Analog electronics module two full note slides

It is analog electronics module 2

1. Frequency Response of Amplifier

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ain

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‘<—— Bandwidth ————,

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Lower cutoff Upper cutoff
frequency frequency

Typical frequency response of an amplifier

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Frequency response of an amplifier is the
graph of its gain V/s the frequency.

+ Cut-off frequencies : the frequencies at which
the voltage gain equals 0.707 of its maximum
value.

+ Mid band : it is the range of frequencies over
which device gain is constant

+ Bandwidth : the band between upper and

lower cut-off frequencies

2. Frequency Response of Amplifier

(Neh
,) |Lowtequency High-frequency
E Tange range
i = Midband range =
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3 Ans Le el
sal Gain falls of due to the effects \\?; gy
E of stray capacitance and
h transistor capacitance effects
3 Gain falls of due to the
) effects of Cc and Ce !
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BW=f,-f, (log scale)
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Phase nlot of RC coupled amolifier

Frequency is specified in logarithmic scale and gain in dB

V,
Gain in dB = 20108 | LE

Vin

Most amplifiers introduce a 180° phase shift between

input and output signals. This is the case only in the

midband region.

At low frequencies, there is a phase shift such that V,

lags V; by an increased angle.

At high frequencies, the phase shift will drop below 180°

At higher and lower frequencies the gain of the amplifier

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Low-frequency High-frequency

Gain falls of due to the effects
of stray capacitance and
transistor capacitance effects

Gain falls of due to the
effects of C¿ and C;

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Phase plot of RC coupled amplifier

+ Outside the midband, the voltage gain can be determined

by these equations:
A= Ania A Ania
A= = 4 mE;
14+(f/f) V4(f/f)
Below mid band Above mid band

+ At low frequency range, the gain falloff due to coupling
capacitors and bypass capacitors.

+ As signal frequency decreases , the reactance of the
coupling capacitor, X, increases - no longer behave as short

circuits.

4. Low-Frequency Response (BJT) Amplifier

Circuit diagram of CE amplifier

In the analysis of the voltage-divider BJT, it will
simply be necessary to find the appropriate
equivalent resistance for the RC combination.
Capacitors Cs, CG, C will determine the low-
frequency response.

Considering the effect of each capacitor
separately, the lower cut-off frequency of a given
common emitter amplifier will be given by the
highest of the individual transistor terminal

circuits

5. -Frequency Response (BJT) Amplifier

« Effect of Cs: the general form of the R-C
configuration is established by the network of
the following Figure.

R, = RyllRallhie
1
2m(R; + RNCS

his

Circuit diagram of CE amplifier

6. -Frequency Response (BJT) Amplifier

+ Effect of C¿: the general form of the R-C
configuration is established by the network of
the following Figure.

Ro = Rello

1
ni + Role

fic

Circuit diagram of CE amplifier

7. -Frequency Response (BJT) Amplifier

+ Effect of C¿: the general form of the R-C
configuration is established by the network of
the following Figure.

(Ri/B) tr.

Re |

R
R= Rell|7 +7

Rs = RsllRl|R,
1
Re,

Circuit diagram of CE amplifier fu

8. -frequency hybrid-pi model

€, = Che + Cm Cu _ Cocl1 + Ap)

1
Co = Coe + Cuz Cm = Coe ( a +)

9. -frequency hybrid-pi model

Raw Rel RI Ral IR, Rano=Rel IRI IF
fn ae = Che + Cpell + An)
4 & fem mx , Co= Cet Cre ( +)
Fa = min fui fro)
Thevenin equivalent circuit Thevenin equivalent circuit

for the input network for the output network

10. M KTUASSIST.IN

High-Frequency Response (BJT Amplifier )

When the reactance of C,, becomes small enough, a
significant amount of the signal voltage is lost due to a
voltage-divider effect of the source resistance and the
reactance of C,..

When the reactance of C,. becomes small enough, a
significant amount of output signal voltage is fed back
out of phase with the input (negative feedback), thus
effectively reducing the voltage gain.

11. High-Frequency Response (BJT Amplifier )

At high frequencies, internal transistor junction

capacitances do come into play, reducing an amplifier's

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gain and introducing phase shift as the signal frequency

increases.

+ At lower frequencies, the internal capacitances have a

very high reactance because of their low capacitance
value and the low frequency value, Therefore, they look
like opens and have no effect on the transistor's

performance.