# Audio Amplifier Need Help....

Discussion in 'The Projects Forum' started by b_frozt, Sep 12, 2007.

1. ### b_frozt Thread Starter New Member

Sep 12, 2007
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i got a preamp design... and i just want to know if it will work right on your simulator, or if the values are correct. and on the output side, i would like to place a Class B amplifier, i don't know what values (resistors,caps,transistors) that i am going to place. tnx a lot for your help! i got the schematic attached.

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2. ### Eduard Munteanu Active Member

Sep 1, 2007
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My sim is Qucs. The amp is driven with a 0.01 peak voltage, 440Hz sine wave (with an added 1 ohm in-series resistor). I did a parameter sweep over $\beta_F$ (same value for both transistors). I loaded the output capacitor with an 10K ohms resistor (that is, set to 100% gain corresponding to your schematic).

Some thoughts on the schematic:
- You'll most likely need a non-polar capacitor at the input.
- Try to make the quiescent point more stable across $\beta_F$ variations, possibly by adding more feedback.
- Add AC feedback to get a predictive, stable gain.
- These variations may also be caused by temperature in real-world.
- Use bigger capacitors. Although I didn't include an AC analysis (because of plotting issues), it didn't look that good.

P.S.: The quiescent output voltage is not measured for the first (left-most) transistor, in case it was not clear.

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3. ### b_frozt Thread Starter New Member

Sep 12, 2007
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tnx alot for that help sir! but i still got problem if that will have produce full swing of signal and how to connect the push pull circuit. hehehe!

4. ### Eduard Munteanu Active Member

Sep 1, 2007
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The problem is the input impedance. The second stage worries me: the emitter equivalent resistance at AC (440 Hz) and beta=100 is just 3K ohms. That gives an input impedance of only about 650 ohms according to my calculations! The voltage drops 3-fold!

The total AC gain (at 440Hz) is about 900 if my calculations are correct. Dividing that by 3, we get 300. So the output peak voltage should be approximately 0.01V x 300 = 3V. Not much different than what we get from the simulation if we account for the crude approximations.

On the other hand, driving the amp with a 5000Hz sine-wave increases the output voltage swing tremendously, by providing more gain (coming from decreased emitter capacitor reactance) to compensate the low input impedance.

5. ### amaynew Member

Jul 9, 2007
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The circuit you presented, is that really a class b amplifier?? i think its a class A

6. ### Eduard Munteanu Active Member

Sep 1, 2007
86
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Yes, it's class A (both stages). He said he was going to add a class B stage, but that one isn't in the schematic.