, and don't even try to hand crank 400 ci. (or was that a model A? I'm not that old).
JR
PS: BTW a single common emitter transistor may be more like half a Norton Amp (ala LM3900). Add a two transistor current mirror and you could have your differential input. I like to think of Norton amps as the back 2/3rds of a proper opamp.
One could hang a nice pnp or p-channel differential in front of a Norton amp to make a hybrid opamp. The LM3900 didn't deserve that much effort but I recall a higher performance LM359 that appeared worthy. No obvious choices for that task, and you couldn't call it discrete if that's important..