laboratory activity 1: frequency analysis

8/10/2019 Laboratory Activity 1: Frequency Analysis

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Procedure:

1. Set-up the designed Voltage divider circuit into a breadboard.

2. Measure the current(Ie) flowing thru Re, by measuring its voltage and applying Ohms law.

3.

Calculate reby using this formula re=.

4. Connect the Oscilloscope channel 1 to Rb2 and to the ground.

5. Connect the Oscilloscope channel 2 to load(RL) to the ground.

6. Observe the graph displayed in the Oscilloscope.

7.

Get the mid frequency, low cut-off frequency and high cut-off frequency by adjusting the input of

the function generator.

8. Compare the results on the experiment to the calculation.

9.

Draw conclusions from the observations.

Calculations:

DC Analysis:

Rth= Rb1||Rb2= 470k|| 330k Vth=

=

Rth= 193875 Vth= 3.7125V

Ib

Ie= ( + 1) Ib = ( + 1)(12.48 A)

Ib= 12.48 A Ie= 1.26 mA

re=

=

re= 20.63


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AC Analysis:

Vi= ( ib) ( ( + 1)(Re)+ ( re) )= ( ib) ( ( + 1)(470 )+ ()20.63 )

Vo= (Rc || Rl)ib= (1.2k || 500 )ib

Av=

=

=

Av= 0.7125

DATA Gathered:

Circuit: Low cut-off frequency:

Mid frequency: High cut-off frequency:

Table 1: Cut-off frequency

Frequency Vi VoLow cut-off frequency 62.97Hz 800mV 340mV

Mid frequency 25.77kHz 760mV 568mV

High cut-off frequency 1.92MHz 520mV 352mV


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Analysis:

In a voltage divider circuit, if we change at least one of the values of the resistors then it will

have an effect to the result of the experiment. In most cases if we want to change the output to a

certain degree, like changing its Av, we change the value of Rc because as we observed it is the most

sensitive one.

Having a load(RL), would mean a lower value of Av based on the formula:

( )(with RL) and

(without RL). So as we can see, both have

the same input but have different output and from that we can say that (Rc||RL)ib ( Rc)ibso a voltage

divider with RLhas a lesser Av compared to a one without.

In finding the value of the capacitor to be used, we were told to assume that the low cut-off

frequency is 100Hz.

By using this formula C=

we can get the value for each capacitor:

Cc=

=

=

=1f

Ci=

=

=

=1f

From our observation if we assumed the low cut-off frequency into another value, lets say

1kHz, the values of the capacitors would vary and eventually the measured low cut-off frequency of

our data will be closer to 1kHz.


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Conclusion:

From this experiment I learned that cut-off frequency is dependent to the values of the

capacitors and resistors used. I also confirmed that our hypothesis was shown to be true which is that

the assumed low cut-off frequency is close to that of the result.

It is possible to have errors due to the materials being used. For example, malfunctioning

oscilloscope, malfunctioning function generator, malfunctioning DC power supply, or any other

possible deficiency which can affect the result of the experiment.

laboratory activity 1: frequency analysis

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