a Zobel network (sometimes called a Boucherot cell) to help compensate for
inductive loads at high frequencies.
The Class G Lifters
are in two complimentary halves – TR600, TR601 and
TR602 plus p-channel power MOSFET M601 for the positive half, and TR603,
TR604 and TR605 plus n-channel power MOSFET M603 for the negative half.
They are arranged as CFPs (complementary feedback pairs) rather than plain
emitter (source) followers so that the MOSFETs can be fully turned on without
needing power supplies that exceed Vcc and Vee.
The two halves operate identically – we will examine the positive (top half)
lifter to see how it works. With only small signals coming from the power
amplifier, node PLD_C remains biased at about +20V (via the network R614,
D600, R602., R603, R605) whilst node +VLIFT_C is held at approximately
+30V (i.e. +Vlo less the 0.2V or so dropped across the Shottky power diode
D100). The emitter of TR602 meanwhile is at about +24.5V, so TR602 is
turned hard off. Thus the base of TR600 is connected via R600 to Vcc and the
emitter is at Vcc – 0.6V. Vgs of M601 is thus -0.6V which is well under the
approx -2.5V required to turn on M601.
When the voltage at node PLD_C exceeds approxi25V this is enough
to turn on TR602. When about 3V is developed across R600 this is enough to
turn on M601 which conducts until the voltage at the emitter of TR602 has
risen enough to stabilise the system. The output voltage then can rapidly track
the input (plus about 5V) as required, reverse biasing the Shottky power diode
D100 and drawing the collector current for TR406A via M600 and the +Vcc
supply. The complementary emitter follower TR600, TR601 ensured the gate
capacitance of the MOSFET can be both charged and discharged very
quickly. C605 provides fast local decoupling to minimise switching transients.