Q
My question is about the load current of amplifiers.
What exactly does this value mean? In datasheet it is given sometime with
+-10mA, sometimes only 10mA.
Means this load current the maximum output current which the amplifier can
offer? E.g. I have an amplifier with +-4Vp at the output. When in the datasheet
is given +-10mA load current, can I only use it up to a minimum load of
4V/10mA = 400 Ohms? Am I right?
A
Amplifiers typically have a short circuit protection that limits the current
that flows out of its output terminal. This would usually be in the case of a
short circuit to prevent damage to the amplifier. However, this is not a
tightly controlled parameter and it is not tested in production. It varies with
temperature and sometimes, with lower voltage supply rails, the amplifier may
not even be capable of delivering such current level due to internal
limitations at lower supply voltages. This means the same regardless of
whether or not the +/- is in the specification. However, this does not mean
that in both polarities the actual limit will be the same. No output stage is
perfectly symmetrical, but since it is not a precise number, it does not matter
as much.
While you are correct in pointing out that the minimum load of the amplifier
will be given per your calculation, you should only use it as a guideline. It
is a good practice to keep a reasonable margin away from that minimum load (for
all the reasons given above). In addition, don’t forget that if you are using
an opamp with feedback (such as in the case of a non-inverting amplifier), the
feedback resistors are also part of the load. Also, bear in mind that at
heavier loads, the amplifier output stage will dissipate more power, thus
increasing the die temperature. In your example, 10mA at 4V output means that
if you are powering your amplifier with 15V, there’s 11V*10mA = 110mW
additional dissipated in the amplifier, which will increase the temperature (in
the case of an SOIC) about 15 degrees C. It is not a big problem but this will
limit the temperature range of operation of the amplifier and may also increase
input-related offsets. However, other amplifiers may have more current drive
and therefore, if you load them to the limit, the power dissipation will be so
much greater.
