BJT Power Amplifier

Developed and Written by Marcus Haigh (Engineering Technician The University of Sheffield)

Bi-Polar Junction Transistors (BJT), normally referred to as just ‘Transistors’ can be used to amplify small signals. To do this they need to be configured with support circuitry, to provide either voltage gain, current gain or both. A circuit that can do both is referred to as a Power Amplifier. Amongst other applications the Power Amplifier is predominantly used in audio systems. Where a small audio signal is amplified to a level capable of driving loudspeakers.
In this exercise you will build a power amplifier that will allow an MP3 player to drive a bookshelf loudspeaker. The chosen physical circuit is shown in Fig 1 and the circuit diagram is in Fig 2.

The objectives of this exercise are:

To understand the basic operation of this circuit.
To build and test a working power amplifier.
Gain more practical experience using lab apparatus, such as oscilloscopes, signal generators and breadboards.

This exercise is a step by step guide. A background knowledge of transistors /amplifiers is not needed. However it is assumed you are familiar with lab bench test equipment (power supplies, oscilloscopes etc).

Fig 1

Fig 2

Basic Operation

Fig 3 shows the full circuit of the power amplifier. The circuit achieves power amplification by boosting the voltage and the current of the ‘Audio IN’ signal. The voltage is increased by the first stage (highlighted in blue) and the current capability increased by the second stage (highlighted in red). The robust signal is then output to the ‘8 Ohm Speaker’.
The circuit itself is active, so it needs its own power supply. This is indicated on the circuit at the top with ‘+Vcc’ and at the bottom by the ‘0V’ and ground symbol. This is showing that the circuit requires a DC supply (positive and ground).

Fig 3

The example circuit in Fig 4 is called a Common Emitter Amplifier. It is basically the same as the first stage in our circuit. To briefly explain its function, Transistor Q1 turns a small current provided by R1 and R2 into a much bigger current travelling through resistor Rc. Rc then presents a voltage. This voltage is the power supply voltage shared with transistor Q1 and Resistor Re. For a C.E amplifier, the circuit designer, when no input signal is present, will choose Rc’s value so Rc and Q1/Re will equally share the voltage (half of the power supply each).
So ‘Vout’ (Fig.4) is now at this mid point in voltage. Being at halfway, then allows a small input signal ‘Vin’ (Fig.4) to push this larger voltage up or down in an equal range. The magnitude of the large voltage shifts compared to the small voltage shifts of Vin is the voltage gain of the circuit. This is determined by the Q1’s own current gain and the values of Rc and Re. In our circuit all these calculations have been done for you (see Fig 6).

Fig 4

Loudspeakers have very low resistance (called impedance as their resistance changes with frequency). The output of the first stage has more resistance than the speaker. Therefore the speaker will need more current than the first stage can offer.
The circuit in Fig 5 is an example of the second stage circuit. It is called a Push/Pull Amplifier. It uses two BJT transistors connected in Common Collector configuration. In this mode they do not create any voltage gain, they can only pass on the voltage level from the first stage. But they do have very low output impedance. Meaning that they can supply a lot more current to keep up with the speaker’s demand. Hence, a current amplifier.
When there is an input signal at the first stage (‘Vin’ Fig.4), the resulting large voltage swings across Rc and Q1/RE (in Fig.5 shown as R, D1,D2 and lower R) impress against T1 and T2. This voltage swing carries through to the speaker and the two Common Collector transistors take care of the current needs. The push pull circuit uses two opposite polarity transistors (NPN and PNP), so one can supply current for the positive half of the voltage shift and the other for the negative half of the shift.

Fig 5

Exercise Part 1: Build The Circuit

Check you have all of the components in your kit using the diagram in Fig 6. Identify all of the components by their part numbers or the coour code chart for the resistors.
Build the circuit in Fig 6 using a breadboard provided. If you have not used a breadboard before, you can copy the example photo (Fig 11) if you wish but still check the connections from circuit diagram. Do not power the circuit yet!
See Figs 7-8 for the pin connections of the transistors.
See Figs 9-10 for the polarity markings of the diodes (D1 and D2) and the capacitors (C1 and C2).
Have your circuit checked by a member of staff.

