[SOLVED] is it possible to design a signal supply op amp with negative bias only (Vee=-5V, Vcc=0V)

[levnu]

Hi,
i use AD8031 and i wonder if its possible to deisign as mentioned in the title,
the reason is becuase i want the op amp to bias a PA Vgg that must be negative,
my op amp input is a DAC Voltage between 0 to 2.5V (i want that at 0V input the opamp will output the most negative voltage, for DAC input to opamp =2.5V i want the minimal negative output, close to zero, i would also would like to have a equation for it and not only simulation as i have today)

Thanks,
Arye

 

[crutschow]

Yes, it can be done.
Do you have a +5V supply available also.

 

[danadakk]

Regards, Dana.

 

[crutschow]

at 0V input the opamp will output the most negative voltage,

Do you mean -5V or -2.5V?

 

[dick_freebird]

You could apply any rail-rail I/O 5V op amp to this.

I've had occasion to design very simple gate bias amps in ASICs for
phased array panel element controllers.

One design attribute that I've seen the RF guys care about, but fail
to express in specs, is that Vgg wants to be low impedance for noise
interests. And I never got an outgoing-noise-amplitude or -profile
requirement. Probably because those parts were IRAD projects and
they didn't have the assigned army of engineers with nothing to do
but analyze to death.

Anyway, because the application is fundamentally constrained, you
might get by with much simpler than "general purpose" solutions,
than an "everything to everybody" catalog op amp.

Since those days, I have seen more MMICs incorporate active bias
control in the guts (where it belongs, and where you can get at
things like die temperature and processing, to optimize more
finely than simple fed-back drain current). I also think it's likely
(but have no occasion to search) that purpose-made active bias
controllers may be on the shelf somewhere.

 

[KlausST]

Hi,

Generally when you read voltage like "-5V" .... then this is with respect to GND.
Now an OPAMP usually has no GND pin. Thus it will never know what's the voltage w.r.t. GND. It just sees the voltages between it's pins (like "-Vs to +Vs"). And as long as these values are within specification everything is fine.
You say the input is from a DAC and us positive ... but most probably this signal goes to a resistor .. and the OPAMP will never see this voltage. The Opamp does not care what voltage is at a resistor pin.

Klaus