PAiA 4740 Envelope Generator Design Analysis
originally publised 1974
see accompanying file: 4740adsr.gif for schematic

The PAIA 4740 Envelope Generator is built around an LM3900
Quad Norton Amplifier. Analysis of this integrated circuit is
beyond the scope of this text, but it should be sufficient to
say that a Norton Amplifier is a type of operational
amplifier that is current sensitive rather Thai voltage
sensitive.

Before triggering, the circuitry is in a stable state with
the output of A1 and A2 near ground, the output of A3 near
supply, C3 discharged and the voltage at the emitter of Q2
near ground for an essentially zero voltage output.

When the trigger button is pressed, or an external voltage is
applied to the triggering input of the module, the current
flow through R3 into Al's "+" input causes this amplifier to
switch to a high level. As A1 switches, a short duration
pulse is coupled through C2 and appears across R8 which
causes a current flow through R9 into the "+" input of A2.
A2's output now goes from near ground to near supply and
current fed back from the output through R12 into the "+"
input holds this amplifier in a high state, At this point,
the trigger signal could be removed and there would be
no change in the circuit because Al's output is held in a
high state by the feed-back current from the output of A2
through R6 into ill's "+" input.

With A2's output near the supply voltage, D4 is forward
biased and current can flow into C3 through R13 and R22. R22
sets the current flow and consequently the rate at which C3
charges. The voltage across C3 is sensed by the darlington
pair emitter follower comprising Q1 and Q2 so that the
voltage at the emitter of Q2 rises along with the capacitor
voltage.

Eventually, the voltage at the emitter of Q2 rises to the
point that the current flow through R11 when added to the
bias current through R27 and R10 exceeds the feed-back
current through R12 and at this point the output of A2
switches back to near ground.  D4 is now reverse biased so no
current can flow from C3 back into the output of A2.

While A2 was high, current flow through R18 into the "+"
input of A3 caused the output of this amplifier to be near
supply. Now that this current is absent, the current flow
into the "-" input of As through Rl5 exceeds the current
through R17 into the "+" input and the output of this
amplifier switches low causing current to drain from C3
through R23, R19 and Dl which is now forward biased.  C3
discharges at the rate set by R23 until the current through
Rl5 and R17 are equal. With A3's two input currents equal the
output of this amplifier stage rises to whatever voltage is
required to keep the currents equal. R20 and R21 provide a
high resistance feed-back path that "closes the loop" for A3.
When the module is not working and the output of A1 is near
ground, diode D3 clamps the middle of this loop to ground so
that there is no current leakage into C3.  During the times
that the output of A1 is high (so that D3 is reverse biased)
the current flow through these high value resistors is so
small compared to the working currents charging and
discharging C3 that it can be ignored. The circuitry is now
in a second stable state with the output of A1 high, A2 low
and A3 at some intermediate level as set by the front panel
sustain control.

When the trigger voltage is removed the circuit begins to
work again and the removal of the current through R3 causes
A1 to switch low. C3 now begins to discharge through R25, R7
and D2 at the final decay rate set by R25. If the trigger
input had been removed while the module was still in the
attack cycle the output of A1 would have remained high as
explained earlier until the attack cycle was complete. At
that point both A3 and A1 would switch low and current from
C3 would be drained through both of these amplifier outputs.
As the output voltage passed through the sustain level, A3
would switch high removing this discharge path so that C3
would continue to discharge toward zero volts at a rate set
by R25 only.

LED 2 is the attack indicator light and is on whenever the
output of A2 is high. LED l is the decay / sustain indicator
lamp which is on when the current through R29 exceeds the sum
of the currents through R31 and R32, a situation that only
happens when the output of A1 is high and A2 is low.

R33 is a current limiting resistor for the light emitting
diodes.  During the times that neither of the status
indicators are lighted, the lack of voltage across R33 causes
transistor Q3 to conduct. As Q3 switches on and off it senses
a current flow through the "dummy load" R35. This current is
of about the same magnitude as the current drawn while the
LEDs are on and consequently serves to keep the current drawn
by the 4740 from the power supply relatively constant
regardless of the ADSR's status.  This constant current drain
reduces power supply regulation requirements,

C4 is an integrating capacitor to keep noisy trigger inputs
from producing multiple triggers.

R1 and C1 provide power supply decoupling.

(c) 1995 PAIA
all rights reserved
phn 403-340-6300
fair use copy only
with this notice