Question
I'd like to use the OPAMP AD823 in a closed loop as amplifier between 0 and
100kHz (gain is 20 or 100).
The datasheet Rev.0 fig.11 shows the dependence of open loop gain vs. output
voltage (parameter=load resistor).
The question is as follows: How does the gain-bandwith-product (GBW) of the
OPAMP depend on the load resistor? Is it independent (about 12MHz) or is it
dependent? If dependent, then how?
Answer
The GBWP should not be affected significantly if the load and output swing are
with in the recommended limits of the amplifier. As shown in fig 11 in the
AD823 datasheet the open loop bandwidth is load dependant to a degree, but this
is more due to the output stage not being able to drive heavy loads. The open
loop bandwidth is relatively independent of load when RL = 1k or 10k, this
makes sense as the loading on the output is relatively light with these loads.
The output stage of the AD823 has a maximum current drive limit when this limit
is reached the output voltage starts to drop as the base current to the output
transistors is fixed and cannot support the output current requirements. This
increases the drop between the supply rail and the output pin (i.e. the output
voltage decreases). At 12MHz it should be relatively independent, if the output
swing and load resistor are within the recommended ranges.
