Class A Design in progress

Hi all,

I'm designing my own 30W class A amplifier or at least trying to do it....:D

At the moment I decided to use Class A CFP as reported on Douglas Self book.

At first I tought to use a current mirror topology and a cascode buffered vas. I tried to implement it but my need of low closed loop gain leads me to have an amplifier with more than 100dB of feedback. This cause me to great problems in compensation and looking for stability.

Now I decided, at opposite, that I will try to have a very linear open loop amplifier and then apply feedback. At the moment I'm using a differential VAS and in this way I can get a closed loop thd20 of about 3e-3 prior to clipping and this quite satisfies me. Do you have some advices on differential VAS since its the first time I use it? :scratch: :scratch:

Thanks,
Michele
 

Attachments

  • driver.jpg
    driver.jpg
    71.4 KB · Views: 527
Hi,

The second version I posted has 70dB open loop gain as opposed to the first version, not reported here that has 130dB open loop gain so I have reduced OLgain of a great amount. I was only looking for an advice about differential vas or better topologies if anyone knows.

Thanks,
Michele
 
Hi,

That's right Q11 and Q12 are dissipating about 120mW. This seems too much also for me. I will change them with a more aggressive type.

For VAS current around 5 mA is what my calculation shows. LTP is biased something around 5 mA (Ic=(1.2-0.67)/Re), subtracting 0.56mA for J503 remains 2.25mA on 1.5k that makes up 3.4 volts over the load resistor. Substituting in the equation for DC bias of the differential VAS you will find a current of about 5 mA.

EDIT: Now trying to add base stopper

Thanks

Michele
 
Hi all,

I tried to substitute the transistor with more aggressive type and the performance remains the same, that's great. The 22R ohm resistor for base stopper lead to a little higher distortion 3.3e-3 instead of 3e-3 but seems safe to use it.

I was thinking if I can improve that data improving the quality of the differential vas. Is there something I can do??

Thanks,
Michele