Fuzz Face biasing - collector vs emitter

[Phend]

Lucky Fuzz Face only has 10 +/- parts, Imagine if it had 20.
Interesting how 2 transistors surrounded by 5 resistors can become __________ !!
(Fill in the blank)

[cab42]

I'm still confused, but now on a much higher level...

Lot's of great information in this thread. Thanks to all for contributing!

[antonis]

I'm still confused, but now on a much higher level...

The more the direct coupled stages the more the confusion..
(tre logic..)

[pacealot]

just a few parts and yet still more stuff going on than I can really comprehend.

As the young'uns say, "big mood"...

[PRR]

So even without the feedback, changing Q1c voltage would re-bias Q2, no?

Without the feedback, Q2C would slap from zero to 8.9V so easy you could never keep it midway for more than a minute.

Most transistor circuits depend on NFB to work usefully.

[Rob Strand]

Quote from: fowl on Today at 04:14:30 AM

    So even without the feedback, changing Q1c voltage would re-bias Q2, no?


Without the feedback, Q2C would slap from zero to 8.9V so easy you could never keep it midway for more than a minute.

Most transistor circuits depend on NFB to work usefully.

If you intended to created such a design you could get Q2 to bias at 4.5V.   The way you would go about it is remove the 100k from the emitter of Q2 and wire it back to the wipe of a trimpot.  The ends of the trimpot would connect to the supply and ground.   The trimpot is then set to give 4.5V at Q2.  This can be done.

The trimpot will be impossible to adjust.  You could finesse the pot arrangement to make it easier to adjust but it's still likely to be an unsuccessful design.   For example if the battery voltage shifted from 9V to 8.5V the DC voltage on Q2's collector would shift 2V!   Temperature effects would also make the Q2 collector voltage unstable.

The point I wanted to make is the DC gain from the base of Q1 to the collector of Q2 is quite high.  Both Q1 and Q2 have *DC* gain.  That means tiny changes is the conditions at the base of Q1 will have large DC consequences on the collector of Q2.   The effect of changing Q1's collector resistor would give quite large changes to the Q2 collector voltage.

When you put DC feedback around the stages the DC gain is much reduced and so when DC parameter change the effect on DC bias point is stabilized.   The thing about the standard Fuzz-Face circuit is the design is quite stable for silicon transistors but the stability is marginal with germanium transistors - hence the complaints of bias shifting under extreme temperatures.