Hi All,
In Class AB amplifiers with mosfet outputs, it is common to see zener diodes from the gate to source to provide current limiting and protect the gates from overvoltage. However, the zener is often shown in series with a normal diode. For example: Rod Elliot P101 Mosfet Amplifer Schematic
What is the purpose of this diode and does it need to be anything special or is a normal signal diode ok?
Many thanks,
Greg
In Class AB amplifiers with mosfet outputs, it is common to see zener diodes from the gate to source to provide current limiting and protect the gates from overvoltage. However, the zener is often shown in series with a normal diode. For example: Rod Elliot P101 Mosfet Amplifer Schematic
What is the purpose of this diode and does it need to be anything special or is a normal signal diode ok?
Many thanks,
Greg
There are two gates to drive in a complementary output. If you put a Zener on one gate, it will clamp the negative drive to 0.65V, not enough for the other gate, so you have to prevent it from clamping the negative (opposite) drive. The situation is complicated by the idle bias voltage, but the idea remains. This means that quasi outputs are different because the two FETs are driven separately, so one Zener (each) is fine.
Thanks for your help, I have some further questions but I know what to look for when simulating now.
Notice the added diode points the opposite way of the "built in" Zener one.
Because with an AC signal, in one way it drops rated Zener voltage , but, as mentioned above, the other way it drops some 0.65V, so that path must be canceled by a series reverse biased one, typically a 1N400* is enough.
You can also use a "signal" one such as 1N4148