Limiter Resistor of MOSFET Power Amp

[ervinl]

In the MOSFET PP power amp (either as source follower or common source), there are usually limitter resistors there, does it really limit the current/in rush or something else? How to calculate the value, and which should be the best position, series to drain or source side?

How it works (working as limitter), as the value usually is really small (<= 1 ohm).

Thanks,

Ervin L

[AndrewT]

which resistor?
Where in the circuit is it located?

[ervinl]

Originally Posted by AndrewT

which resistor?
Where in the circuit is it located?

Like in Legend Quasar or Pass (not sure which version). There is "Power R" in series before going to speaker/output bus, usually around 0.1 - 1 Ohm.

[nigelwright7557]

They are balancing resistors to balance the current through each output transistor.
Without them one transistor would take all the current and go into thermal runaway.

[ervinl]

Originally Posted by nigelwright7557

They are balancing resistors to balance the current through each output transistor.
Without them one transistor would take all the current and go into thermal runaway.

I still can't catch the idea. If the resistor is to balance all output transistor/MOSFET, why it can be so with same value, could it be done automatically (like auto bias/auto compensate)?

Regarding balance, what I understand is to match all output MOSFET (either N - P or in the same block/parallel side), and the bias will balance up and down device block.

If the R is to limit, the value could be too small to limit current. Or could it be only as additional backup of limiter, so it is safe to remove all (unless R series at gate as stopper, must be in its own due to avoid parasitic oscilation between device).

Ervin L

[nigelwright7557]

They arent current limiting resistors.

Transistors dont all switch on at the same voltage.
If the resistors werent there then one transistor would take all the current.
This means it would get hot and the switch on voltage would drop causing thermal runaway.

With resistors when one transistor switches on first the voltage across the resistor increases and so allows the other transistors to switch on and pass current.

[ervinl]

Originally Posted by nigelwright7557

They arent current limiting resistors.

Transistors dont all switch on at the same voltage.
If the resistors werent there then one transistor would take all the current.
This means it would get hot and the switch on voltage would drop causing thermal runaway.

With resistors when one transistor switches on first the voltage across the resistor increases and so allows the other transistors to switch on and pass current.

OK, then in MOSFET, which placement is best, source or drain, or both are OK (no matter either at source or drain)? Of course, if placed at source, the gate bias should be adjusted due to voltage lift by R (V = i * R).

Ervin L

[AndrewT]

FETs need source resistors.
BJTs need emitter resistors.

A 1pair output stage with Lateral FETs could be run without emitter resistors.
I can't think of any other exceptions, but maybe someone could come in and point out more for us.

[ervinl]

Originally Posted by AndrewT

FETs need source resistors.
BJTs need emitter resistors.

A 1pair output stage with Lateral FETs could be run without emitter resistors.
I can't think of any other exceptions, but maybe someone could come in and point out more for us.

If I put R at drain/collector side, will it also work as expected (as limitter)? As the current usually flows from source-drain (or emittor-collector BJT).

Ervin L

[sangram]

Read the Pass Labs F5 documentation for a nice explanation on common-source and common-drain topologies, it should answer your question.