I will use my Total Harmonic Analyzer for LTSPICE to perform these tests. Control script is slightly modified, to decrease test (simulation) time. It is possible to do so, because all schematics are settling fast and we are not looking into very small THD numbers. Signal amplitudes are also of moderate values.
Inputs of all 4 stages are connected together and transfer function of every stage is modified to have Gain=1 with corresponding loads and at test frequencies. The reason for this adjustment – I would like to make comparison of all stages at the same signal amplitude at the output. This is done by changing transconductance value of driving AC current source, while keeping DC current constant. This is pretty much the same as changing input signal amplitude.
To perform THD test with my single channel analyzer, I have to connect its input to corresponding output for every test. This limits the way how results are displayed to one stage at the time. So, 4 plots will represent test for these particular test conditions.
And, one picture worth a thousand words, so I will try to keep comments at minimum and post more results. Most of results will comment themselves pretty well.
First set of tests is related to optimization of stage bias. All stages are working in AB mode and I’d like to evaluate stage performance with different quiescent currents, while changing signal amplitudes. Output amplitudes are changed from 0.3V to 35V. Frequency is 1 KHz.
I used 20mA, 40mA, 60mA, 80mA, 110mA, 140mA, 190mA, 240mA and 300mA quiescent currents. Color legend for used BIAS is represented by the following picture:
As you can see, Stage 3 shows the worst performance in THD tests.
Stages 2 and 4 are the best performers, showing almost identical results.
They are much better than Stage 1, especially with big signals.
Next set of tests does not correspond exactly to any standard THD measurement. What I would like to evaluate is how signal linearity is affected by DC offset. It is somehow related to intermodulation test, but without emphasis to any particular signal ratio.
To perform this test, generator amplitude is kept constant and DC offset is stepped from 0V to 30V. This test is performed for different signal amplitudes from 0.3V to 10V.
Results are displayed as THD vs. offset plots. Amplitude of each signal is represented by separate line with different color.
Color legend looks as following:
Total Harmonic Distortions vs. Offset for all 4 stages look like: