Low frequency response High frequency response Miller ...

Amplifier: Frequency Response

ƒ Bode plot
ƒ Miller effect
ƒ High frequency response
ƒ Low frequency response

Figure 8.1 Low-pass RC filter.

Figure 8.2 Logarithmic frequency scale.

Figure 8.3 Bode plot for the low-pass RC filter.

Figure 8.4 Bode plot for phase of the low-pass RC filter.

Figure 8.5 Circuit for Example 8.1.

Figure 8.6 Bode plots of the terms on the right-hand side of Equation (8.19).

Figure 8.7 Bode plot of the magnitude of Av for the circuit of Figure 8.5.

Figure 8.8 Approximate plots of the terms of Equation (8.20).

Figure 8.9 Bode phase plot of the voltage-transfer function for the circuit of Figure 8.5.

High frequency FET model with
parasitic capacitances

• Small signal model should include the
parasitic capacitances of the device to
determine the frequency response.

High-frequency FET equivalent circuit.

• We can draw the small signal equivalent
circuit for the common source amplifier given
below.

Common-source amplifier.

• Using same node equations as before, we
can find the transfer function from input to
output.

• The transfer function will contain poles and
zeros due to the parasitic capacitances
shown.

Small-signal equivalent circuit of the common-source amplifier.

• Combined response of individual poles and
zeros would determine the overall frequency
response of the amplifier.

Bode plot of voltage gain for a typical common-source stage.



• An impedance Zf connected from the input of an Z
amplifier to the output can be replaced by anZin,Miller = −
impedance 1 f
across the input terminals Zf .Av
Av −1 Av

and impedanZcouet,Miller =

across the output terminals (next).

A feedback impedance can be replaced by impedances in parallel with the input and output terminals.

A feedback impedance can be replaced by impedances in parallel with the input and output terminals.

Miller Effect Applied to Feedback
Capacitance

• Miller theorem proves very useful since it is
much easier to determine poles and zeros by
splitting the Cgd.

• Cgd will reflect to input side as CMiller= (1-
Av)Cgd (next).

Miller Equivalent Circuit

The rπ-β model

The rp– b model for the BJT.

The two-port hybrid model

Common-emitter h-parameter small-signal equivalent circuit. (Note: hie is resistance And hoe is conductance).

The hybrid-π equivalent circuit

• The arecsciosutanntsceforrx is called base-spreading resistance
and the ohmic resistance of the base
region.

• Twhidethremsiosdtaunlacteiornµ accounts for the effect of base-
on the input characteristic.

• CjuµnicstitohneadnedplCeπtioisn capacitance of base-collector
base-emitter diffusion capacitance.

• Tfrehqeuteranncsyi)tiocnanftfr=be2eqπurwπe(Crnβiµtct+eyCn(π u)ansi;ty current gain

Hybrid-p equivalent circuit.