BJT and JFET

1
Chapter 4

• BJT and JFET
• Frequency Response

2
Contents

• Logarithm and dB
• Low frequency analysis-Bode plot
• Low frequency response-BJT and FET amplifiers
• Miller effect capacitance
• High frequency response-BJT and FET amplifiers
• Multistage frequency effects
• Square wave testing

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Low Frequency Response BJT Amplifier
At low frequencies Coupling capacitors (Cs, CC)
and Bypass capacitors (CE) will have capacitive
reactance (XC) that affect the circuit impedances.
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Coupling Capacitor - CS

• The cutoff frequency due
• to CS can be calculated
• using

5
Coupling Capacitor - CC

• The cutoff frequency due
• to CC can be calculated
• using

6
Bypass Capacitor - CE

• The cutoff frequency due to CE
• can be calculated
• using
• where

7
Example

• Determine the lower cutoff freq. for the network
  of Fig. 1 using the following parameters
• Cs 10µF, CE 20µF, Cc 1µF
• Rs 1KO, R1 40KO, R2 10KO,
• RE 2kO, RL 2.2KO,
• ß 100, r0 8O, Vcc 20V
• Sketch the frequency response using a Bode plot

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Bode Plot of Low Frequency Response BJT
Amplifier
The Bode plot indicates that each capacitor may
have a different cutoff frequency. It is the
device that has the highest of the low cutoff
frequency (fL) that dominates the overall
frequency response of the amplifier (fLE).
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Roll-off of Gain in the Bode Plot
The Bode plot not only indicates the cutoff
frequencies of the various capacitors it also
indicates the amount of attenuation (loss in
gain) at these frequencies. The amount of
attenuation is sometimes referred to as
roll-off. The roll-off is described as dB
loss-per-octave or dB loss-per-decade.
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-dB/Decade
-dB/Decade refers to the attenuation for every
10-fold change in frequency. For Low Frequency
Response attenuations it refers to the loss in
gain from the lower cutoff frequency to a
frequency 1/10th the lower cutoff frequency. In
the above drawn example fLS 9kHz gain is
0dB fLS/10 .9kHz gain is 20dB Therefore the
roll-off is 20dB/decade. The gain decreases by
20dB/Decade.
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-dB/Octave
-dB/Octave refers to the attenuation for every
2-fold change in frequency. For Low Frequency
Response attenuations it refers to the loss in
gain from the lower cutoff frequency to a
frequency 1/2 the lower cutoff frequency. In the
above drawn example fLS 9kHz gain is 0dB fLS/2
4.5kHz gain is 6dB Therefore the roll-off is
6dB/octave. This is a little difficult to see on
this graph because the horizontal scale is a
logarithmic scale.
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Low Frequency Response FET Amplifier
At low frequencies Coupling capacitors (CG, CC)
and Bypass capacitors (CS) will have capacitive
reactances (XC) that affect the circuit
impedances.
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Coupling Capacitor - CG

• The cutoff frequency due
• to CG can be calculated
• using

14
Coupling Capacitor - CC

• The cutoff frequency due
• to CC can be calculated
• using

15
Bypass Capacitor - CS

• The cutoff frequency due to CS can
• be calculated
• 
  
• where
• using

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Example

• Determine the lower cutoff freq. for the network
  of Fig. 2 using the following parameters
• CG 0.01µF, Cc 0.5µF, Cs 2µF
• Rsig 10KO, RG 1MO, RD 4.7KO,
• RS 1kO, RL 2.2KO, IDSS 8mA,
• Vp -4V, rd 8O, VDD 20V
• Sketch the frequency response using a Bode plot

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Bode Plot of Low Frequency Response FET
Amplifier
The Bode plot indicates that each capacitor may
have a different cutoff frequency. The
capacitor that has the highest lower cutoff
frequency (fL) is closest to the actual cutoff
frequency of the amplifier.
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Miller Effect Capacitance
Any P-N junction can develop capacitance. This
was mentioned in the chapter on diodes. In a BJT
amplifier this capacitance becomes noticeable
between the Base-Collector junction at high
frequencies in CE BJT amplifier configurations
and the Gate-Drain junction at high frequencies
in CS FET amplifier configurations. It is
called the Miller Capacitance. It effects the
input and output circuits.
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Miller Input Capacitance (CMi)
It can be calculated Formula 11.42 Note
that the amount of Miller Capacitance is
dependent on interelectrode capacitance from
input to output (Cf) and the gain (Av).
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Miller Output Capacitance (CMo)
It can be calculated Formula 11.43 If
the gain (Av) is considerably greater than 1
Formula 11.44
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High-Frequency Response BJT Amplifiers
Capacitances that will affect the high-frequency
response Cbe, Cbc, Cce internal
capacitances Cwi, Cwo wiring capacitances
CS, CC coupling capacitors CE bypass
capacitor
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High-Frequency Cutoff Input Network (fHi)
using and
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High-Frequency Cutoff Output Network (fHo)
using and
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High-Frequency Response FET Amplifier
Capacitances that will affect the high-frequency
response Cgs, Cgd, Cds junction
capacitances Cwi, Cwo wiring capacitances
CG, CC coupling capacitors CS bypass
capacitor
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High-Frequency Cutoff Input Network (fHi)
using and
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High-Frequency Cutoff Output Network (fHo)
using and
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Multistage Frequency Effects
Each stage will have its own frequency response.
But the output of one stage will be affected by
capacitances in the subsequent stage. This is
especially so when determining the high frequency
response. For example, the output capacitance
(Co) will be affected by the input Miller
Capacitance (CMi) of the next stage.
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Total Frequency Response of a Multistage Amplifier
Once the cutoff frequencies have been determined
for each stage (taking into account the shared
capacitances), they can be plotted. Again note
the highest Lower Cutoff Frequency (fL) and the
lowest Upper Cutoff Frequency (fH) are closest
to the actual response of the amplifier.
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Square Wave Testing
In order to determine the frequency response of
an amplifier by experimentation, you must apply a
wide range of frequencies to the amplifier. One
way to accomplish this is to apply a square wave.
A square wave consists of multiple frequencies
(by Fourier Analysis it consists of odd
harmonics).
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Square Wave Response Waveforms
If the output of the amplifier is not a perfect
square wave then the amplifier is cutting off
certain frequency components of the square wave.