How to build base biased BJT amplifier on breadboard and test with PC soundcard based oscilloscope

Here a base biased BJT amplifier is constructed on a breadboard and tested with PC based sound card scope. The transistor used is the general purpose 2n3904 small signal bipolar transistor. A PC based sound card scope software is used to both generate sine wave which is fed into the BJT amplifier and also used to acquire and plot the input and output amplified signal on the PC oscilloscope.


The PC based sound card scope software generates 10mV, 1KHz sine wave at the PC speaker which is then fed using a male to male 3..5mm jack into the amplifier. The output from the amplifier is then sent back to PC using the male to male 3.5mm jack connected to the line in(mic) input of the PC. This is as shown in the figure below.

BJT amplifier on breadboard with PC line in and speaker jack

The base biased BJT amplifier alone on the breadboard is shown below.

biased BJT amplifier on breadboard

The circuit schematic for the base biased amplifier using 2n3904 is shown below.

base biased BJT amplifier schematic diagram

The calculation of the resistor values and capacitor value was shown in the tutorial How to bias a BJT using base bias. You can also use the online BJT amplifier design calculator for base biased method to calculate the resistor and capacitor values. In the amplifier shown on the breadboard above, three resistor in series 47KOhm, 33Kohm and 4.7KOhm which adds up to 84.7KOhm were used for resistor Rb because single 85KOhm was not available. Similarly, 400Ohm resistor is not available so 390Ohm was used. For the capacitor C1 with theoretical value of3.37uF was not available so a 4.7uF was used. Also for C2 4.14uF capacitor was not available so a 10uF capacitor was used. Both were electrolytic capacitors.

The soundcard scope software used in this tutorial can be obtained for private and educational use from the following download line.

 After installing the software we can use the signal generator tab to generate 10mV amplitude signal with 1KHz frequency as shown in the figure below.

signal generation using PC oscilloscope software

 After sending this signal through the bipolar junction transistor amplifier we get the following input and output signal on the oscilloscope.

input and amplified signal on oscilloscope with base biased amplifier

The following video demonstrates the working of base biased BJT amplifier with input and output signal on the PC oscilloscope.

In this tutorial we illustrated how one can build a base biased amplifier on a breadboard and how one can test the constructed amplifier using free PC oscilloscope software. Also from the above output signal waveform view and from the waveform view of the amplified signal from a voltage divider biasing method shown in the tutorial How to Build BJT amplifier and test with Soundcard based PC Oscilloscope we can observe that the signal using base biased is much more amplified than using the voltage divider biasing. But the drawback of base biased amplifier is that the output is not stable as much as in voltage divider biasing.

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