Simple Small Amplifier Circuit using Transistors

This is one Simple Small Amplifier Circuit using Transistors which work like basic audio amplifier.

It makes weak sound signal more strong so sound come better like in car radios.

Now lets see how this circuit is build and how it work.

Circuit Working:

Parts List:

This amplifier circuit takes a weak audio signal from source.

The source can be anything like radio tuner or other audio output.

Inside a circuit there is one source stage which is biased.

Bias help both BJTs transistors to get same voltage from source so both work same.

Circuit uses two output BJTs in push-pull setup.

Push-pull means when one transistor is ON and working then other one is OFF and then they switch.

By this way it makes circuit more power efficient.

Two diodes are placed between base of BJTs and they control and adjust the bias when transistors are working.

Diodes help in keeping bias steady and safe.

One 3.3 ohm resistor connect with emitter of output transistors and this give stability.

If bias current increases it help stop sudden changes from heat or changes in transistor and diode working.

More bias current means voltage between emitter and base goes down so transistor work less.

This help to control how much sound is amplified.

Circuit has input impedance near 500 ohms and voltage gain is about 5 when 8 ohm speaker is used.

This show how good circuit can increase sound signal.

Then the amplified sound goes to 8 ohm speaker.

Speaker gets 2V signal with low noise so sound is more clear and better.

Power output is around 50 milliwatt.

If we increase supply voltage and put heat sink on output transistors the power can go up.

Circuit uses around 30 milliamp current from 9V battery and this shows how much power it takes.

Formulas:

Here are some easy formulas for common emitter amplifier circuit:

1. Voltage Gain (Av):

This show how much output voltage (Vout) is bigger than input voltage (Vin).

Formula is:

Av = – RL / RE

where:

• RL is the load resistance like in speaker
• RE is the emitter resistor

This formula work when transistor is in active mode.

But real gain is little less because transistor has its own inside resistance.

2. Biasing Resistor (R1):

This resistor make base voltage (Vb) for transistor.

Base voltage control collector current (Ic).

Formula is:

R1 = (Vcc – Vb) / Ib

where:

• Vcc is the power supply voltage
• Vb is the base voltage which is normally 0.7V for silicon transistor
• Ib is the base current

3. Collector Current (Ic):

This is current going from collector side of transistor.

It depends on base current and transistor gain (called hFE).

Formula is:

Ic = hFE × Ib

where,

• hFE is written in transistor datasheet.

4. Emitter Resistor (RE):

This resistor help control gain and make circuit more stable.

RE is usually small value when compared with RL.

Note:

These formulas help when testing simple common emitter amplifier.

Real amplifier circuit can be little different and more complex.

How to Build:

Follow these steps to make a simple small amplifier circuit using transistors:

• First collect all parts and be sure parts match with what is needed in circuit design.
• Place resistors, diodes, BJTs and other parts on PCB board.
• Use jumper wires to make connections same like in circuit diagram.
• Connect 9V power supply with positive and negative wires connect to correct place on PCB and then check polarity is correct.
• To share source voltage same between both BJTs apply biasing to source stage.
• Connect two diodes between base of both BJTs and this will help to control bias when temperature changes.
• Use two BJTs in push-pull type connection for output stage.
• For stability connect 3.3 ohm resistor in series with emitters of both output BJTs.
• Connect audio input signal to right place in the circuit.
• Calculate input impedance it should be near 500 ohms.
• If we connect 8 ohm speaker then voltage gain should be around 5.
• Connect wires from circuit to 8 ohm speaker.
• Speaker will get voltage swing around 2V with low noise so sound will be good.
• Power output should be around 50 milliwatt and check this.
• Circuit should take around 30 milliamp current from 9V battery and check this too.
• If needed put heat sink on output BJTs to keep them cool.
• If we want more power output then we can increase supply voltage.
• For that heat sink is very important.

Testing:

• Turn ON the circuit and see if amplifier is working.
• The speaker should give clear sound with no noise or distortion.

Adjustments if needed:

If there is any problem when testing then try to fix it.
We can change some resistor values and check all wire connections or fix output signal problem.

Conclusion:

Making this Simple Small Amplifier Circuit using Transistors need to work carefully.

If needed we can adjust the circuit later to make it work better.

Take proper care while working with electronics.

References:

Lecture12-Small Signal Model-BJT

Datasheet BD139

Datasheet BD14