Can someone help me understand what is the Miller effect? And is this effect undesirable or desirable?

Thanks.

 

Can someone help me understand what is the Miller effect? And is this effect undesirable or desirable?

Thanks.

The miller effect is UN-desirable. It quite simply is the magnification of the capacitance between the base and collector of a bipolar transistor (or the gate and drain of a FET) due to the gain of the amplifier configuration.

For example, if you have a common-emitter amplifier circuit with a voltage gain of -10 (because common emitter amplifiers always have an inverting gain), the capacitance seen at the input (the base) will be affected by the gain. If the transistor has a base-collector capacitance of 5pF, it will look like (and behave like) 50pF because it is multiplied by the gain of the amplifier.

 

I see, so in what way will it affect the transistor itself? Will it reduce it's gain?

 

It's a bit long and requires some serious thought, but this classic paper, written by the discoverer, is a monument to critical thinking!

Well worth the read.

Eric

 

I see, so in what way will it affect the transistor itself? Will it reduce it's gain?

It acts like a low-pass filter and will roll off the gain at a frequency determined by the base-collector capacitance, the circuit gain, and the input source impedance.

Note that the miller capacitance can be useful where the miller capacitance with an input resistor can be used to build an integrator.

 

There is more than one intentional use for the Miller Effect. Some IC amplifiers use it for stability at low gains, and some designs use it to limit bandwidth.

This is just a characteristic of active amplifiers and good designs always try to use the characteristics of the parts to the best advantage. Whether it's good or bad depends on what you're trying to do.

 

There is more than one intentional use for the Miller Effect. Some IC amplifiers use it for stability at low gains, and some designs use it to limit bandwidth.

This is just a characteristic of active amplifiers and good designs always try to use the characteristics of the parts to the best advantage. Whether it's good or bad depends on what you're trying to do.

It all depends on what your input & output circuits look like---sometimes
it offers just the opposite to stability,like in a TPTG Oscillator.