Buffered Opamp

Sep 17, 2008 5 Replies

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idkfaidkfaidkfa 17 years ago

I'm studying buffered opamp and i've found this configuration

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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

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Phil Allison 17 years ago

** 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

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John Popelish 17 years ago

"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.

Regards, John Popelish

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John Popelish 17 years ago

By the way, "reduces output load" refers to current, not resistance.

Regards, John Popelish

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George Herold 17 years ago

"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

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hahn.alan 17 years ago

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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