why do want a match pair of germanium transistors for a fuzz circuit??

prorms · 22nd August 2008, 10:56 PM

cerrem · 27th August 2008, 08:03 AM

The Fuzz-Face circuit does not need "matched" transisitors...
The circuit toplogy is offically called a Shunt-Series-Feedback-Pair...
When the circuit is "properly" designed it is Beta independent, meaning that the Beta of the transisitors becomes third order in the gain equation....
The first transisitor is where most of the gain and fuzz occurs due to the assemytrical biasing.... This makes for poor temperature dependency.... I tend to re-bias the first transistor closer to the center of the load-line swing, this way temperature won't push the quiescent into shut-off...
The second transisitor is key to have as high a beta as possible...this is beacuse the base current will be smaller with higher Beta, thus is has less loading effect on the first transistor...
Keep in mind that the effective load of the first transisitor is the collector resisitor in parallel with the input impedance of the second transistor... Normally this is approximately emitter resistor of Q2 times Beta... BUT when Q2 emiiter resistor is fully bypassed by the cap, then all bets are off....This is when the fuzz control is maxed out...then the input resistance of Q2 become little "R'e" times Beta and then skews the Q1 load-line further...and the feedback loop goes to hell..

Chris

22nd August 2008, 10:56 PM	#1
prorms diyAudio Member Join Date: Nov 2006	why do want a match pair of germanium transistors for a fuzz circuit?? why do want a match pair of germanium transistors for a fuzz circuit??

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27th August 2008, 08:03 AM	#2
cerrem diyAudio Member Join Date: Jun 2005 Location: San Diego, CA	The Fuzz-Face circuit does not need "matched" transisitors... The circuit toplogy is offically called a Shunt-Series-Feedback-Pair... When the circuit is "properly" designed it is Beta independent, meaning that the Beta of the transisitors becomes third order in the gain equation.... The first transisitor is where most of the gain and fuzz occurs due to the assemytrical biasing.... This makes for poor temperature dependency.... I tend to re-bias the first transistor closer to the center of the load-line swing, this way temperature won't push the quiescent into shut-off... The second transisitor is key to have as high a beta as possible...this is beacuse the base current will be smaller with higher Beta, thus is has less loading effect on the first transistor... Keep in mind that the effective load of the first transisitor is the collector resisitor in parallel with the input impedance of the second transistor... Normally this is approximately emitter resistor of Q2 times Beta... BUT when Q2 emiiter resistor is fully bypassed by the cap, then all bets are off....This is when the fuzz control is maxed out...then the input resistance of Q2 become little "R'e" times Beta and then skews the Q1 load-line further...and the feedback loop goes to hell.. Chris

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