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View Full Version : Can anyone simply explain the Miller Effect?



terenkleo
02-20-2012, 09:33 PM
In my microelectronics class we are studying how to make a "Miller Approximation." I can understand what to do with the circuit, but I don't know exactly what *IS* the Miller Effect, and when is it appropriate to make a Miller Approximation.

My only request is that you explain it in your own words, without linking to Wikipedia or anything like that. Thank you!

jmg
02-20-2012, 09:59 PM
What part of Wiki do you not understand ?
I'll admit it is somewhat verbose.

I prefer to use just i = c*dV/dT, and that shows that a simple shunt C, on the input, appears as (say10pf), but when you flip that same value as a Ip-Op leakage cap on an amplifier, you now have a magnified slew rate, so dV/dT is higher and thus the 'apparent' C is larger
- you can model that as a larger C value, to ground, if you wish.

Phil Pilgrim (PhiPi)
02-20-2012, 10:36 PM
I'll try. Consider the following transistor inverting amplifier:

http://forums.parallax.com/attachment.php?attachmentid=89740&d=1329776174

A transistor exhibits capacitance at both the collector-base junction (Ccb) and the base-emitter junction (Cbe). A rising voltage on the input will produce a falling voltage on the output whose rate of change is proportional to the gain (G) of the transistor. But, due to feedback through Ccb, the falling voltage on the collector will counteract the rising voltage on the base, thus reducing the amplifier's overall gain. The faster the input changes, the more effect this negative feedback will have, since it's capacitively coupled through Ccb. Because the rate of voltage change on the collector that drives the feedback is amplified by the transistor's gain, the effect will be the same as if a smaller voltage change were being fed back through a much larger capacitor. In fact, it's as if a voltage change equal to the input voltage change were being fed back through a capacitance G times as big as Ccb. This apparent magnification of Ccb is the "Miller effect" and is equivalent to replacing Ccb with a capacitor equal to (G + 1)*Ccb (the "Miller capacitance") in parallel with Cbe.

'Make sense?

-Phil

bill190
02-20-2012, 11:19 PM
FYI - When I want to read what people have to say about something, I search google.com with the words along with the word FORUM. Then this will bring up forum web pages where someone is discussing that. So in this case, search for the following words including quotes...

"Miller Effect" forum

I mainly use that trick for my "consumer reports". I search for a product and the word forum, then I can read what people have to say about it.

Or use the word book to find books on a topic.
Or use the word cart to find where I can buy something.