Arcam Alpha 10 Servicehandleiding - Pagina 2

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ALPHA 10/10P SERVICE MANUAL

Figure 1Block Diagram of Power Amplifier

The bias is regulated in two modes, one where these is no signal and one when signal is being split between the high

and low side MOSFETs when driving a alternating signal into a load.

Under static conditions, Z8 simply compares the sensed current, which includes the driver stage current, with a d.c.

mode reference of 13mV. The sense resistor is 0.11 so this corresponds to a current of about 120mA, some of which

is driver current. The current through the MOSFETs is about 80mA at this point. The comparator output is level shifted

to drive the integrating current servos Z3 and 4. This adjusts the amplifier current so that, on average, the bias level is

held at the reference point.

Under dynamic conditions, the low side drive will definitely be conducting more current than the reference (80mA) for

half the signal cycle and will be switched off for the other half. The result is a rectangle wave output from the

comparator, Z4.

When the output stage is biassed correctly (class AB operation) the comparator output toggles when the current

through the sense resistor for the -ve MOSFET is equal to the d.c. mode static reference level and at the half way

point of the signal. The result is a perfect square wave output from the comparator. Under these conditions, the +ve

MOSFET is also conducting the same amount of current at this point. If the bias level is lower than the reference, say,

at zero (class B operation) then the -ve MOSFET will spend slightly less than half the time conducting at or above the

reference level resulting in a rectangle wave output from the comparator at a duty cycle slightly less than 50%. If the

bias level is higher than the reference then the -ve MOSFET will spend slightly more than half the time conducting at

or above the reference level resulting in a rectangle wave output at a duty cycle slightly more than 50%. The change

in duty cycle away from 50% causes the integrator, Z3 and Z4 to adjust the bias level via the bias adjusting

transistors, Q22 and 29.

The integrator has a reference, the a.c. mode dynamic reference, for a bias point slightly higher than for a 50% duty

cycle. This eliminates the possibility of the bias being slowly throttled due to component tolerance mismatch resulting

in a reference which would pull the servo down. A high dynamic bias reference level simply stabilises the bias slightly

higher than the static reference but a low dynamic reference causes the bias to drift down to complete throttle.

This system works if the signal is a.c., symmetrical and is not a rectangle wave. Certainly, only a.c. signals are passed

Sense Resistor

-ve MOSFET

Measured Current (blue)

Other MOSFET

Comparitor Output

50%

49%

52%

Bias correct

Under Biassed

Over Biassed

Figure 2 Auto-bias under dynamic conditions