This is a 4 transistor audio amplifier circuit. Which is a 4-transistors complementary push-pull amplifier, that shows the basics of audio amplifier design. This circuit saving on battery current, which is quite low with middle volume, rising to 25 -30mA as a volume is increased.
This gives us a 250 mW amplifier, enough to drive a loudspeaker to the same volume as a mobile phone or MP3 player. The input must be about 100-500 mV to drive amplifier fully. Previously, you may like LM386 amplifier circuit. But you may also like this circuit as well.
The working of 4 transistor amplifier circuit
This type of circuit use few components needs no transformers and provides very good results. The 4 transistors are directly coupled and DC feedback loops help stabilize working of the circuit.
4 transistors working
Both transistors Q3 and Q4 are arranged as a complementary pair operating in push-pull. Each output transistor deals with one half of the audio cycle, one being cut-off when the other conduct.
Next, the Q1 and Q2 transistors run as a pre-amp to increase the incoming voltage to drive the output pair. Then, at Q1 the bias of the whole circuit commences with the voltage divider made up of the 56K and 100K resistors. This provides the base with a bias voltage of 5.5V. The emitter voltage of 0.6V less than this, and will be 4.9V.
And next, The Q2-transistor is biased so that it provides a voltage across the 270 ohms load resistor, which will give the output transistors. There is a voltage differential of 0.6V between their base and emitter leads.
This needs to reduce cross-over distortion which occurs whenever two transistors are connected in push-pull. The 100uF electrolytic protect DC from appearing on the speaker, the require the speaker to oscillate around this new position.
4 transistor audio amplifier circuit
Parts you will needs
First of all, You need all parts below
Q1,Q3 = BC547 or equivalent, 45V 100mA NPN Transistor,Quantity: 2
Q2,Q4 = BC557 or equivalent, 45V 100mA PNP Transistor,Quantity: 2
R1 = 56K, 1/4W Resistors tolerance: 5%,Quantity: 1
R2 = 100K, 1/4W Resistors tolerance: 5%,Quantity: 1
R3 = 33K, 1/4W Resistors tolerance: 5%,Quantity: 1
R4 = 470 ohms, 1/4W Resistors tolerance: 5%,Quantity: 1
R5 = 270 ohms, 1/4W Resistors tolerance: 5%,Quantity: 1
R6 = 1.5K, 1/4W Resistors tolerance: 5%,Quantity: 1
R7 = 10K, 1/4W Resistors tolerance: 5%,Quantity: 1
C1 = 10µF 25V,Electrolytic Capacitors,Quantity: 1
C2 = 1µF 25V,Electrolytic Capacitors,Quantity: 1
C3 = 100µF 25V,Electrolytic Capacitors,Quantity: 1
B1 = 9-volt batteries, Quantity: 1.
SP1 = 8 ohms 0.25” Speaker
Build the 9V mini amplifier
This circuit is a small size you can assemble them on the breadboard. Or You can assemble the amplifier circuit on A small piece of universal PCB Board 20 – 25 holes. Make a layout diagram first which very nearly follows the schematic diagram before attempting any soldering.
With care you will find you will not have to cut any of the universal PCB Board tracks and most the parts will fit neatly onto the board as they all 0.1” spacing.
When you complete soldering, Next, connect the battery via a milli-ammeter to check, that the current is within 30mA and most probably is 5-15mA with no input signal.
You may like to trace through the amplifier with the signal tracer project.
Obviously, all the step cannot provide the good amplifier, since a transistor has a gain of at least 20 times, and sometimes an in-circuit gain of 100. Thus, this gain of amplifier about 20 x 20 x 20 or 8,000 times! But this is not so. If the input about 250mV, the amplifier needs to provide a gain of about 40 to 80 times.
Your signal tracer will indicate which transistor is providing this gain. See for yourself. In all, these combined 4 transistors projects should give you hours of fun. Although, this 4 transistor audio amplifier circuit is ancient, it still suitable for children’s learning and good sound.
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