improving my VAS

[djdamix]

Originally posted by Upupa Epops
Hi David, you see distortion, because peak driving current in negative halfperiode is low ( only a few mA ). Colector load of VAS must be bootstraped or as currrent source.

I try that and you are right, seems are better now I have less distortion on negative halfperiode. I've also solve my DC problem (bad transistor). With the extra stage (IRF 610 & IRF9610) , I get about 50Khz "clean" bandwith with three output pairs. I'll had some more and rise the voltage. But before I must get a MPS42 to replace the MJE15031 with can't stand the voltage.


David

[djdamix]

in fact I'll try to get a MJ15035 (350V) instead of an MPSA92

[tmblack]

Anyone try to use a small MOSFET for the VAS?
I orrdered some MJE15032/33 to replace the old one that blew.

But I got a small enhancement type VP0808 and wonder if there is any reason not to try it if bias can be set up correctly?

Tom

[sajti]

And what do you think about the local feedback in the VAS? The local feedback reduce the distortion, and the output impedance, without very large overall feedback.

sajti

[richie00boy]

Give the MOSFET a try. The only major issue is to make sure that it's capacitance is low so that it doesn't load down the diff pair. I have thought about using a MOSFET for that position before as it would allow me to run the VAS at a relatively high current thus drive several output MOSFETs directly.

I always use degeneration on the VAS to linearise it, I think it's good to have local feedback at every stage and then just a little overall loop feedback is required.

[sajti]

I think to use parallel voltage feedback. I attached the schematics, I planned. The R11 and R12 do this feedback.

sajti

[richie00boy]

Nice. Should R5 and R8 be there though?

[sajti]

Originally posted by richie00boy
Nice. Should R5 and R8 be there though?

The R5, and R8 set the bias current for the input transistors. The gain of the VAS is about R11/R6 or R12/R7. With this values, the result is about 33.

sajti