Gain calculations

[ian_elvar]

Hi Ian

In the equation you are using the Rc is limited by the parallel equivalent of the VAS stage and the load. Effective output resistances of CE stages may be in the region of a few 100 k. BUt if you have an output transistor and driver transistor with an 8 ohm load, the Rc load will be about 8 x100 x100 = 80k

Remember these figures are all "small signal" equivalents. You can achieve a high Rc by cascoding but you still have the output load...

cheers
John

A good point, well made. Essentially the loading becomes dominant if the CCS has a high output impedance is what you're saying. I can see where you're coming from there. Is that calculation strictly true
for the CFP though? It's easy to see that it would hold true for an EF topology, and assuming both driver and OP transistor have hfe = 100. But with CFP you dont have two base-emitter junctions in series with the load like you do with EF. Time to do some pondering and calculation

 

[ian_elvar]

I played with SPICE briefly, years ago, but recently i started poking around with the basics (ONLY) in Multisim. It's very good. The aggravating things about simulations is that the device I like to use is not in the library even though, say, Multisim has over 20000 models. Also, they are just for spotting our mistakes (or incompetence) circuit designs. The detailed accuracy is not there, IMHO. For an example, 2SD1609 I suspect perform a lot better than 2SC2240 as a gain device, regarding THD. But simulation will not give us that. The models are just not detailed enough, IMHO.

I've managed to get most of what I need by visiting a few manufacturers websites and sending a few emails requesting datasheets. I've always been a bit wary of simulation software, but people do seem to get some good results with it. I guess it's very rigidly a garbage-in-garbage-out thing, and requires some pretty serious thinking about what's going to be happening in a circuit, rather than simply plugging in the schematic and hoping for the best. I do find it a bit daunting, but I'm sure I'll get used to it.