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Buffered Opamp

I'm studying buffered opamp and i've found this configuration
http://img156.imageshack.us/img156/5640/afsdafen7.png
i don't understand second statement
"reduces output load at the drains of M6 and M7"
I don't understand why a large resistance at the drains of M6 and M7
is a bad thing...if i consider voltage gain of second stage, it is
similar to
(gm6+gm7)*output resistance
so, if output resistence is high, the voltage gain of the second stage
is high and the opamp votage gain is high....i think this is a good
thing....
What's the problem?
Thanks in advance
 
P

Phil Allison

Jan 1, 1970
0
I'm studying buffered opamp and i've found this configuration
http://img156.imageshack.us/img156/5640/afsdafen7.png

i don't understand second statement
"reduces output load at the drains of M6 and M7"

I don't understand why a large resistance at the drains of M6 and M7
is a bad thing...


** When the " load " on some source is " reduced " - it means the load
impedance seen by that source is increased.

The logic is simple - if you add load to the back of a horse, the horse is
not so happy about it.




...... Phil
 
E

Eeyore

Jan 1, 1970
0
John said:
"reduces output load at the drains of M6 and M7" is listed
as one of the good things (purposes) of M8,9, not a bad thing.

It would be a bad thing if M6 and M7 weren't buffered.

Graham
 
G

George Herold

Jan 1, 1970
0
I'm studying buffered opamp and i've found this configurationhttp://img156.imageshack.us/img156/5640/afsdafen7.png
i don't understand second statement
"reduces output load at the drains of M6 and M7"
I don't understand why a large resistance at the drains of M6 and M7
is a bad thing...if i consider voltage gain of second stage, it is
similar to
(gm6+gm7)*output resistance
so, if output resistence is high, the voltage gain of the second stage
is high and the opamp votage gain is high....i think this is a good
thing....
What's the problem?
Thanks in advance


"reduces output load at the drains of M6 and M7"
I don't understand why a large resistance at the drains of M6 and M7
is a bad thing...

A large resistance here is a good thing!! That is what the output
stage looks like to the FET's driving it. Perhaps you are confused by
what "reduces output load" means. This means an increase in the
impedance.
George
 
I'm studying buffered opamp and i've found this configurationhttp://img156.imageshack.us/img156/5640/afsdafen7.png
i don't understand second statement
"reduces output load at the drains of M6 and M7"
I don't understand why a large resistance at the drains of M6 and M7
is a bad thing...if i consider voltage gain of second stage, it is
similar to
(gm6+gm7)*output resistance
so, if output resistence is high, the voltage gain of the second stage
is high and the opamp votage gain is high....i think this is a good
thing....
What's the problem?
Thanks in advance

M8 and M9 form a simple low impedance circuit that both source and
sink base current in Q19 to reduce the miller effect, It's not rocket
science
 
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