Simple Small Amplifier Circuit using Transistors

The simple small amplifier circuit using transistors functions as a basic audio amplifier designed to amplify weak signals and enhance the audio experience particularly in car radios.

Let us find out into the working and construction details of this circuit:

Circuit Working:

Parts List:

The circuit for the amplifier receives the weak audio signal.

This could be any audio sources output including a radio tuner.

There is a source stage in the circuit that is biased.

Biasing ensures that two BJTs with matching outputs receive equal amounts of the source voltage.

The circuit employs two output BJTs in a push pull configuration.

This means that while one BJT is in the active state working the other is in the off state and vice versa.

This push pull arrangement enhances efficiency.

Diodes placed between the bases of the BJTs serve to moderate the bias during conduction.

They are involved in controlling and maintaining the transistors biasing.

A 3.3 ohm resistor is connected in series with the output driver transistors emitters to provide stability.

By increasing the bias current this lowers the possibility of fluctuations brought on by temperature changes or modifications to the workings of transistors and diodes.

When the bias current increases the voltage between the emitter and base decreases leading to a decrease in working.

This mechanism allows for control over the amplification process.

The circuit shows an input impedance of approx 500 ohms and the voltage gain is around 5 when an 8 ohm speaker is connected.

These values indicate how effectively the circuit raises the input signal.

The amplified audio signal is then delivered to an 8 ohm speaker.

The speaker experiences a voltage fluctuation of 2V with minimal noise resulting in an improved audio output.

The power output of the circuit is in the 50 milliwatt range.

Increasing the supply voltage and adding heat sinks to the output driver transistors can boost the power output.

The circuit draws approx 30 milliamps from a 9V supply indicating the power consumption of the amplifier.

Formulas:

Here are some of the simple formulas for common emitter amplifier circuit:

Voltage gain (Av):

This is the ratio of the output voltage Vout to the input voltage Vin.

It is given by the following formula:

A_v = – R_L / R_E

where,

• RL is the load resistance for speaker in this circuit
• RE is the emitter resistor

The transistor is assumed to be in the active zone in this simple formula.

The transistors internal resistance will cause the real gain to be less.

Biasing resistor (R1): This resistor sets the base voltage Vb of the transistor which in turn controls the collector current (Ic).

It is calculated using the following formula:

R1 = (V_cc – V_b) / I_b

where,

• Vcc is the supply voltage
• Vb is the desired base voltage which is 0.7 V for silicon transistors
• Ib is the base current

Collector current (Ic):

This is the current that passes through the transistors collector.

It is calculated by the transistors current gain (hFE) a datasheet settings in relation to the base current (Ib).

I_c = h_FE * I_b

Emitter resistor (RE): This resistor helps set the gain and gives the circuit stability.

Usually a small value is selected in relation to RL.

Note:

These formulas are used for circuit testing of common emitter amplifiers.

Transistor amplifier circuit designs can really be more difficult and include other parts and factors.

How to Build:

Follow these connections to construct a simple amplifier circuit using transistors:

• Assemble the necessary parts making sure they match to the requirements stated in the circuit design.
• Put the resistors, diodes, BJTs and other parts on the PCB.
• To create connections in line with with the circuit diagram use jumper wires.
• Connect the 9V power supply positive and negative connections to the correct PCB positions.
• Check the correct polarity.
• To ensure that the source voltage flows uniformly among the output BJTs, apply biasing to the source stage.
• Diodes should be connected between the BJT bases in order to reduce bias during heat transfer.
• Use two BJTs in a push-pull arrangement to set up the output stage.
• To ensure stability connect the output driver transistors emitters in series with the emitter resistor which is 3.3 ohm.
• Connect the source of the input signal to the right spot in the circuit.
• Calculate the input impedance which should to be approximately 500 ohms.
• If you connect an 8 ohm speaker the voltage gain should be about 5.
• Wire the circuit to the 8 ohm speaker.
• With low distortion the speaker will encounter a voltage fluctuation of about 2 volts.
• Check the circuits power output which should to be within the 50 milliwatt range.
• The current should be around 30 milliamperes from the 9V source, so one needs to check it.
• Add heat sinks to the output driver transistors if necessary.
• This step is especially important if you plan to raise the supply voltage in order to increase the power output.

Testing:

• Turn the circuit on and check the amplifiers functioning.
• The speakers amplified audio output should be clean and distortion-free.

Adjustments if needed:

• If any problems are found during testing think about modifying the circuit as needed.
• Changing resistor values, checking connections or fixing any issues with the output signal could all be part of this.

Conclusion:

Building this simple small amplifier circuit using transistors involves careful placement and connection of components on a PCB.

Ensure that the connections are secure and the components are properly focused

Take precautions to prevent short circuits and be careful of the heat generated during circuit operation especially if heat sinks are not used.

Adjustments to the circuit can be made based on measurements and performance testing.

References

Lecture12-Small Signal Model-BJT

Datasheet BD139

Datasheet BD14