Questions on MOSFET followers.
Oct 26, 2009 at 6:55 AM Thread Starter Post #1 of 4

ph33lix

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Hi guys.
I'd like to ask for some advice for a circuit I would like to build based on figure 9b of the following article.


I realized that it is possible to bias the NPN transistors in an op-amp to class A provided that the opamp is driving a high input impedance buffer based on tangentsoft's detailed article on biasing op-amps.


My question is whether the input impedance of the mosfet follower's section is almost or equally high enough to permit biasing of the opamp into class A without going into over-current protection. And whether it is practical to do so or not.

I tried running a search here, headwize's projects page and in google, but I seem to only get articles on circuit schematics featuring mosfets and not much about what I asked above.

Many thanks in advance.
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Oct 26, 2009 at 7:47 AM Post #2 of 4

amb

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I wouldn't advise doing that to this circuit because the quiescent output voltage of the opamp is not zero, but sits at Vgs above 0 (around +4V for the IRF510). If you pull a current down to V- at the gate of the MOSFET it will affect the overall output DC offset.
 
Oct 26, 2009 at 8:03 AM Post #3 of 4

ph33lix

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Phew thanks for that. Looks like I could've messed things up.

So I guess I'm better off just putting a CCS at the source of the MOSFET as described in that headwize article.

Also, is it right to assume that the stability of the OPA627 op-amps that I am planning to implement will be dependent mainly on layout design from this point forth?
In the schematic, the low-pass in the feedback loop should theoretically eliminate any chance of HF oscillations, right?

I tried using those gems in a Cmoy circuit with a poor layout (very n00b of me) which obviously didn't work out all too well and had to add a complex rail-splitter + buffer to at least keep the rails stable. And I still had oscillations to contend with, based on the fact that the opamps felt warm to the touch.
 
Oct 26, 2009 at 8:16 AM Post #4 of 4

amb

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The stability of the opamp depends on circuit layout and supply bypassing. The compensation cap in the feedback path is more to "tune" the square wave behavior of the amp than to quell all chances of instability.
 

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