Tweaker's Asylum

Posted by Jon Risch (i) on March 24, 2001 at 15:45:35

In Reply to: Op Amps running Class A? posted by AudioTweaker on March 22, 2001 at 16:29:28:

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?

Figure how much current will flow with a 2 volt output (maximum for a CD player, and typically the hottest line level signal in the chain), lets say as an example, that a CD player is faced with a 10 kohm input impedance for a preamp. 2 V out into 10 kohm is 0.2 mA. Now, you want to bias the op-amp with no more than about twice this current level, or approx. 0.4 mA. To draw 0.4 mA from an op-amp, you would connect a 37.5 kohm resistor to the negative rail, assumming a 15 volt PS.

This much bias will keep the op-amp output in Class A up to a 2.8 V RMS output level. Now, there is the issue of peak RMS voltages, and so on, and not getting too close to zero output current when the signal drives against the current load, so twice as much current as indicated for a 2 V output covers this nicely.

Remember, the total load on the op-amp will be the input Z of the next stage/component, in parallel wiuth the effective current load resistance. In the above cited example, the 10 kohm component load in parallel with the 37.5 kohm load will end up at approx. 7.9 kohm total load, so make sure that the op-amp can handle this amount of total loading in terms of the output stage.

Of course, just like many power amps, the harder/deeper you run an amp into Class A, up to 1/2 it's full current output, the sweeter it tends to sound (usually). But for op-amps, there is a very real issue of heat dissipation in the IC die, and keeping the thermal situation under control, so somewhere between the above recommended loading and 1/2 the max output current is going to be the place to experiment.

Using the above recommended 0.4 mA for most op-amps will usually sound better than operating Class AB, and not cause any over heating problems.

Personally, I have not found too many op-amps that need 2 mA to sound their best, as this level of bias is a bit much in most cases. Use of a 5 kohm ressitor to the negative rail will result in approx.3 mA of load current, and an effective loiadon the op amp fo the 5 kohm in parallel with the next circuit/component load. If this were 10 kohms, then ther total laod will be on the order of 3.3 kohms, which is a difficult load for most op-amps, in terms of maintaining absolute linearity and freedom from HF distortion.

So I would try to temper how much current load is placed on any given op-amp, especially op-amps that do not have a robust output capability.

One trick that can be used, is to help stabilize the load current, and you can do this by using a split resistance, say a 10 kohm resistor from the rail, then a series of by pass caps, and then from that node, a 27kohm resistor to the output/s. This filters any HF noise off the PS rails, and loads the op-amp with a 27 kohm AC load, and a 37 kohm DC load. This should not be necessary with a FET based current load or current diode, etc.

Jon Risch