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RE: Optimum bias point?

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In Reply to: RE: Optimum bias point? posted by rick_m on May 04, 2008 at 16:57:18

Doug Self talks about this in his book, and it was also the topic of a paper by a guy from HP named Oliver, back in the 1970s. I have PDFs of these if you're interested.

The problem can be looked at in several different ways, but with the same end result. Self uses SPICE to look at a DC swept plot of output voltage vs. input voltage. For various emitter resistors (0.1 Ohm, 0.22 Ohm, 0.33 Ohm, 0.47 Ohm) he plots curves of the derivative of Vout with respect to Vin for various bias currents. He then finds the optimum bias current for each emitter resistor that makes the derivative as constant as possible. They are as follows:

0.1 Ohm: 215 mA
0.22 Ohm: 107 mA
0.33 Ohm: 74 mA
0.47 Ohm: 59 mA

He also does this procedure (varying bias current for a fixed RE) and looks at the residual from a distortion analyzer, with the same results.

Oliver looks at this a different way. He does it analytically, as his paper was before SPICE was created I think. He looks at the incremental output impedance vs. output current and derives some formulas that strive to keep this as constant as possible. The formulas get really messy though.

The other way of looking at this with SPICE is to apply a swept DC current source to the output and plot the derivative of Vout with respect to Iout. This gives optimum bias currents that match Self's optimum. This can be done with LTspice by using the undocumented d() plot function operator. If the output node is called "out", the plot expression would be d(V(out)).

Of course, if you crank up the bias current by a large amount, you'll get full class A which will decrease the distortion again. But in the class AB region there is that local minimum.

This doesn't seem to apply to MOSFET output stages, for which it seems that the higher the bias current, the better.

Follow Ups:

RE: Optimum bias point? - rick_m 20:13:38 05/04/08 (3)
- Yet another way of looking at it... - andy_c 21:17:32 05/04/08 (2)
  - RE: Yet another way of looking at it... - rick_m 17:41:03 05/10/08 (1)
    - RE: Yet another way of looking at it... - andy_c 18:28:09 05/10/08 (0)

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