Let’s try the 3 transistors Audio Amplifier circuits (MONO)

Do you like a transistor? Today I am going to 3 transistors audio amplifier circuits. Why use transistors in the amplifier? Transistors are devices made of semiconductors.

It has many benefits, but the most important and common use is to use it as an amplifier. How? Must … follow to see.

3 transistors Audio Amplifier circuit

Although currently, we will popular to use an IC in more power amplifier circuits.

But transistors are still widely used. Because they are small in size. And the high gain of current and voltage. It depends on the bias, which can be done easily.

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Basic principles of transistors

In general, we can divide the working range of the transistor into 3 ranges:

1. Cut off ( transistor stop) There will not be both the base current (IB) and the collector (IC) flowing through the transistor. There will be some leakage currents which are very rare.

2. Saturated range. There is electricity flowing through the transistor fully Until it saturated. And the current will not increase any more than this. Which we can limit the current flowing through the connection of the resistors.

3. The active range is the period that the transistor operates (conducts current). By driving the collector current (IC) proportional to the base current (IB).

The ratio of these two currents. These two values can find the current gain (hFE) from:

hFE = IC / IB

Therefore, when using the transistor audio amplifier, the circuit works in the active phase. In this experiment, it will take you to learn a simple amplifier circuit. Let’s get started.

These circuits are simple amplifiers you can look other HERE Small transistor amplifier circuits

We have 2 interesting experiments.

Simple Microphone audio amplifier

Look at the circuit below. This is the experiment process no.1. We called a simple audio amplifier. It will increase the single from a microphone.

Microphone 3 transistors audio amplifier
Figure1: Simple Microphone audio amplifier Circuit

Circuit experiment precess

  1. Connect the equipment according to the circuit in Figure 1 onto the breadboard. But be careful of the polarity of the device too. Do not connect to the wrong polarity.
  2. When finished, connect the positive and negative wires, from the 6V power supply.
  3. Now tap on the MIC1, 2-3 times. We will hear the “popping” sound from the speaker. After that, try to speak on the microphone. You will hear your sound extended through the speakers.

How does it work?

Please look back at Figure 1 again. It is an amplifier circuit from a microphone. When there is an audio signal coming into the MIC1.

When a sound signal comes through the MIC1. And, it is transformed into a small electrical signal fed to Q1 at point A.

In order to amplify the signal first. Which we set as a common emitter amplifier.

There are R1 and R2 to divide the voltage into bias to Q1. But the signal is not strong enough.

At point B. When the signal is increased. Then, send it to both Q2 and Q3 that are connected to a Darlington compound amplifier circuit.

To amplify the signal once more, strong enough to drive through the speaker.

Read others: 3 transistor audio amplifier

Darlington compound amplifier

Figure2: Darlington compound amplifier Connection

See in Figure 2 shows the Darlington transistor connection. We connect 2 transistors with exactly the same characteristics. According to the circuit characteristics as in the Figure.

For example:

If the gain of each transistor is equal to 100, the total gain will be equal to (100×100) = 10,000.

Related: High impedance small amplifier circuits

AM radio receiver with 3 transistor amplifier circuit

This is another example of a simple Experiment 2. Look at Figure 3. It is a simple experiment with an AM radio receiver circuit.

AM radio receiver with 3 transistor amplifier circuit

How it works

By using the L1 coil wrapped on a ferrite rod. One end connects to the anode of D1. The other end of the coil is grounded. There is an adjustable capacitor (VC1) attached to this coil.

When the values of L1 and VC1 resonance with that frequency are complete. The diode D1 detects only the audio signal. To forward to the Q1, amplifying according to the previous principles.

If the sound is very quiet. Try to use headphones instead of speakers. Which can be heard more clearly from the radio station. And helps to cut out outside noise.

One disadvantage is that this circuit is a simple basic tuning circuit, so it may be difficult to tune a station.

Read also: Very simple amplifier circuit using transistor 2N3904

Feedback circuit of a transistor amplifier

In addition to learning to connect the audio transistor. We have also studied how to organize the bias circuit to have feedback. There are two types of feedback:

  • Positive feedback
    Which we will notice from the general audio amplifier. When we turn the microphone towards the speaker We will hear the whistle. Or, the sounds of the feedback signal from the output to the input.

    In the direction that is added to the input signal or has the same phase. Causing the signal to be too strong to cause distortion.
  • Negative Feedback
    The signals from the output fed back to the inputs will cancel out or have opposite phases. Therefore, the sum of the signal is reduced. It helps to improve the stability of the amplifier.

Ways to improve the circuit

Look at Figure 5. We divide R4 values into R4A and R4B, with C2 connecting at the center of both R. Then the other side is to the ground. It will provide a little more feedback.

improve the feedback circuit
Figure 5: improve the feedback circuit

Which will give good results even if the magnification decreases somewhat.

However, the sum of the expansions in the circuit is limited. We then add C2 to reduce negative feedback.

Recommended: 40W transistor audio amplifier circuit with PCB

This C2, also helps to make the voltage at the emitter pin or the potential voltage at point D more smooth (from Figure 1).

