Help!!! Question about the two stage Amplifier output stage

Hi, I am designing a very simple two stage differential amplifier that posted on razavi's book, but the load is a 8 ohm resistor parallel with a 500fF cap, but when i connect it with the output, the AC response result become totally different!! It seems current flows in second stage flows into the resistor, and the DC level at output also changes a lot!! How can I solve this problem? Do I need a output stage for my circuit? Can anybody give me some suggestions plz? Thanks very much!!!!!!

Please post your schematic.

Ok I just post it~!

Your circuit looks like a low power very simple opamp that does not have enough power to drive an 8 ohm resistor in parallel with a 500pF capacitor.
Why don't you use an audio power amplifier instead?

audio power amplifier? I am a freshman in this area... can you give me some examples or materials to design such kind of amplifer?

DC level at output also changes a lot!!

Click to expand...

This suggests rogue variations in current flow through your output transistors.

Or perhaps one of them turns off completely?

The idea of the op amp is that its output is never at high impedance.

This means the output is supposed to be at a low resistance, regardless what volt level it's outputting.

This means one output transistor might be mostly on and the other mostly off.

But what's supposed to happen when output voltage goes to zero? Should both output transistors turn off? No.

Can you make sure that your output transistors are never both entirely off?

This means adjusting a few things. Current flow at the bottom of the long-tailed pair. Bias adjustments, etc. Maybe it means adjusting a lot of things.

It will be easier if you attach less load at first.

An 8 ohm load is low ohms. To drive it will require similar low ohms through the output transistors (in order to create voltage on the load). Thus you want to find a balanced range of operation between the supply rails.

Make adjustments on a light load, until you get the desired performance. Then progress to a heavier load.

It is impossible to design this amplifier without having:
1) Detailed spec's for the amplifier.
2) A list of calculated required voltages and currents.
3) Suitable Mosfet spec's.
4) Suitable Mosfet part numbers.

The 8 ohm resistor is probably a speaker. Then the circuit must be an audio power amplifier.
What is the most important spec for an audio power amplifier? Its rated output power!
But you forgot to tell us how much power it must produce.
Then you must know how much voltage gain the amplifier must have, its frequency response and its maximum amount of noise and distortion.