Thermal Compensation Bias

[hawkfeather]

Can someone explain to me why there is a thermal compensation bias in the attached schematic of a MOSFET amp? Couldn't this just be replaced with a single resistor?

[ilimzn]

Originally posted by hawkfeather
Can someone explain to me why there is a thermal compensation bias in the attached schematic of a MOSFET amp? Couldn't this just be replaced with a single resistor?

No it couldn't. First of all, the configuration is a combination of a BJT/MOSFET 'darlington' + CF pair, typical for quasicomplementary designs. Both the BJT and the vertical MOSFETs used require thermal compensation, with the BJTs requiring somewhat more in this particular amp, so a BJT was chosen for the thermally compensated bias generator.
Even if lateral MOSFETs were used, it would still require thermal compensation, because the driver BJTs would require it.

[AndrewT]

Hi,
Vfets have a positive temp coefficient upto a very high Id.
Check the datasheet to see at what current it becomes negative temp. coef.
They will destroy themselves if you don't look after them.
This schematic looks like it has been designed properly.

You posted a schematic without a credit for the designer or simpler to post a link instead.

[ilimzn]

I know it's not popular to quote oneself, but I really have to reiterate this:

Even if lateral MOSFETs were used, it would still require thermal compensation, because the driver BJTs would require it.

Lateral MOSFETs have a negative tempco ovar about 100mA drain current, which s usually where the quiescnt current is set anyway. BUT even if they were used, instead of the verticals in the design, which indeed have a positive tempco, a temerature compensated bias would in theory still be required because it is the driver transistor's Vbe temperature coefficient that would still require compensating. Fortunately, because of the emitter degeneration, it is unlikely you would have runaway, but the total biac current tempco would still be positive.

[darkfenriz]

better to have to-126 bias generator, in the schematic is to-92