[HomeBrew] Class B Small Signal Amplifier Question

[Loren Moline WA7SKT]

Gene,

Were you using AC or DC coupling to check the circuit. I'm sure that if you used DC you would see a sine wave in which the voltage is always above ground by a certain voltage.

 
Loren   WA7SKT

 
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> Date: Fri, 16 Oct 2009 22:03:16 -0700
> From: ejcristjr at gmail.com
> To: homebrew at mailman.qth.net
> Subject: [HomeBrew] Class B Small Signal Amplifier Question
> 
> ** Please do NOT cross-post messages when posting to HOMEBREW **
> 
> 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)
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