Filter circuit question - unusual design?

#12

Joined Nov 30, 2010
18,223
Q5 and Q6 are acting as clipping diodes - which is unusual,
I have a theory about why. Notice there are no diodes used for any other purpose in that circuit? Using transistors for diodes means one less part in inventory, and that's worth money in a production line factory.

An extreme case is represented by a place I worked which used 1% resistors in several circuits, but the factory did not stock all 96 values per decade. It would require 576 bins to hold all the values from 1 ohm to 976K. Lots of money saved by only stocking the 2 or 3 dozen values actually used in their circuits.;)
 

MrAl

Joined Jun 17, 2014
8,483
Hi,

The Ge diodes have lower forward voltage than the Si diodes so the sustain should pick up somewhat but the gain will also be reduced somewhat. A little gain boost somewhere would correct that if needed.
Also, Schottky diodes would work too as they have lower forward voltage drop also.

Transistors used as diodes supposedly have a more ideal response than diodes, but that's when they are connected properly which for this circuit they dont seem to be.
How much difference this makes for a 'fuzz' circuit is very questionable :)
I would also think Schottky diodes would work as good as transistors.
 

Audioguru

Joined Dec 20, 2007
11,249
Transistors used as diodes supposedly have a more ideal response than diodes, but that's when they are connected properly which for this circuit they dont seem to be.
I also thought that saturated transistors make better diodes but these transistors are not saturated.
 

MrAl

Joined Jun 17, 2014
8,483
The collector voltage is as low as it can go. The base current is very low so the Vbe and also the collector voltage are also fairly low.
Hi,

Oh so you mean the transistor is actually in saturation, ok thanks.

But then what about when it is not in saturation, it does not act like a diode anymore (?), and if not, why not?
Diodes dont always conduct either so i was wondering about this.
 

Audioguru

Joined Dec 20, 2007
11,249
I was talking about a transistor that has its base connected to its collector that I said is saturated when it is forward biased. The distortion transistors in this thread have the collector connected to the emitter so each transistor is 2 diodes in parallel.
It makes a thermometer where there is more fuzz when it gets warmer.
 
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