Amplifiers
Simple class AB current amplifier with common base, common emitters
The circuit: an NPN and a PNP in a complementary pair, each conducting for half the input waveform into the load.
(But with 10uF capacitor @ input and 1000uF at output, Vcc is 10V.
Input on yellow and output on blue at 1kHz. The flat step through each zero crossing on the blue trace is the crossover distortion.
The crossover distortion is clearly visible.
Now with two diodes to bias the transistors:
The biased version: two 1N4148s between the bases hold them about 1.2V apart so both transistors idle just on.
With the diodes in place the output follows the input cleanly through zero, at 3.77V peak to peak against 4.40V in.
Oscillation
With tip31/tip32 the bloody thing oscillates:
Swapping in a TIP31 and TIP32 makes it oscillate: every zero crossing sets off a burst on all three traces.
Beautiful pictures 
But slightly unwanted
Zooming in:
Zoomed to 50ns per division with the cursors on one burst: 44.7ns between peaks, which is 22.36MHz.
We see a pulse of 44.7ns, equaling to a 22.4MHz oscillation.
