question on boot-strapping for gain

[mordechai]

I ran across Tim Escobedo's website and found his schematics for "stealing" output gain and sending it into the collector path of Q1:

http://folkurban.com/Site/BootstrappingforGain-692.html

In these schematics, the "boot-strapped" signal is directed between two resistors on the collector path -- 33k/33k in one, and 47k/47k in the other.  Can this technique be done with a circuit where the collector path of q1 just uses ONE resistor up to the 9V power rail?  Or do these 33k/33k and 47k/47k resistors result from the addition of another identical resistor to the collector path BECAUSE of the boot-strapped signal (i.e., without the boot-strapped signal, there would only be a single 33k or 47k resistor, and the boot-strap technique demands the addition of another identical resistor)?   Could I do this between the emitter of Q2 and the collector path of Q1 in a Fuzz Face?  If so, would some modifications be required?

[DavenPaget]

I believe it's a bias point .

[gritz]

The examples of audio bootstrapping I've seen are always like this - the positive feedback is applied to the halfway point of the load resistance. I guess it may have something to do with bootstrapping being rather incestuous - in order to generate a feedback signal at the emitter of Q2, first you need a signal at the collector of Q1. If you tried to apply feedback from Q2's emitter direct to Q1's collector then Q1's collector would be "clamped" through the10uF capacitor. Instead the feedback is applied to the midpoint of Q!'s collector resistor, which initially "sees" a resistance of 33k - which is high enough for it to pass an initial signal to Q2, which buffers the signal and feeds it back via the capacitor. That feedback means that the two ends of the resistor are at the same potential, so Q1 sees an almost infinitely high resistance to A.C. resulting in lots of gain.

Hope this makes sense!

Remember that the feedback source needs to have a fairly low impedance and that the gain should be no more that unity (otherwise it'll oscillate). The emitter follower of Q2 ticks both these boxes. If you want to do this to a Fuzz Face type circuit you'll need an exra transistor to do the unity gain follower's job as the output from the emitter of Q2 in a FF isn't unity gain and in any case it gets bypassed by the cap to ground.

[DavenPaget]

In any case , if you want to save on components and love JFETs , you have to buy J201's in bulk , i mean really bulk , next time you want to have 4 jfets in one circuit , match them , i recently bought like 70 of them 

[PRR]

What gritz said: "positive feedback is applied to the halfway point of the load resistance." Cut the first stage load in half (or replace the one resistor with two half-value resistors), shove signal in there.

The "half" is not magic. If the follower has large power the "upper" resistor can be smaller. For single stages "half" is a good guide; when you get to loudspeaker amps you may find a 1:4 ratio.

> bootstrapping being rather incestuous

Yes. In fact it converts a CE-CC pair into a CE-CE pair. The "follower" actually becomes a gain stage, adding distortion. "Usually" you can get a cleaner design with a simple CE-CE pair without bootstrapping.

The capacitor value must be quite large. Normally we can accept 70% fall-off to pass signal; but to bootstrap we need 90%-99% signal.

Also note that at today's prices, an additional transistor stage may be cheaper, smaller, and much more gain than bootstrapping.