Miller effect

Miller effect as the increase of the input capacitance of the inverting amplifier is known, which occurs due to the effective enhancement of the capacitance between the input and output of this amplifier. This effect is generally disturbing, but it can also be used to create larger effective capacitance values ​​advantageous. The effect is named after John Milton Miller, who discovered it in 1919. A generalization of the Miller effect is the Millertheorem.

Description

In the picture above, an amplifier with voltage gain A = Ua / Ue is illustrated, there is a Miller capacitance Cm between input and output. This capacity acts on the input of the amplifier with the size of Ce, which depends on the gain A, and is given by the expression

Of an inverting amplifier, the gain is negative, so that there is an increase in the capacity. It is now that A < 0:

And Millerkapaziät displayed by a factor of (1 | A | ) increases at the input.

For an arbitrary gain A can be replaced by the corresponding capacity of the Miller capacitance at the input of the amplifier, whereby the effect of the capacitance is illustrated between the input terminals. This is illustrated in the picture below.

Miller effect transistor circuits

In order to realize a voltage amplifier with transistors using the emitter connection (or at the common source field effect transistors), which operates inverting. Between the terminals of the transistors always existing parasitic capacitances such as the collector- base capacitance CCB the bipolar transistor or the gate -drain capacitance in the MOSFET, so that the Miller effect occurs here.

In the picture an emitter circuit is shown, with a Miller capacitance Cm was explicitly drawn. This capacity is representative of the existing parasitic capacitance of the transistor. This appears due to the Miller capacitance effect to the amount of amplification at the input plus one more.

The effective capacitance at the input of the amplifier Cin is therefore given by the expression

Where Cm represents the collector -base capacitance in this case, and A is the gain of the emitter circuit, which is negative.

This effect is most undesirable because it limits the use of the emitter or source connection of low to medium frequencies. In operational amplifiers, however, this effect is used for easy frequency compensation. In power MOSFETs (power MOSFET ) and ring oscillators, the effect increases the switching time considerably, which ultimately results in a low cut-off frequency.

A cascode circuit is an effective way to suppress the Miller effect because the cascode reduces the feedback of the amplifier output to the input.

If an inductor connected to the amplifier output, the Miller capacitances may cause oscillation instead of a negative feedback. See Miller effect oscillator.