Fig 6

Fig 7 shows the pin connections for the small transistor in the voltage stage (2N3904)
Fig 8 shows the pin connections for both of the current stage transistors (TIP41 and TIP42).
Fig 9 shows the polarity markings of the diodes (D1 and D2).
Fig 10 shows the polarity markings of the capacitors (C1 and C2).

Fig 11

Fig 12

Exercise Part 2: Test The Circuit (prepare the circuit for testing)

Fig 13

To begin testing, fit the test loops to the power rails, the input and the output (see the author’s circuit in Fig 13).
For correct testing, an amplifier needs to be connected to a load. However it is unwise to connect a loudspeaker at this point because if there was a fault it could potentially damage the speaker. So instead we add a resistor in its place with the same resistance/impedance as the speaker. We call this a dummy load. You’ll see that it is bigger than the other resistors. It is a power resistor, for dealing with the large currents delivered by the current amplifier. (See Fig 14).

Fig 14

Exercise Part 2: Test The Circuit (connect power, signal source and oscilloscope)

Fig 15

Fig 16

As in Fig 15 connect the power supply to the circuit top rail and ground using croc clip leads hooked on to the test loops. Again using croc clip leads, connect the signal generator to the input of the circuit and ground as in Fig.16.
Hook up an oscilloscope to the output via its probe and ground wire. See Fig 17.

Fig 17

Exercise Part 2: Test The Circuit (check the quiescent voltages)

Switch the 12Vdc power supply output on. During no input signal conditions there will be static DC voltages around the circuit (quiescent voltages).
Using a multimeter check your quiescent voltages match the ones in Fig 18. If they are a few millivolts different, it doesn’t matter.
As long as they are roughly the same. If any are clearly different, turn off the power supply output and check your circuit. Ask for help if needed.

Fig 18

Exercise Part 2: Test The Circuit (input a 1kHz signal)

Fig 19

If all of the quiescent voltages roughly match, it’s time to feed a signal into the circuit. We need a known signal that is easy to assess. For this we will feed a 1kHz sine wave into the circuit and observe the output on an oscilloscope. First set a signal generator to give a 1kHz sine wave at an amplitude of 0.1Vp-p (see Fig 19).Switch the generator’s output on.
Switch on the Oscilloscope. Now on the scope you should see a sine wave (Fig.20). Enlarge or trigger the wave if you have to. Bring up the onscreen measurements and look for peak to peak voltage and frequency. The Vp-p should be about 1Vp-p and the frequency 1kHz (Fig 20)
If this is correct, proceed by increasing the signal generator amplitude to 0.8 Vp-p. Now on the scope, it should give an output voltage of about 8Vp-p. If this is true then your amplifier circuit is working. Return the signal generator amplitude to 0.1Vp-p. Turn off the signal generator output and the power supply output.

Fig 20

Exercise Part 2: Test The Circuit (play a 1kHz tone)

Remove the dummy load resistor and the oscilloscope probe. Connect in its place the bookshelf speaker provided in the lab. Red wire to the output and black to the ground section. See Fig 21.
Ensure the signal generator is set back to 0.1Vp-p. Switch the power supply back on and then the signal generator output back on. Now the speaker should be emitting a 1kHz tone. If not, turn the sig gen output and power supply output off and check your breadboard connections. If there is the correct tone, increase the amplitude of the signal generator to 0.8 Vp-p. Do this only briefly as the sound will become very loud.
Turn the signal generator output off and the power supply output off.

Fig 21

Fig 22

Exercise Part 2: Test The Circuit (play music)

Caution! While playing music, the power transistors will become very hot. Do not touch them with bare skin.

Fig 23

Fig 24

Remove the signal generator croc clips from the breadboard. Leave the power supply croc clips connected. Next, take the jack to breadboard audio adaptor (in your kit) and connect the red and black pins to the signal input on the breadboard (where the signal generator was connected). See Fig 23 and Fig 24 . Plug the jack plug of the adaptor into an mp3 player (provided in the Lab). See Fig 25.

Please do not use your own phone or any other portable music player. If there was a fault with your circuit, it could damage your equipment.
Switch the power supply output on and then switch the mp3 player on. Set the volume to about 50%. Play a music track and increase the volume to a comfortable hearing level. You should hear clear, relatively loud music.
Exercise complete, well done!

Fig 25

Fig 26

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