Crown Micro-Tech MT-1000 Informations techniques télécharger pdf (Page 24)

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Micro-Tech 600/1200/2400 Power Amplifiers

From the error amp, the voltage translator stage routes

the signal to the Last Voltage Amplifiers (LVAs) based

on signal polarity. The +LVA (Q105) and the –LVA

(Q110), with their push-pull effect through the bias servo

Q318, drive the fully complementary output stage.

The bias servo Q318 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. D301, D302, D303, and D304 re-

move the charge on the unused portion of the output

stage based on the polarity of the output signal.

With the voltage swing provided by the LVAs, the sig-

nal then gains current amplification through the

Darlington emitter-follower output stage.

The bridge-balanced circuit (U104-D) receives a sig-

nal from the output of the amplifier and compares it to

the signal at the Vcc supply. The bridge-balanced cir-

cuit then develops a voltage to drive the bridge-bal-

anced output stage. This results in the Vcc supply

having exactly one half of the output voltage added to

their quiescent voltage. D309, D310, D311 and a trim-

mer resistor set the quiescent current point for the

bridge-balanced output stage.

The protection mechanisms that affect the signal path

are implemented to protect the amplifier under real-

world conditions. These conditions are high instanta-

neous current, excessive temperature, and output

device operation outside safe conditions.

Q107 and Q108 sense output current and act as a

common current limiter. When instantaneous current

exceeds the design criteria, the limiters remove the drive

from the LVAs to limit output current to safe levels.

To further protect the output stages, the patented

ODEP

circuitry produces an analog output proportional to the

always changing die temperature of the output transis-

tor. This output controls the translator stage previously

mentioned, removing any further drive that may exceed

the

safe operating area

of the output stage.

Thermal sensors S100 and S200 give the

ODEP

cir-

cuits vital information on the operating temperature of

the heat sink on which the output devices are mounted.

Should the amplifier fail in a way that would cause DC

across the output lead, the DC protection circuit senses

this on the negative feedback loop and shuts down the

output stage drive until the DC is removed.

5.2.2 Bridge-Mono Operation

By setting the back panel stereo/mono switch to Bridge-

Mono, you can convert a

Micro-Tech

amplifier for

bridged-mono operation. With a signal applied to the

channel 1 input and the load connected between the

positive (+) output terminals, twice the voltage can be

delivered to the load.

The channel 1 output feeds the channel 2 error amp

U204-A. The signal feeding channel 2 is inverted so

the channel 2 output will have the opposite polarity of

channel 1. This makes it possible to deliver twice as

much voltage to the load while the protection mecha-

nisms for each channel continue to work independently.

5.2.3 Parallel-Mono Operation

With the stereo/mono switch set to Parallel-Mono, the

output of channel 2 is paralleled with that of channel 1.

A suitable jumper capable of handling high current must

be connected across the positive (+) output terminals

to gain the benefits of this operating mode.

The signal path for channel 1 is the same as previously

discussed, except that channel 1 also drives the out-

put stage of channel 2. The balanced input, error amp,

translators, and LVAs of channel 2 are disconnected

and no longer control the channel 2 output stage. The

channel 2 output stage and protection mechanisms

are also coupled through S1 and function as one.

In Parallel-Mono mode, the amplifier can deliver twice

the current of a single channel. Because the channel 2

ODEP

circuit is coupled through S1, the amplifier gains

additional protection if a fault occurs in the channel 2

output stage. The channel 2

ODEP

circuit will limit the

output of both output stages by removing the drive from

the channel 1 voltage translator.

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Crown Micro-Tech MT-1000 Informations techniques Page 24

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