[HomeBrew] Class B Small Signal Amplifier Question

[Eugene Crist]

I've been experimenting with small signal amplifiers on a breadboard in an
effort to understand how they work. I built a simple LC oscillator that
generates a sine wave at around 7 MHz as my signal source, and I've been
coupling the oscillator to the amps via a 100 pF capacitor. The amps consist
of a 2N2222 NPN transistor with a voltage divider at the base and resistors
on the emitter and collector. The configuration is a common emitter amp. The
first amp I put together was a class A. I biased the base of the 2N2222 with
about 1.5 VDC/900 mVAC, and the resulting sine wave on the collector was a
faithful reproduction of the input sine wave but at 420 mVAC which makes
sense since the load (amps) on the collector should be higher.

The next amp I attempted to make was a class B and I expected to get a
clipped wave on the collector. To accomplish this I removed the DC bias
completely from the base and the AC signal was all that remained. To my
surprise though, the sine wave on the collector wasn't clipped at all. In
fact it looked exactly like the wave on the class A amp. This doesn't make
sense since the base voltage and load are oscillating which should cause the
transistor to oscillate between cutoff and somewhere higher on the load
line. This has baffled me for days. If anyone has any ideas or can point me
in the right direction I'd certainly appreciate it.

One other area of confusion is that the waves on the collector of both the
class A and B amplifiers had both positive and negative polarities which I
also don't understand. I thought the base wave would be a typical sine wave
with positive and negative but the collector wave on the class A would
oscillate in the positive polarity region only on the scope. Any help in
explaining this would also be appreciated.

Thanks, Gene (KB3ONA)