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[SOLVED] Push-pull emitter follower amp (LTSpice)

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PelleS

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Hi. I'm fairly new to electronics and tried to simulate a circuit from The Art of Electronics in LTSpice. It's supposed to be a simple sound amp. It did not go well, and I am wondering if anybody could help. The circuit is very simple and images are below.

Circuit.png
voltageIn.png
voltageOut.png
 

I don't believe that you got this flawed circuit from the book. Please review the text and/or tell the chapter and page.

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Are you trying to implement this circuit? If so, find the differences!

1324280500_1391859332.gif


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Your transistors need some BIAS. That is a small voltage (~ .8V) to turn on (open up) the base emitter junction. The top transistor needs its base at 1/2 Vcc(V2) + .8V, the lower needs 1/2 Vcc(V1) -.8V.
Click to expand...
Yes, the figure 2.55 circuit is used to demonstrate the problem, figure 2.57 is solving it.
 
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    PelleS

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I see. Thought the first circuit worked. Thanks for the help :)
 

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  • Circuit2.png
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The second circuit does not work as a current amplifier, because the power supply ground connection is missing. You might not realize this in the simulation as the input source will drive the output instead.
 

Circuit3.png

So this is the right way to do it, or am I completely off track?
 

PelleS said:
View attachment 101954

So this is the right way to do it, or am I completely off track?
Click to expand...
R2 in your circuit is usually the collector of an NPN transistor so it can swing the base of the PNP output transistor completely to ground.
R3 is usually two resistors in series with their junction connected to a capacitor from the output for bootstrapping. Then the base of the NPN output transistor can swing a little higher than the supply voltage. The bootstrapped resistors are a high impedance current source for high voltage gain from the driver transistor.

The two diodes might not match the base-emitters of the output transistors so usually they are replaced with a transistor with adjustable bias.
 

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