# No Miller effect in differential amplifiers

Why is there no Miller effect in differential amplifier and cascode circuits? (from the book Art of Electronics)

• understand what Miller effect is, and look at the voltage swings around the input devices in these arrangements, and you might see the answer...
– user16324
Commented Jan 24, 2016 at 18:42
• Using cascode configuration substantially reduces Miller effect. Some datasheets show internal circuit of the opamp, upon inspection you will notice these cascodes. Commented Jan 24, 2016 at 18:44
• Which edition, what chapter/page? Commented Jan 24, 2016 at 18:45

Brian provided a great hint, but here's some more info for reference:

The Miller effect says that if a capacitor is connected between two nodes with a negative voltage gain between them (-A), then the input capacitance looking into the input node will be approximately A times bigger than the capacitance between the nodes.

As a rough analogy think of adding water to a swimming pool with a bucket (That's the input node). At the same time somebody on the other end is taking water out with a bigger bucket (the node pulling the other end of the capacitor in the opposite direction of your input). Now you need a lot more water to fill the pool.

In a cascode (bipolar for example) you're using a common base stage to keep the collector voltage of the common emitter stage relatively constant. So the voltage across the base-collector capacitance isn't changing nearly as much as it would if the collector weren't clamped by the cascade device. "A" is close to 1. Therefore the Miller effect is greatly reduced.

• From the above answer one could conclude that the MILLER effect would be relevant for capacitances only. However, this effect reduces not only a capacitive but also a resistive path between two points with a negative gain in between.
– LvW
Commented Jan 25, 2016 at 8:02

Regarding the differential amplifier, there is no MILLER effect if (a) one collector node (Q1) is directly connected to the supply voltage and (b) because the other transistor (Q2) can be seen to work in common-base configuration. This is similar to the cascode stage where the upper transistor also works in common base configuration.

Note that the whole diff. amplifier circuit can be regarded as a series connection of a common collector and a common base stage. By the way, this view is a good method for evaluating the gain properties of the circuit.