Fender Passport PD-250 Service Manual page 6

PASSPORT
POWERED PORTABLE
SOUND SYSTEM
THEORY OF OPERATION
Power Amplifier
NOTE: In cases where the amplifier circuits are identical between the left and right channel. Only components relat-
ed to the left channel will be discussed for simplicity.
The first stage of the power amplifier after the processor is comprised of FET Q201 which performs muting during
power up and down of the unit. This is accomplished by controlling the gate voltage of the FET Q201 and the
charge on C203. At initial power up the FET is biased on as the voltage across the gate is basically 0Vdc. As the
negative rail increases, Z201 and R204 apply a negative voltage to the gate biasing Q201 into the off state. At
power down, the gate of Q201 is pulled high with R203 and D201 as the negative rail collapses.
The first stage fo voltage amplification is performed by a discrete differential circuit comprised of Q202, 203, 204,
205. Each end of the differential pair provides drive to pre-driver complimentary transistors Q206, 209 and Q207,
210, the outputs of which are effectively summed through biasing transistor Q208 and VR202. It is interesting to
note that if VR202 should become open circuited, bias current will be minimized and not destroy the output devices.
Adjustment of VR202 is mainly to minimize crossover distortion at low signal levels as with conventional amplifiers.
Gain of this stage is determined by R217 (4.7k) / R205 (120) or 4.7K /120=39.17. In terms of dB we have, 20 x log
(base 10) of 39.17 = 31.85 dB. Distortion is minimized with negative feedback to one side of the differential pair
allowing for low distortion. Typical distortion may be as low as 0.05% THD up to clipping.
The driver stage is a proprietary design which performs three main functions and is comprised of Q211, 213. Firstly,
this stage utilizes a bootstrap circuit made up of Z202, 203, C210, 211, D202, 203 and R228. This circuit allows the
driver transistors to swing above the rails eliminating the need for a higher voltage tap from the power supply and at
the same time, minimizing the power dissipation in the drivers. Trickle current is supplied to the drivers during initial
power up through R225, 226 after which the bootstrap takes over. Secondly, having the drivers in this configuration
allow the output to swing closer to the rails and keeps the drivers from clipping before the output devices. The third
function of this circuit is that if the speaker outputs are shorted to ground, the drive to the output devices is mini-
mized since the bootstrap circuit is discharged and only the trickle current is supplied to the driver stage, thus pro-
tecting the amplifier SOA.
The output section is comprised of Q212, 215 and Q214, 216 in a two up and two down configuration further
increasing the amplifier SOA. Additional output protection is provided by the switched mode power supply which is
discussed later.
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