# Internal impedance of Class AB amplifier?

This was a challenge problem that my instructor gave. Here is what a class AB amplifier looks like: I've looked everywhere to find out about internal impedance but I got nothing. This is the problem I'm trying to solve:

Determine the maximum peak to peak signal that can be applied to a amplifier and the efficiency rating or nstage for the amp. Assume the amp is driven by a sine source and has an internal impedance of 600 ohms.

Given:

• $\text{hfe} = \text{hFE} = 100$
• $V_{be} = 0.7$
• $E_s = 32.093~V_{p-p}$ at 1 kHz
• $R_L = 8~\Omega$
• $R_1 = R_2 = 1~k\Omega$

I've tried all that I've been taught so far. I took the internal impedance as $Z_{input}$. Then, I used $Z_{input}$ and $E_s$ to find $V_{in}$, since $V_{in} = V_{out}$ is the voltage at the load. Finally, I used that answer to find the efficiency as $P_L / P_{dc} \cdot 100$.

I calculated an efficiency over 200%, which must be wrong. What am I doing wrong?

• You will notice that if you put some more care in writing the question it will receive much better feedback. I've edited it as an example. – clabacchio Oct 6 '14 at 12:56
• Sorry I was in a rush for class an this is my first post so forgive me for my negligence but your advice is well noted. – Deathkamp Oct 6 '14 at 13:00
• I've only done a few push pulls, but those caps on the input bother me. Will this work? – George Herold Oct 6 '14 at 13:35
• Sorry I took a look at my circuit and u r right there is no caps just what you see above since I changed it. Inputs signal connects directly to circuit with no Rx resistor but I still don't understand how an internal impedence affects the vload or how much voltage the amp can be put through – Deathkamp Oct 6 '14 at 14:58
• What is Es, and why is it specified to 5 significant digits? It seems it might be the input voltage, but that doesn't make sense since that is what you are supposed to solve for. – Olin Lathrop Oct 6 '14 at 15:05