Ecler MPA4-150, Service Manual

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52-0006-0101 EP05-99 anglès.doc 1 of 2
The amplifying stage basic structure is actually the one commonly used until now, this is, a
push-pull mounted A-B class amplifier, using P-type (IRFP9240) and N-type (IRFP240) mosfets.
The system's controlling core is a NE5534 OpAmp, which is internally compensated in order to
obtain an amplifying gain ratio equal or greater than 3. The amplifier's feedback runs through a
resistor and a capacitor associated to the OpAmp's non-inverting input.
Transistors BF471 and BF472 are common-base configured, becoming actually a current
source structure. They accomplish a dual function: on one hand, they polarise the mosfet's
gate-source junction, keeping them on their conduction knee. On the other hand, they carry out
the OpAmp's output voltage variations, referred to signal ground.
The polarisation current adjustment is fixed by a 2k5 trimming potentiometer connected to the
BF transistors base. This current is added to the current source's output, which passes through
the BF-transistors load resistors. The bias current stability against temperature is fixed through
the BD437 transistors. Their temperature- dependent base-emitter voltage curve is used to alter
adequately the current source's reference voltage. As a consequence, if the temperature rises,
the reference voltage decreases, thus the gate-source voltage also does, and finally the bias
current also decreases.
The Zobel network, formed by a resistor-inductor-capacitor group, and which is located at the
amplifier's output, intends to keep the amplifier's load impedance as constant as possible, no
matter which load is connected to the stage's output, or which signal frequency is to be
amplified, in order to prevent an inverted-phase feedback signal.
In order to avoid a DC offset on the output signal, a diac-triac tandem system is used, which
shorts the output to signal ground when the DC level is enough to get the diac triggered. To
prevent this from happening while carrying audio signal (sine-wave, music), the diac's reference
voltage is taken from a filter formed by resistor 33k2 and capacitor 1µ.
The protection circuitry supervises at any time the power consumed by the MOSFETS. The
circuitry basically consists on two sections: MOSFET's drain current (Id) monitoring and drain-
source voltage (Vds) monitoring.
When Id exceeds a fixed values a control transistor in every branch starts to conduct like a
switch, apliying a parallel resistor to BF's load resisitor, reduring the gate-source voltage, and
also reducing Id.
If the MOSFET's drain-source voltage (Vds) drops too low, a second circuitry actuates to alter
the control-transistor's triggering level, obtaining a SOA-like curve section and a current stage,
which can be adjusted adequately in order to maintain the MOSFET's power consumption as
close as possible to its SOA.
author:
J. Colomines
date:
050401
project:
EP05-99
product:
MPA4-150
ECLER
approved:
num:
52.0006
version:
01.01
title:
FUNCTIONING DESCRIPTION