Doing opamp educational stuff - suggestions wanted

[DrGonzoDK]

So, I've been contributing some time to a project that teaches high school-age (15-18) kids about electronics - mostly analog with a bit of boolean logic thrown in for good measure.

And, one thing that is always a focus of interest is amplification, so I've been working on a "how does an opamp work" thing, and for this thing, I'd really like to sort of build up something basic that works and elaborate that a bit.

Now, I've done some basic material on a very basic, three-transistor opamp with about ~15x gain top closed-loop gain, with a reasonably nice bandwidth (at least 1MHz G*BW). I've attached a simple photo (I've translated the annotations to English so you guys can actually understand it) of the first iteration design they're gonna construct...

So, to my question, what would be a good next step? Introduce current sources, current mirrors? Or would adding a buffer stage to the output be a better thing to explain?

EDIT: Oh, and before I get a lot of "why the f**k is there a germanium transistor in there", we happened to get our hands across a large lot of PNP Ge's for almost no money, so that's why. 

[danadak]

The OpAmp with buffer / output driving stage much more useful.

But so are current sources and mirrors, although most of the time
they come with a part internally, one does not need to design them
external.


Regards, Dana.

[Ian.M]

Maybe take a look at the GAP/R P65 OPAMP schematic.  Its a very early solid state silicon six transistor OPAMP module.  See Op Amp History - Analog Devices page 33 (H.31).  Also see http://www.philbrickarchive.org/pp65a.htm for specs and how to trim it (as the PP65A potted module used external trim components).

[DrGonzoDK]

Maybe take a look at the GAP/R P65 OPAMP schematic.  Its a very early solid state silicon six transistor OPAMP module.  See Op Amp History - Analog Devices page 33 (H.31).  Also see http://www.philbrickarchive.org/pp65a.htm for specs and how to trim it (as the PP65A potted module used external trim components).

Yeah, I've read that before, it's an excellent piece of writing! And thanks for reminding me of it. Good idea! Obviously one thing that has to be discussed from this is, well, supply voltage. These discrete opamp designs are really for one set of supply voltages - you have to design your desired supply voltage into the value of the resistors to the rails and from the output transistor.

I'll check out the fabled Section H document again!!