... hmm yeah nice analysis but we sort of knew that in advance did we..
Sooo, does that mean we'll be using Sziklai buffer now? With a N-Channel JFET first and a PNP second? Or just a regular old BJT-BJT Sziklai? The combines hfe should be large enough for decently high input impedance. If not, there is always bootstraping, Cornish buffer style.
The decision the make the first transistor JFET or BJT changes things a bit. At the end of the day making the first stage a JFET is most likely to give the best noise performance.
When the first transistor is a BJT it is best to operate it with a low current to keep the noise down. So that means the output impedance goes up and it is helped by having a second transistor. So with the constraint of operating Q1 at low current the Sziklai case looks pretty good.
When the first transistor is a JFET it is best to operate the JFET at a high-ish current to keep the noise down. When you do that the output impedance of the JFET is relatively low but not great. If you have a JFET with a current source load, even without the second transistor like the Waza, the performance is actually pretty good ie. low buffer loss and also low distortion. I compared the simple JFET+CS case to the JFET+BJT feedback amp (ie. the JFET version of the Sziklai) and the JFET+BJT has more distortion but lower loss than JFET+CS.
It's good to keep in mind some of the differences are fairly academic. There's also some real differences. We would need to watch out for low impedance loads on the simple JFET + CS. The JFET+BJT Feedback amp is probably the most likely to have oscillation issues and perhaps will need good supply bypassing.
The thing about the JFET+BJT feedback amp is you have a second variable to pin down: the collector/source resistor at the output. Since we are operating the JFET at high-ish current and the JFET source current flow from thought the output resistor, if the output resistor is chosen too high the BJT will operate at low current and essentially the circuit will fall back to a JFET + source resistor buffer. In this case the BJT is kind of benign. From what I can see there is a value of the output resistor which gives the least buffer loss. Then decreasing that resistor further increases the buffer loss again but the distortion decreases and the supply current increases - so here we have the option to trade-off performance specs.
The fact we need to operate the transistor at a minimum current and the JFET is operating at a high-ish current means this configuration is likely to draw a little more current.
Notice in the JFET+BJT feedback amp the BE resistor sets the JFET operating current. We don't need a current source to set the JFET current. It only sets the current when the BJT current is not so low as to make the BJT benign.
I've been messing around with Darlington and Sziklai buffers for a while, purely out of interest and for educational purposes. But I don't really see the value in them above that. Surely any op-amp would outperform the lot, and with far fewer components. Am I missing something?
As far as getting the most out of the circuit for a guitar I'm not so fond of the single-part Darlington transistor solution since it doesn't allow you to set the bias current of the first transistor. If you make your own Darlington then you can tune the operating current using the BE resistor. I only see the Darlington being an advantage if you have a low impedance load. The Sziklai does seem to have some advantages over the Darlington in general.
You can definitely get lower noise and lower distortion opamps but the good ones quite often operate at higher supply currents. The discrete designs are pretty good and are probably better than your average opamp. The thing discrete designs offer is they are likely to operate at lower currents, maybe a bit more swing. You can also build a single opamp, so no wasted current.
If we go back to the original problem:
BJT buffer has higher noise with the high impedance source of a guitar
==> JFET buffer low noise but JFET+source resistor has a volume drop
==> So to fix either of those problems you have to do something.
Probably best to keep the JFET input from noise perspective.
After that, all the options have small differences in performance.
The combines hfe should be large enough for decently high input impedance. If not, there is always bootstraping, Cornish buffer style.
Bootstrapping gives you higher input impedance but it doesn't solve the input noise currents of a BJT design.
