improving my VAS

[djdamix]

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]

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