question about input stage

[demons_wing]

i would like to know what are the advantages of having an input double differential pair with cascode connections?

[E007]

I think it will improve the linearity of the input differential pair, it also placed on the voltage amplifer stage in some circuitry I had saw.

[demons_wing]

cheers mate

[azira]

Originally posted by demons_wing
i would like to know what are the advantages of having an input double differential pair with cascode connections?

I don't know about the linearity and such but cascoding the input stage produces better high frequency response. This is because the CE amplifier only has a gain of 1 which nullifies the miller multiplication of Cbc. The CB amplifier then has all the gain, but it, instead, miller multiplies Cce which is typically much lower than Cbc.

W/rt to the dual differential. Having a complementary VAS can cancel 2nd harmonic distortion. But in order to drive a complementary VAS, you have to provide two signals 180deg out of phase from each other. The signals also have to be at different biases. If you use a single differential pair, your inverted output is at the same high bias as your noninverted signal. This will have to be level shifted if it is to be used for the lower VAS. That means extra devices in one path vs the other path. A dual diff pair, ideally, has the exact same number of devices in each path.

If you have a single ended VAS, there is no use for the 2nd diff pair.
--
Danny

[hunter audio]

hi

dual differential inputs has been more or less senn in european and us amps

single differential and diamond differential are generally seen in Japanese amps

I am talking about the good days of 70 s where amps circuirts were best devoloped

basically the dual differential are european / us patents

therefore the ultimate sansui amps had to be designed around a diamond differential stage

What id like to say is there is lots of material on this - the pros and cons of each input stage - by manufactureres who have done - a lot of research on this - there is a lot of litereature fro sansui ets

if you can get hold of this info - mostly published in the 70 and 80 s there is a lot to learn from it

suranjan

[audiofan]

you may read leach superamp text at :


http://users.ece.gatech.edu/~mleach/lowtim/instage.html

[demons_wing]

thanks guys...the help is very appreciated...audio is much harder than i thought

[thanh]

a diamond differential stage

Can you post it's circuit?
You can't buy any array-bjt in my country.I think if i use cascode input satge ,the offset will increase