Lateral Type Mosfets

Diogo said:
Why does the Anthony, Variable power amp is suitable only for lateral type mosfets?
What's the diference of this typre of mosfet?

1 I don't know, maybe he designed the amp a while ago or simply likes these parts (I also like these old goodies)?

2 Normally you can't replace lateral type with vertical ones just like that.

Lateral has lower capacitances (good)

Lateral has lower Vgs turn on voltage, often good

Lateral has postive temperature coefficent at more than 200-300 mA (higher temp-> decreasing current), good thing. You can take advantage of this fact. Vertical types switches the sign at several amperes, not very useful.

Lateral has nearly constant gm, good for distortion figures but bad for output impedance which is rather high.

Lateral has rather high losses, high Rdson

To be short: if you switch from lateral to vertical you must first of all do some redesigning of the bias circuits. You will need approx. -10mV/deg C for the bias circuits.. The additional capacitances can also cause trouble (high freguency oscillations).

Check the Mark Alexander amp for ideas of how you can do the bias circuitry. A simple Vbe-multiplier (one transistor and two resistors) will also do I suppose.
http://www.analog.com/library/applicationNotes/AdAudio/AN211.pdf

BTW: This amp isn't particular well regarded (here) but I have listened to it and I couldn't find any I disliked.