Vbe... Vgs multiplyer with MOSfets?

[Citizen124032]

Almost everywhere a transistor is used as a Vbe multiplyer in class A-A/B main amplifiers.
As the BJT is forward biased, so is the (enhencement) MOSfet.
Anyone ever bumped into a MOSfet Vgs multiplyer?
Any input welcome.

 

[jan.didden]

Do you mean the bias generator for a MOSFET output stage?
As far as I know, everyone has one, the concept is not different from a BJT output stage.
Or do you mean a bias generator using a FET instead of a BJT?

Jan

 

[lineup]

It is possible to use mosfet as multiplier.
But why do it.
BJT like BD139 does the job. And mount at heatsink it will keep the bias stable.

Nelson Pass have used also TL431 to set the bias of the output mosfets.

 

[Citizen124032]

Or do you mean a bias generator using a FET instead of a BJT?

Ja Jan, dat is exact wat ik bedoel.
Sorry folks:
Yes Jan, that is exactly what I mean.

It is possible to use mosfet as multiplier.
But why do it.

It is not why, but if.
Same conditions else.

Anyone with a valid application?

 

[Mark Tillotson]

Well the typical Vbe multiplier for bias generation is thermally bonded to the devices its biasing so that the temperature coefficients track each other. This makes it a natural fit for BJT output stages - multiply by 2-ish or 4-ish (for darlingtons), and somewhat more if drivers are involved. There are two different multiplication factors you ideally want, one for the tempco, one for the voltage span (CFP v. EF outputs are somewhat different in this).

A MOSFET bias generator would work for a MOSFET output stage fairly naturally perhaps.

But once you know the tempcos you could use either type to bias an output stage, assuming you can multiply by the right factor and adjust for the right voltage span.

 

[Citizen124032]

...once you know the tempcos...

That part of the story is clear, but not what I'm aiming for.
It is if and how such a Vgs multiplier is used.
Not necessary for bias settings.

 

[jan.didden]

Well then you need to give some more info what exactly you mean by 'Vgs multiplier'.
Do you have an example circuit.?

Jan

 

[lineup]

The multipier choice to use is dependent on what output devices you use.
Are you meaning to use HEXFET, Laterals or what as output devices?
Please give some more info.

 

[jan.didden]

@lineup, you should really read previous posts. He said 'Not necessary for bias settings.' .

Jan

 

[MarcelvdG]

V_GS usually spreads more than V_BE, especially with discrete devices.

In integrated circuits, it is not unusual to use diode-connected MOSFETs to set the gate-source bias voltage of matching larger MOSFETs.

 

[Citizen124032]

Anyone ever bumped into a MOSfet Vgs multiplyer?

It is if and how such a Vgs multiplier is used.

Do you have an example circuit.?

Anywhere, anyhow, ever.

 

[jan.didden]

With reference to the previous circuit - what's the actual question??
It looks like you've figured it out already.

Jan

 

[MarcelvdG]

Whether anyone actually uses it, and if so, what for, if I understood it correctly.

 

[jan.didden]

So he doesn't know what it is used for?
Why doesn't he say so?

Jan

 

[Citizen124032]

It looks like you've figured it out already.

Anyone ever bumped into a MOSfet Vgs multiplyer?

H A S A N Y O N E E V E R S E E N S U C H A C I R C U I T A N Y W H E R E ? ? ?

 

[Citizen124032]

So he doesn't know what it is used for?
It's in every power amp schematic.

...

 

[Citizen124032]

So he doesn't know what it is used for?
Why doesn't he say so?

...

 

[MarcelvdG]

So he doesn't know what it is used for?
Why doesn't he say so?

Jan

See post 6, and substitute "necessarily" for "necessary".

 

[Citizen124032]

See post 6, and substitute "necessarily" for "necessary".

My native tongue is dutch, sorry.
What did I type?

 

[MarcelvdG]

Anyway, I think it is rarely (if at all) used in discrete circuits because of the relatively large spread of V_GS.

On an IC, it is also unusual, because you would normally just stack the appropriate number of diode-connected NMOS and PMOS transistors.