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.

 

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.

 

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!

 

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.

 

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

 

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.