Principles of Operation
LV 3620
4–3
4.3 Circuit principles
Refer to the block diagram in Illustration 4–4 at the end of this section.
4.3.1 Dual channel operation
For simplicity, the discussion of dual channel operation will refer to one
channel only. Single operations will be explained in a separate discussion.
The input signal at the barrier block input terminals passes directly into the
balanced gain stage. The balanced input stage converts the signal to an
unbalanced signal with a difference amplifier.
The error amp amplifies the difference between the output signal and the
input signal, and drives the voltage
translator stage.
The voltage
translator stage channels the signal to the Last Voltage
Amplifiers (LVAs), depending on the signal polarity, from the error amp. The
+LVA and the –LVA, with their push-pull effect through the bias servo, drive
the fully complementary first output stage.
The bias servo is thermally coupled to the heat sink and sets the quiescent
bias current in the output stage to lower the distortion in the crossover region
of the output signal.
With the voltage swing provided by the LVAs, the signal then gains current
amplification through the Darlington emitter-follower output stage.
The bridge-balanced circuit receives a signal from the output of the amplifier,
and differences it with the signal at the Vcc supply. The bridge-balanced
circuit then develops a voltage to drive the bridge-balanced first output stage.
This results in the Vcc supply having exactly one
half of the output voltage
added to its quiescent voltage.
The protection mechanisms that affect the signal path are implemented to
protect the amplifier from high instantaneous current, excessive temperature,
and operation of the output transistors outside safe conditions.
Currents in the output stage are sensed by conventional current limiters. The
allowable current level is also adjusted as a function of voltage. When current
at any instant exceeds the design criteria, the limiters remove the drive from
the LVAs, thus limiting current in the output stage to a safe level.
To further protect the output stages, the
ODEP
(Output Device Emulation
Protection) circuit is used. It produces an analog output proportional to the
always-changing
safe operating area
of the output transistors. This output
controls the translator stage by removing any drive that exceeds the
safe
operating area
of the output transistors.
A solid state thermal sensor gives the
ODEP
circuits vital information on the
operating temperature of the heat sink on which the output transistors are
mounted.
