Question

what is parasitic capacitance and miller capacitance and how to overcome it?

Answer 1

Parasitic capacitance is an unwanted capacitance that exists between electronic components.

Generally when two electrical conductors which are having different voltage levels are closed together ,then the electric field between them causes electric charges which is called as parasitic capacitance.

The parasitic capacitance between the turns of the inductor is termed as self capacitance.

At low frequencies the parasitic capacitance is neglected where as in high frequencies it is a major problem.

The following are the methods for reducing parasitic capacitance

1. Avoiding parallel routing of metals

2. Using higher metals

In 1st method , parallel routing means one metal on another metal completely i.e Signal A (Metal 2) is routed over signal B (Metal 1) in parallel, and causes a large area between the plates A and B, which in turn causes higher parasitic capacitance between the plates as shown below   With parallel routing there is a maximum area between the two metals so there is maximum capacitance between the two as well. Hence to avoid that the below figure is used

In 2nd method , the parasitic capacitance between higher metal and the lower metals into account i.e. parasitic capacitance between signal A and other signals (B, C, D, E, F etc.) By using higher metal we are actually increasing the distance between the plates which ultimately decreases the parasitic capacitance as shown below

Similary the Miller capacitance normally refers to capacitance between anode and cathode .In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals.

In order to overcome miller capacitance the following methods are used

1. Current buffer stage may be added at the output to lower the gain I.e in  between the input and output terminals of the amplifier. For example, a common base may be used as a current buffer at the output of a common emitter stage. This will typically reduce the Miller effect and increase the bandwidth of the amplifier. 2. Alternatively, a voltage buffer may be used before the amplifier input, reducing the effective source impedance seen by the input terminals which lowers the time constant of the circuit and typically increases the bandwidth.