Alexander fast FET

This design is very close to that by Mark Alexander ('The Alexander Current-Feedback Audio Power Amplifier', Analog Devices Application Note AN-211, Audio Products 4-57/71) which contains much detail including PCB, component and wiring layouts whilst quoting a lower slew-rate, >200V/µS. If the reader is considering constructing this amplifier, perusal and study of the application note is recommended.

(Precision Monolithics Inc SSM-2131 datasheet, 6/89, rev A, p7).

With V+/V++ and V-/V-- ±40V and quiescent set at 300mA,
THD (50W into 8 ohms) = 0.002% @ 1kHz and 0.007% @ 20kHz.
IMD = 0.0017% (50W into 8 ohms).
Flat to 1MHz (-3dB).
Slew rate = 300V/µS.
Gain = 24.
DC output offset = 1mV.
No RC compensation to ground or output inductor required.
All unspecified Rs 1%, MF, ¼W.

Input buffer gain can be increased from 1.5 to 10 before bandwidth drops (33R2 resistor set to 1R8, or 16R5 to 299). Capacitive load compensation can be given, if needed, with components marked 'TBD'. Operation to ±100V feasible with additional output pairs, for outputs of several hundred watts.

The driver load of 1µF paralleled with 200R can, given output layout, be conveniently built up using paralleled components, eg; 10 x 0µ1F //'d with 5 x 1k.

A very compact build that lacks complexity, given the potential for power and speed, including a distinct lack of output filters. Compare to another fast (300V/µS) design and Kurodas' example.

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