Question

What is the Miller effect? Explain how it arises in an NPN transistor?

Answer 1

Miller effect represents the increment in the equivalent input capacitance of an inverting voltage amplifier due to amplification of capacitance effects of input and output terminals. It is an extension of the Miller Theorem. A point to note is that capacitive impedence decrease at high frequencies.

Capacitances exists between the terminals of transistors due to the depletion regions on the both sides of junctions. C_EB - the emitter-base capacitance and C_CB - the collector-base capacitance decrease the gain of a common emitter amplifier circuit at higher frequencies. In a common emitter amplifier circuit, the capacitive feedback from collector to base increses C_CB by according to the Miller theorem. The amount of negative gain reducing feednack is related to both current gain and C_CB. This is called Miller Effect.

Consider an ideal inverting voltage amplifier.

Gain = A_V

Impedence between input and output node = Z

Output voltage V_o = - A_V * V_i

Being an ideal amplifier, input draws no current. All of the input current flows through the impedence Z

.

The input impedance of the circuit is

.

Z represents a capacitor with impedance

,

The resulting input impedance is

.

Thus the effective or Miller capacitance C_M is the physical C multiplied by the factor (1 + A_V).

This increment in input capacitance can reduce the bandwidth of amplifiers, restricting amplifier operration at lower ranges of frequencies.