Am

plifier: Frequency Response

Bode plot

Miller effect

High frequency response

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 RCfilter.

Figure 8.4 Bode plot for phase of the low

-pass RCfilter.

Figure 8.5 Circuit for Exam

ple 8.1.

Figure 8.6 Bode plots of the term

s on the right-hand side of Equation (8.19).

Figure 8.7 Bode plot of the m

agnitude of Av for the circuit of Figure 8.5.

Figure 8.8 Approxim

ate 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 equivalent circuit.

High frequency FET m

odel with

parasitic capacitances•

Small signal m

odel should include the parasitic capacitances of the device to determ

ine the frequency response.

Com

mon-source am

plifier.

•W

e can draw the sm

all signal equivalent circuit for the com

mon source am

plifier given below

.

Small-signal equivalent circuit of the com

mon-source am

plifier.

•U

sing same node equations as before, w

e can find the transfer function from

input to output.

•The transfer function w

ill contain poles and zeros due to the parasitic capacitances show

n.

Bode plot of voltage gain for a typical com

mon-source stage.

•C

ombined response of individual poles and

zeros would determ

ine the overall frequency response of the am

plifier.

A feedback im

pedance can be replaced by impedances in parallel w

ith the input and output terminals.

•An im

pedance Zfconnected from the input of an

amplifier to the output can be replaced by an

impedance

across the input terminals and im

pedance across the output term

inals (next).

v

fM

illerin

AZ

Z−

=1

,

1.

,−

=v

vf

Miller

outA

AZ

Z

A feedback im

pedance can be replaced by impedances in parallel w

ith the input and output terminals.

Miller Effect Applied to Feedback

Capacitance

•M

iller theorem proves very useful since it is

much easier to determ

ine poles and zeros by splitting the C

gd.•

Cgd

will reflect to input side as C

Miller = (1-

Av)Cgd (next).

Miller E

quivalent Circuit

The rp –b

model for the B

JT.

The rπ -βm

odel

Com

mon-em

itter h-parameter sm

all-signal equivalent circuit. (Note:h

ie is resistance And h

oe is conductance).

The two-port hybrid m

odel

Hybrid-p

equivalent circuit.

The hybrid-πequivalent circuit

•The resistance rx is called base-spreading resistance and accounts for the ohm

icresistance of the base

region.•

The resistance rµ accounts for the effect of base-w

idth modulation on the input characteristic.

•C

µ is the depletion capacitance of base-collector junction and C

πis base-em

itter diffusion capacitance. •

The transition frequency (unity current gain frequency) can be w

ritten as; )

(2

πµ

ππ

βC

Cr

ft+

=