Miller Effect in Amplifier Frequency Response:

According to this effect, when viewed from the input base terminal of the CE-connected transistor the capacitance C_cb appears as (1 – A_v)C_cb i.e., it is amplified by a factor of (1 – A_v). In fact Miller effect takes into account the feedback from the collector to base and vice versa due to C_cb.

When input signal of voltage V_in is applied to the base of the transistor, the change in collector voltage,

The negative sign is due to phase shift inherent in a CE amplifier. As seen, collector voltage V_C is reduced by (A_vV_in) when base voltage is increased by V_in. Thus total reduction in collector-base voltage is

This also represents the change of voltage across C_cb, the capacitance between collector and base. Now charge supplied to the input of the circuit,

Thus, “looking into” the base, it is seen that the collector-base capacitance appears to be (1 – A_v)C_cb; i.e., the capacitance is amplified by a factor of (1 – A_v). This amplification is known as the Miller effect.

The total input capacitance C_in to the transistor is (1 – A_v)C_cb in parallel with C_be. Thus

At high frequencies, the value of C_in reduces the input impedance Z_in of the circuit and affects the frequency response.

Noteworthy point is that Miller effect occurs only with a circuit in which there is a phase reversal such as in CE configuration. It does not occur in CB and CC configurations.

In the same way, the device inter-terminal capacitances (C_gs, C_gd and C_ds) are important quantities in determination of the performance of a FET circuit, and these are used in the same way as the BJT capacitances. The input capacitance is amplified by the Miller effect in the case of a common source circuit (an inverting amplifier). Equation (15.53) may be written in the FET circuit

where C_M is the Miller capacitance and is equal to (1 – A_v)C_gd.