# max symmetrical swing

#### automagp68

Joined Nov 13, 2011
81
Can someone help me understand how to get an AC swing from a BJT NPN

This is just something i want to practice on my own. I am familiar with the operation

Im choosing an NPN just for simplification. Say a 15-20v supply either bipolar or single ended

Gain not important. I just want to see the swing.
Say 6v PP swing so 3 volt peak swing.

#### MikeML

Joined Oct 2, 2009
5,444
(I see Jony beat me to it)

Common-Emitter NPN transistor amplifer. Bias voltage is 3Vdc. Input is +-50mV, riding on the bias. Emitter is grounded for AC signal. Gain makes ~6Vpp at collector, but it rides on the quiescent output voltage of ~9Vdc. Use a DC blocking capacitor to make a symmetric output swing (around ground) of +-3V.

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#### automagp68

Joined Nov 13, 2011
81
ok thanks for the help

So i understand the swing a little bit better
What I'm fooling with is a simple power BJT

I got the swing i wanted, now I'm trying to get it to drive an 8 ohm load
This is just a sim and will never be built so practicality is not a concern

I built a simple little circuit and got the swing i wanted but when i attach my 8 ohm load i loose my swing.

The load is capacitivley coupled from the collector

I designed for a 3v peak swing or 6v pp

with an 8 ohm load i biased the bjt for about 265 mA

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#### MikeML

Joined Oct 2, 2009
5,444
If your amplifier is a small-signal NPN with a collector load resistor of a few KΩ, then the amplifier can only drive a load which at least as high as the collector resistor, meaning that an 8Ω will just totally overload the whimpy amplifier.

If you had an old "transistor radio output transformer", with an input impedance of like 500Ω, and an output impedance of 8Ω, you could make it work.

Direct-drive of an 8Ω speaker takes a much different type of amplifier...

#### automagp68

Joined Nov 13, 2011
81
If your amplifier is a small-signal NPN with a collector load resistor of a few KΩ, then the amplifier can only drive a load which at least as high as the collector resistor, meaning that an 8Ω will just totally overload the whimpy amplifier.

If you had an old "transistor radio output transformer", with an input impedance of like 500Ω, and an output impedance of 8Ω, you could make it work.

Direct-drive of an 8Ω speaker takes a much different type of amplifier...

I thought of a transformer also

so just for quick and dirty practices and because i don't know anything about push pull design or power amps
Just to get a quick simulation working what do u think my best bet is?