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Query on bias in
Ultra-LD Mk2

The description of the bias compensation circuit for the Ultra-LD Power Amplifier Mk.2 in the August 2008 issue states that a voltage of 70-100mV across the 0.1W emitter resistors gives a quiescent current of 70-100mA per transistor. In fact the current would be 700mA-1A. This would in turn suggest that the amplifier was dissipating around 200W at idle.

Wouldn’t the actual voltage be 7-10mV? Furthermore, in the absence of an adjustment, how can you guarantee that the quiescent current settles at a level that is optimum for crossover distortion purposes?

Wouldn’t the actual bias current be highly dependent on the characteristics of Q10 and Q11? Would a standard Vbe multiplier arrangement where DQ12/DQ13 and DQ14/DQ15 were wired in parallel and Q10/Q11 were at ambient temperature be a better way to do this? Wouldn’t that guarantee optimal thermal tracking? (F. Z., Barrack Heights, NSW).

  • The voltage across the emitter resistors should be 7-10mV (not 70-100mV). This was corrected in the September 2008 issue.

    Also in September, we included details of how to vary the bias, if necessary, because we have found that there can be a larger than expected variation in the voltage drop across the internal diodes of the transistors. This adjustment will cope with any variations in the Vbe of the drivers as well.

    We have not used a trimpot for this adjustment – we prefer to do without trimpots if possible and we think that, in most cases, no adjustment will be needed.

    By the way, our circuit is not much different in principle from the On Semiconductor application note AND8196/D which can be downloaded as a pdf from the onsemi website: http://onsemi.com

    In fact, the published bias circuit will always give much tighter compensation than a Vbe multiplier which effectively monitors the average of the Vbe drops (of driver and output transistors) at "arms’ length" rather the very precise tracking made possible by the internal diodes.

    In any case, regardless of which bias stabilisation scheme is used in a class-B amplifier, the driver transistors should be on the same heatsink as the output transistors because any variation in the Vbe of the driver transistors will affect the overall quiescent current.

    Using the speed control with shunt motors

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