Does open-loop gain of an amplifier change with load condition? Is it worth to investigate? Does it provide us valuable information for improving an amplifier?
Output impedance is non zero (voltage amp model). Heavier loads need higher input level to maintain the same output level. This is effectively a lower open-loop gain. Right?
An input voltage will produce a certain amount of current at output.
Most amplifiers (open loop) produce a current that is more or less proportional to the input voltage.
Putting output current across a load with X Ohm will make a voltage on the load.
Current x Ohm = Voltage
When measure Open Loop Voltage Gain, OLG, I think the usual is to use Nominal load.
What the amplifier is intended to drive.
For example 8 Ohm or 4 Ohm.
Another number to measure would be Open Loop Current Gain / Input voltage.
This would be less dependant of the load
but still somewhat different at different loading.
Most amplifiers (open loop) produce a current that is more or less proportional to the input voltage.
Putting output current across a load with X Ohm will make a voltage on the load.
Current x Ohm = Voltage
When measure Open Loop Voltage Gain, OLG, I think the usual is to use Nominal load.
What the amplifier is intended to drive.
For example 8 Ohm or 4 Ohm.
Another number to measure would be Open Loop Current Gain / Input voltage.
This would be less dependant of the load
but still somewhat different at different loading.
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If you design one which doesn't do it I'd hurry up and get it patented.
Spot on.
Yes and no. If the change in o/l gain is static (i.e. independent of output voltage and current) there's nothing interesting going on--a reduction in loop gain then simply corresponds to a (usually pretty subtle) change in closed-loop gain. Much more interesting are dynamic o/l gain changes, which are just another way of saying that there is some distortion going on.
I suggest you also look at NSC AN-1485.pdf (big file!). Note that the thermal output-input coupling of IC opamps does not apply to discrete power amps to any significant degree but still the measurements are worth looking at.
Samuel
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