We only limit the sum of the feedback. Because in the basic amplifier circuit, High magnification is more important than high clarity.

How Application it

We like to use this circuit. Used in an intercom. All you need is to do this audio amplifier, add another set. Then connect the cable from the speaker. But should not be extended more than 20 meters.

If you really want to build it for use. Look at the circuit below.

transistors intercom circuit

Read next: simple transistor intercom circuits

Only then can it be able to interact with each other. Even being far apart. Playing is not difficult at all.

Not only that we still have 3 transistors amplifier.

Recommended: LM386 audio amplifier

Mono amplifier two speaker outputs using transistors

In normal stereo amplifier is 2 speakers. And if a mono amplifier is a single speaker.

But this circuit is a special Mono amplifier. It can drive 2 speakers at the same time.

Without parallel or serial access. Because It causes the impedance of the speaker changes.

But in this circuit, we continue the speakers instead of the collector resistor (RC) of the transistor.

So, It can amplify out of 2 speakers.

two speaker mono amplifier

How it works

First, enter the power supply to the circuit and the audio signal to the input terminal. Then, the audio signal coupling to through the C1 and R1 to amplifies by transistor-Q1.

The Q1 is the first Preamplifier to increase a little signal up. Before sending it to Q2.

Next, the Q2 is connected to the emitter follower circuit. Its function is as a driver amplifier to boost up a signal from the preamplifier section. To more power to drive the Q3 runs well.

mono amplifier two speaker

And Q3 is a power amplifier out of the speaker.

It has the feedback of an audio signal through the VR1 and R2 to the pin B of Q2. To control the stability of working for good.

This circuit is an output of 40 milliwatts of distortion of the signal rate is at 0.1 percent. And frequency response from 15 Hz – 200 kHz.
The power supply voltage of 9V to 20V.

You are looking for Mono amplifier. You have many choices. For example:

Parts you will need

0.25W 5% resistors
R1: 27K
R2: 47K
R3: 12K
R4: 10 ohms
VR1: 50K Potentiometer

C1: 0.22uF 50V Ceramic
C2: 5pF 50V Ceramic
C3, C4: 10pF 50V Ceramic
C5: 1000uF 16V Electrolytic

Q1: BC558, 45V 0.1A PNP transistor
Q2: BD140,80V 1.5A PNP Transistor
Q3: TIP2955, 60V, 15A PNP transistor

Note: I have published this circuit for a long time. But I haven’t tried it. Found many mistakes. I found this website, put this content into the website. And change the transistor number. This is a good idea. Thanks.

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6 thoughts on “Let’s try the 3 transistors Audio Amplifier circuits (MONO)”

  1. Transistors have 3 states of operation….1) cutoff…Vce=Vcc 2) Saturation…Vce=.2 volts and Normal…Vce=1/2 x Vcc …. where Vcc=Voltage supplied to Transistor(Bias)

  2. I was building this same circuit and i was frustrated by the distortion at the output. Your neat trick in fig. 5 solved it for me. Thank you.

    • Hello Muse,
      I am happy to hear that. The transistor amplifier is fun that test and improves the circuit we can adapter it. Electronics! It’s challenging and fun to solve circuit problems. It will get rid of your loneliness, but don’t get discouraged if the circuit fails. It is the process of learning. When my daughter grows in electronics. I will teach her to learn more.

  3. Hello, It’s Desmond again.
    I constructed the above circuit and I used it to amplify the sound output from a Raspberry Pi 2. The result is horrible. I put the circuit into an emulator and found that the output waveform was seriously distorted. Based on the simulation result I changed the components:
    C1: 0.01 uF
    Q2, Q3: 2N2222
    R1: 100 kOhm
    R2: 330 kOhm
    R3: 220 Ohm
    R4: 470 Ohm
    R5: 68 Ohm
    and the impedance of the speaker is 4 Ohm. The quality of sound produced improved a lot, but the power was quite low.
    I am planning to increase the output power to about 5-10W (enough for me) by changing the components, especially the transistors. I may use TIP41C or BD139. Do you have any suggestion? Thank you.

    • Hello again my friend
      I’m glad you made the amplifier circuit. It was quite fun to experiment with different circuits. In the past (when I was a child) I liked to build various circuits. I like modifying it, which is quite challenging. Even though it was less successful than it was successful. But I still like it. Right now, I’m quite busy with my kids. each of his ages There are details that we must always pay attention to.

      I would like to review your message again. You have an audio amplifier with a power output of 5W to 10W, but this circuit only has a supply voltage of 6V. It will have a maximum output power of about 2W at 4 ohms. P = (VxV)/R V = 3V because it is OTL form.

      You should change to use another circuit.

      It’s a pity. I’m not free at this time. I want to teach my children to Build a 10W audio amplifier circuit using transistors. It’s quite challenging. Although the transistor has low efficiency, it has the charm of being able to modify equipment a lot.
      Sorry, I couldn’t help you much. Next time I’ll try to create a better circuit (uses about 4-5 transistors).


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