S L I G H T L Y N A S T Y E L E C T R O N I C S A D E L A I D E , A U S T R A L I A
7
Ci rcui t O ve rvi ew
The 1411's input amplifier is a classic three-transistor opamp circuit consisting of a
differential pair (Q101 and Q102) that control an output transistor (Q105). The
best way to understand the operation of the differential pair is to consider the
fact that they both share a common emitter resistor (R108) - if one transistor
has a higher voltage at the base, it will conduct more current through this
resistor, raising the voltage across it and consequently reducing the base-emitter
voltage on the other transistor,
As Q102 passes current, it increases the voltage across R109, pulling the base of
Q105 down and turning it on. Because Q105 has no resistor on its emitter to
provide negative feedback, it provides a massive gain boost to the signal,
essentially switching on hard any time a minor current flows through R109/Q102.
What controls this large gain and makes the output voltage across R110 a
meaningful value is the negative feedback path flowing through R105/R123.
As an opamp, the overall circuit around these three transistors can be
understood as a basic inverting summing amplifier as illustrated on the right.
This topology provides isolation between the three inputs by feeding them all
into a "virtual ground" - a common summing node that the opamp automatically
adjusts to be equal to 0v regardless of the currents entering through the input
resistors. Because of this, even though the inputs are all tied to a single point,
there is no way for an individual input signal to bleed into any other as occurs in
purely passive mixing circuits.
Because input C is provided as an inverting input, it bypasses the input amplifier
in LINEAR mode so that it only gœs through a single inverting amplifier stage. In
DISTORTION mode it is rerouted to the front of the input amplifier and becomes
a normal non-inverting input.
The soft-clipping stage is a very simple diode-clipper that takes advantage of the
virtual ground node of the output amplifier to avoid needing to explicitly scale
the signal down and then back up again as is normally required in clipping
circuits. The best way to understand how this works is to imagine that R112 and
R113 are a single normal input resistor feeding an inverting summing amplifier -
we know that at one end will be the input signal, and at the other end will be
the 0V virtual ground - therefore we know that across the length of the resistor
there should be a signal that steadily descreases in amplitude from unity down
to nothing. If we want to gently clip the signal, we need only to "tap off" the
M O D E L 2 2 3 1 A s y m m e t r i c S l e w L i m i t e r
I N P U T A M P L I F I E R
S O F T - C L I P P I N G S T A G E
Typical topology of a summing
amplifier. The negative feedback
through Rf cancels any voltage shift at
the opamp's inverting "-" input and
thus holds it at the same voltage as
the non-inverting "+" input. This
means that each input signal sees only
a resistor connected to 0v, even
though multiple input signals are
connected to the same circuit node.
One way to think of this is that the
signals are each converted into a
current by the input resistors, and the
opamp adjusts the current through Rf
to make it equal to the sum of all the
input currents. As the same current is
flowing both into and out of the node
connected to the opamp's inverting
input, the voltage at that point cannot
change.
D u a l D i s c r e t e M i x e r
M O D E L 1 4 1 1
Voltage response of the soft clip stage
with the input in blue and the output
in red.
