[sdiy] Function of 4148s in this 555 envelope

Harry Bissell harrybissell at wowway.com
Tue Jun 28 14:55:10 CEST 2011

D1 prevents Q1 from damage by negative voltages reverse biasing the B-E junction, most transistors
cannot have more than 5 - 7 volts before they break down in this mode.

D2 has the dual purpose of protecting against reverse bias of Q2, and also giving a reset path
for differentiator cap C1.  Without D2, the cap would not reset until the B-E junction of Q2 breaks down, something you don't want to happen. 
That would be around 5V instead of .7V

H^) harry

----- Original Message -----
From: Mark Rivera <marr at lumin.us>
To: Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 28 Jun 2011 08:41:24 -0400 (EDT)
Subject: [sdiy] Function of 4148s in this 555 envelope


What do D1 and D2 (schematic coord A2 and C2) accomplish in this 555-based ADSR?

A reverse voltage here, for a low pass RC filter (with diode having
~1pF capacitance) is something like 15MHz.. is that it? Or is this a
form or protection..?
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