Hi

I am just wondering what's the purpose of the zener between the s and g of the mosfet, a 2kHz signal is fed to the G.

thanks

Zhuhua

 

Hello,

The zener diode is there to protect the mosfet.
It will probably be a 12 Volts zener, as a gate voltage higher as 15 Volts will damage the mosfet.

Bertus

 

so what would be the cause of the higher voltage? the signal fed to the mosfet should be 12v as swell, the whole circuit is power by 12v (other part of the circuit is not in the pic)

Zhuhua

 

As bertus said, it's just an additional (maybe unnecessary protection (under normal circumstances)).

VGSmax for this transistor is +-30V.

 

It is interesting that VN66AF apparently used to be made with an integral zener, but not now.

 

Speakers are inductive loads and they can cause transiets that can destroy FETs.

 

Say what?!? Uh, No. While they have inductive properties it is very small compared to the total resistance. These are not relays, which do have substantial inductance.

 

Say what?!? Uh, No. While they have inductive properties it is very small compared to the total resistance. These are not relays, which do have substantial inductance.

Especially with a resistor in series, as in the schematic.

 

Static discharge would be a concern. Also transients can happen.