Electronics II Lab
EELE 3120
Experiments #4
Frequency Response of BJT
1) Objectives:
To study the frequency response and bandwidth of the common emitter CE-BJT, the
common collector CC-BJT, and the common base CB-BJT amplifiers.
2) Introduction:
Most amplifiers have relatively constant gain across a range or band of frequencies, this
band of frequencies is referred to as the bandwidth (BW) of the circuit.
Bandwidth means the difference between the upper and lower frequencies of the
frequency response as shown in the figure 4.1
Figure 4.1: the frequency response of BJT
When operated within its bandwidth, the values of 𝐴𝑣 & 𝐴𝑖 for an amplifier are calculated
as shown earlier, these values are referred to as mid-band gain values, and are
designated as 𝐴𝑣𝑚𝑖𝑑 & 𝐴𝑖𝑚𝑖𝑑 .
The frequency response is a graphical representation of the relationship between
amplifier gain and operating frequency for input signal, as an example sinusoidal signal.
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Electronics II Lab
EELE 3120
So why we need this type of analysis for an amplifier, because we need to determine the:
-
To determine the stability region for amplifier systems.
To design an amplifier systems that met the required specifications.
3) AC & DC Analysis:
a) DC Analysis:
As we know the frequency in DC is equal zero because there is no oscillation in DC
signal so, the reactance of capacitor is equal infinity that mean the capacitors work as
open circuit.
b) AC Analysis:
In this analysis we will study the effect of capacitors in different level of frequencies
as:



Low frequency
Middle frequency
High frequency
Before we start our analysis we need first know the names of capacitors and its
locations, there are two type of capacitor:


Practical capacitors: Cin, Cout, CE these capacitors are connected as shown in
figure 4.1 and it’s effective in low frequency.
Virtual capacitors: Cwi, Cwo, CBE, CBC, CCE these capacitors are appears and
effectives on the high frequency because its value in Nano-farad.
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Electronics II Lab
EELE 3120
Cases of capacitor:



At very low frequency the capacitor work as open circuit.
At very high frequency the capacitor work as short circuit.
At normal frequency the capacitor have a magnitude and effect on the
frequency response.
𝑋𝑐 =
1
2𝜋𝑓𝑐
Now, we will show the relationship between the voltage gain and varying frequency
and defined the regions on this curve as shown in figure 4.2
Figure 4.2: The frequency response of BJT amplifier
Figure 4.3: The sample circuit diagram of BJT amplifier
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Electronics II Lab

EELE 3120
Low frequency response of BJT amplifier: (𝟎 → 𝑭𝒍𝒐𝒘 )
At this region all capacitors will be open circuit because its reactance will be
equal infinity at 𝑓 = 0 and 𝐴𝑣 = 0.
Now, if the frequency increase slightly the voltage gain will be increase also until
the frequency reach to low cut frequency, after this critical point the capacitors
will be short circuit and voltage gain raise to the maximum value (𝐴𝑣(𝑚𝑖𝑑) ).
1- 𝐶𝑖𝑛 𝑎𝑡 𝐶𝑜𝑢𝑡 & 𝐶𝐸 short circuit:
𝑓𝑙𝐶𝑖𝑛 =
1
2𝜋(𝑅𝑠 + 𝑍𝑖)𝐶𝑖𝑛
Where: 𝑍𝑖𝑛 = 𝑅1//𝑅2//𝐵𝑟𝑒
2- 𝐶𝑜𝑢𝑡 𝑎𝑡 𝐶𝑖𝑛 & 𝐶𝐸 short circuit:
𝑓𝑙𝐶𝑜𝑢𝑡 =
1
2𝜋(𝑅𝑜 + 𝑅𝑙)𝐶𝑜𝑢𝑡
3- 𝐶𝐸 𝑎𝑡 𝐶𝑖𝑛 & 𝐶𝑜𝑢𝑡 short circuit:
𝑓𝑙𝐶𝐸 =
1
𝑅`𝑠 + 𝐵𝑟𝑒
2𝜋 (𝑅𝐸 +
) 𝐶𝐸
𝐵
Where: 𝑅`𝑠 = 𝑅𝑠//𝑅1//𝑅2

