The first four oscilloscope photos show the results of amplifying the output across the 1 ohm emitter resistor to give some indication of the open-loop distortion level. This test method and the interpretation of the results is described in the 'Simple Amplifier Test Method' page. The voltages quoted are peak to peak. More conventional test results may be added later, the intention here being to show that only a signal generator and oscilloscope are needed to obtain useful information, and even these are unnecessary if a music signal and speaker are used, and the 'error' signal can be amplified, recorded and listened to.
Input 740mV 1kHz sine wave. Testing this with a pc spectrum analyser revealed about 4% distortion, mostly 2nd harmonic. The emitter signal voltage is 0.061mV, corresponding to a loop gain of 12,130, and so the closed loop second harmonic distortion is 4/12130 = 0.00033%.
At 20kHz the harmonics are beyond the range of the pc based spectrum analyser, but the wave is close enough to a triangular wave to guess that there is about 11% third harmonic, and then the signal level of 0.39mV shows that loop gain is about 1900, and closed loop distortion is about 0.006% third harmonic plus lower levels of other harmonics.
The input signal is now a 10kHz square wave. The peak amplitude is 0.42mV negative and 0.33mV positive. The clipping level of the input stage is +/- 12mV, so the square wave after passing through the input low-pass filter is well below the slew rate limit.
The test output is compared here with the signal generator output at 1kHz to show the low distortion. In effect we have increased the amplifier closed-loop percentage distortion by over 80dB, and still have a fairly good sine wave signal.
Square wave input at 10kHz. This is the result at the amplifier output before the inductor with a load of 7.5ohms with parallel 4uF capacitor. There is no ringing visible with this or any lower capacitance tried.
As above, but after the inductor. The slight ringing is just the inductor and capacitor resonating, not amplifier instability. HOME.