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 operational details of this circuit:

Circuit Working:

Parts List:

Operational details of the circuit are mentioned below:

The weak audio signal is fed into the amplifier circuit.

This could be the output from a radio tuner or any other audio source.

The circuit includes a source stage that undergoes biasing.

Biasing ensures that the source voltage is evenly distributed between two matching output BJTs.

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

This means that while one BJT is in the active state conducting, 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 play a role in controlling and stabilizing the biasing of the transistors.

To maintain stability, a 3.3 ohm resistor is placed in series with the emitters of the output driver transistors.

This reinforces the bias current, reducing the risk of variations due to temperature changes or alterations in transistor/diode properties.

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

This mechanism allows for control over the amplification process.

The circuit exhibits 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 amplifies 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 distortion 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 relevant formulas for common emitter amplifier circuit:

1. Voltage gain (Av):

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

It is given by:

A_v = – R_L / R_E

where,

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

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

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

2. 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 (typically 0.7 V for silicon transistors)
• Ib is the base current

3. Collector current (Ic):

This is the current that passes through the transistors collector.

It is determined by the transistors current gain (hFE), a datasheet parameter, in relation to the base current (Ib).

I_c = h_FE * I_b

4. 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 are but a handful of the formulas available for circuit analysis of common emitter amplifiers.

Transistor amplifier circuit designs can really be more intricate and contain other parts and factors.

Construction Details:

• Collect all the required components ensuring they match the specifications mentioned in the circuit design.
• Place the BJTs, resistors, diodes and other components on the PCB.
• Use jumper wires to establish connections according to the circuit diagram.
• Connect the positive and negative terminals of the 9V power supply to the appropriate points on the PCB.
• Ensure proper polarity.
• Implement biasing for the source stage to distribute the source voltage evenly between the output BJTs.
• Connect diodes between the bases of the BJTs to moderate bias during conduction.
• Set up the output stage using two BJTs in a push pull configuration.
• Connect the emitter resistor 3.3 ohm in series with the emitters of the output driver transistors for stability.
• Connect the input signal source to the appropriate point in the circuit.
• Measure the input impedance, which should be around 500 ohms.
• Check the voltage gain, which should be approx 5 when an 8 ohm speaker is connected.
• Connect the 8 ohm speaker to the circuit.
• The speaker will experience a voltage fluctuation of around 2 volts with minimal distortion.
• Measure the power output of the circuit, which should be in the 50 milliwatt range.
• Check the current draw, which is approximately 30 milliamps from the 9V supply.
• If desired add heat sinks to the output driver transistors.
• This step is particularly relevant if you intend to increase the supply voltage to boost power output.

Testing

• Power on the circuit and test the amplifiers performance.
• Ensure that the amplified audio output from the speaker is clear and free from distortion.

Adjustments if needed

• If any issues are identified during testing consider making adjustments to the circuit.
• This could involve tweaking resistor values, checking connections, or addressing any anomalies in the output signal.

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 oriented.

Take precautions to prevent short circuits and be mindful of the heat generated during 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