Op Amps Running Class A? ? by Jon Risch
As has been noted, the purpose of running an op-amp output stage in Class A is to run it single ended, using only the (usually faster) NPN transistors.
By drawing a current referenced off the negative supply, from the output stage, the op-amp will be biased to output positive current equal to it to achieve a net voltage output of 0 volts with no signal present. This forces the NPN portion of the output stage to carry the entire signal without reaching the zero current point, as long as the signal output current does not reach that of the load current amount .
The less this current load varies, the less it will be modulated with the signal loading. Hence, a true current load is going to sound and work better. A simple resistor load, while electronically simple and fool-proof, has the least steady current loading, as it's loading varies anytime the supply voltage varies. A FET based current load will have a much steadier current under varying conditions, and therefore work and sound better.
Some notes on implementing a current load on an op-amp:
Figure the loading on the op-amp now, whether internal or if it is the source amp for the output of a component. What is the next stage or component input impedance?
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