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CLASS-G TRANS•ANA AMPLIFIER The transistor Q8 is configured as a switch, which controls the current source Q5, of the input differential amplifier Q11 and Q12. If the power On/Off, Signal Sensing, and Thermal Protection circuits have all stopped pulling to -15V, the turn-on process begins, and the voltage at the emitter of O8 slowly ramps up to zero Volts according to the charging time of soft start components R13, C93, and C94. 15V is divided between R40 and R65, and imposed across the base-emitter of Q5 and R35, causing current source Q5 to turn the amplifier on.
The input signal is connected across the differential amp transistors Q11 and Q12. U2B is configured as a DC- servo integrater to null any DC at the output.
The output of the differential amplifier
is fed to the positive pre-driver Q18, and to the current mirror which mirrors the inverted input current of Q6 into O7 and delivers it to the negative pre-driver Q17. The pre-driver current is split between the three current paths of the bias and the driver circuit according to the factory bias setting of trimpot R9, and the temperature of Q37. Q37 is thermally linked to the heatsink, and compensates for the gate-voltage temperature coefficient of the output MOSFETs. Q19 drives the positive out- put MOSFET Q4, and Q21 drives the negative output MOSFET Q10. Local feedback components C47 and R86 help to sta- bilize this output stage.
At low output voltages, current flows to the woofer from the lower voltage rail +LV or -LV through the diodes CR1/CR17 and output MOSFETs Q4/Q10. When the output voltage approaches or exceeds these lower voltage rails, output current flows from the higher voltage rail +HV or -HV through CLASS-G MOSFET Q9/Q1 and output MOSFETS Q4/Q10.
Positive G driver Q13 turns on class G MOSFET Q9 when the base-emitter voltage exceeds 0.7V. This occurs when
(OUT: 1.4–5.1V) – (+LV: 0.7–10) > 0.7V, which reduces to OUT > +LV -3.5V. A similar equation can be applied to the nega- tive class G driver Q23. The resulting behavior can be observed as the Class-G rail on the source of the output MOSFET tracking 3.5V higher (or lower) than the output signal.
SOFT START CIRCUIT
The amplifier soft start circuit is controlled by three On/Off circuits: Power On/Off, Signal Sensing, and Thermal Protection. The "diode OR" connection of CR5, CR6, and CR3 causes the amplifier to turn off if power On/Off is low or Signal Sensing is low or the Thermal Protection is low. The amplifier will turn on only if all these circuit outputs are high. In this all-high condition, CR5, CR6, and CR3 are open circuited or back-biased, allowing R13 to turn on Q2 and Q8, illuminating LED CR8 and turning on the amplifier. Q8 is turned on slowly, according to the time constant of R13 and C93 and C94. This "soft start" behavior eliminates audible turn on transients.
