Audio Amplifier

[fantabulous68]

I have a test on friday. Im trying to answering this tutorial:



1. Assuming all components are ideal and that the BJTs are linear

8. How would you alter a Class B amplifier to make it into a Class A amplifier. Give full reasoning for ALL the changes you might effect. [10]

I said that you should add a bias voltage to the base of the 2 complementary BJTs in the output stage. Adjust the bias until the current flowing through a BJT=half the peak load current.

The peak load current= 2 IQ hence IQ running down on average will make a class A amplifier.

9. If the devices were now to be considered to be real devices, what would change and by how much? [10]

Uhmmm this is were im totally stumped. Somebody please lead me in the right direction....

 

[mneary]

Larger heat sink?
Temperature compensation?

 

[fantabulous68]

Oh yes! you right! Thanks....

Need a heatsink that can get rid of 5 times as much dissipated power and a transformer that can deliver 5 times as much power.
{since the PD of a class B is 5 times better than a class A amplifier}

The power handling capabilities of the bjts in a class A amplifier is hard since they have to soak up twice the output power. There is always a constant delivery of power into a class A stage even when there is no signal hence will need bigger bjts as well.

 

[fantabulous68]

4. Given that the stage in the attached diagram is running on the limit of Class A, calculate the maximum input power and hence determine the efficiency at:

a) 5% output voltage [2]



My Answer:

efficiency= Pout/Pin

Pout= (Vcc^2)/ 2RL

Pin=(Vcc^2)/RL

Therefore using the above equations:

Pout=(0.05Vcc)^2 / 2RL = Vcc^2 / 800 RL

Pin=(Vcc^2)/RL

efficiency= Pout/Pin=0,125%


Is my solution right? do i use 5% or 95%