High frequency response of BJT amplifier: (𝑭𝒉𝒊𝒈𝒉 → ∞)
In this region will be appear the effect of virtual capacitors which called parasitic
effect (𝐶𝑤𝑖, 𝐶𝑤𝑜, 𝐶𝐵𝐸, 𝐶𝐶𝐸, 𝐶𝐶𝐵), and its value in Nano-farad.
These capacitors at very high frequency will be short circuit and it reactance equal
zero, and the voltage gain goes to zero.
In this region all practical capacitors are short circuit also.
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Electronics II Lab
EELE 3120
1- At 𝐶𝑖𝑛, 𝐶𝑜𝑢𝑡, 𝐶𝐸 are short
circuit:
𝐶𝑖 = 𝐶𝑤𝑖//𝐶𝐵𝐸//𝐶𝑚𝑖
𝐶𝑚𝑖 = 𝐶𝐵𝐸[1 − 𝐴𝑣]
𝐶𝑖 = 𝐶𝑤𝑖 + 𝐶𝐵𝐸 + 𝐶𝑚𝑖
𝐶𝑜 = 𝐶𝑤𝑜//𝐶𝐶𝐸//𝐶𝑚𝑜
𝐶𝑚𝑜 = 𝐶𝐵𝐶[1 −
1
]
𝐴𝑣
𝐶𝑜 = 𝐶𝑤𝑜 + 𝐶𝐶𝐸 + 𝐶𝑚𝑜
2- High frequency law:
𝑓ℎ𝑖 =
1
2𝜋𝑅𝑡ℎ𝑖 ∗ 𝐶𝑖
𝑓ℎ𝑜 =
1
2𝜋𝑅𝑡ℎ𝑜 ∗ 𝐶𝑜
𝑅𝑡ℎ𝑖 = 𝐵𝑟𝑒//𝑅1//𝑅2//𝑅𝑠
𝑅𝑡ℎ𝑜 = 𝑅𝐿//𝑅𝑐//𝑟𝑜
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Electronics II Lab
EELE 3120
Before we are going to lab work we need to know some important definitions
which we need them in design.
Logarithm:
𝑥 = log 𝑏 𝑎 , 𝑎 = 𝑏 𝑥
Why we need log scale? To represent a large scale of frequency start from 𝐻𝑧 and
goes to 𝑀𝐻𝑧.
There are two modes of log scale:
-
Semi Log: that mean the x-axis is logarithm increment and y-axis is linear
increment.
Double Log: that mean the x-axis and y-axis are logarithm increment.
Decibels: (dB)
1 𝑑𝐵 = 10 log10
𝑝2
𝑝1
1 𝑑𝐵 = 20 log10 𝐴𝑣
Notes:
-
To find the Low cut off frequency 𝑓𝐿 = 𝑀𝐴𝑋(𝑓𝐿𝑐𝑖𝑛 , 𝑓𝐿𝑐𝑜𝑢𝑡 , 𝑓𝐿𝑐𝑒 ).
To find the upper cut off frequency 𝑓𝐻 = 𝑀𝐼𝑁(𝑓𝐻𝑖 , 𝑓𝐻𝑜 )
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Electronics II Lab
EELE 3120
4) Lab work:
Figure 4.4: Common Emitter Amplifier
1- Connect the circuit in figure 4.4.
2- Adjust the DC power supply at 20Vdc.
3- Adjust the function generator to sinusoidal of 𝑉𝑝 = 1𝑉𝑎𝑐 at a frequency 1𝐾𝐻𝑧.
4- Measure the output voltage Vo RMS value (AC mode in Multi-meter device).
5- Calculate 𝐴𝑣𝑚𝑖𝑑 & 0.707𝐴𝑣𝑚𝑖𝑑 which be equal 0.707𝑉𝑜 because the input voltage
equal 1𝑉𝑎𝑐.
6- Now decrease the frequency of input signal to get 𝑉`𝑜 = 0.707𝑉𝑜 on Multi-meter
screen to find 𝑓𝐿 .
7- Then increase the frequency of input signal to get 𝑉`𝑜 = 0.707𝑉𝑜 on Multi-meter
screen to find 𝑓𝐻 .
8- Now calculate the Bandwidth 𝐵𝑊 = 𝑓𝐻 − 𝑓𝐿 .
9- Now varying the frequency of function generator according to table 1 shown below
and measure the variable on each level.
10- Plot the voltage gain 𝐴𝑣 relative to frequency.
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Electronics II Lab
EELE 3120
Frequency (Hz)
Vin (RMS)
Vo (RMS)
𝑨𝒗 = 𝑽𝒐⁄𝑽𝒊𝒏
1
10
100
1k
10k
50k
100k
1M
2M
5M
Table 1: Common Emitter Amplifier result
5) Exercise:
1- Repeat all steps for common emitter amplifier using Orcad.
2- Repeat all steps for emitter follower amplifier which shown below using Orcad.
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