# Buffered Opamp

Discussion in 'Electronic Design' started by [email protected], Sep 17, 2008.

1. ### Guest

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?

2. ### Phil AllisonGuest

** 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

...... Phil

3. ### EeyoreGuest

Then you need to read up on what a source follower does. Or emitter
follower in the case of bipolar. It's very important.

Graham

4. ### EeyoreGuest

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

Graham

5. ### George HeroldGuest

"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

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

6. ### Guest

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