Simple 1.5V Audio Amplifier Circuit

A 1.5V audio amplifier circuit is a circuit that amplifies audio signals using a power supply of 1.5V.

Such a low voltage amplifier is often used in battery powered devices or applications where a higher voltage supply is not available or desired.

Circuit Working:

Parts List:

This experimental class A audio power amplifier using only three transistors, can deliver 25mW into an 8Ω load or 50mW into a 4Ω load from a 1.5V power source.

Despite the relatively low power output the sound volume is sufficient and would be quite loud when using earphones.

However, loudspeaker efficiency and enclosure acoustics also play crucial roles in determining the output volume.

The frequency response is good due to direct coupling which limits it to 8Hz on the low end and well beyond 50kHz on the high end.

It is worth noting that the amplifiers quiescent current flows through the loudspeaker making it unsuitable for higher power applications.

The direct coupling of the transistor arrangement NPN, PNP, NPN provides superior base drive essential for saturating the output transistor.

The amplifiers efficiency is about 20% close to the theoretical maximum of 25% for a class A amplifier.

Unlike class B or AB amplifiers the class A amplifiers DC supply current remains constant regardless of the output signal level making it inefficient in terms of battery consumption.

Negative feedback is essential for reducing distortion as observed in the non sinusoidal voltage of the base drive to Q3.

This feedback reduces the voltage gain Av and distortion by a factor of 190/5.

The amplifiers low voltage gain at each stage is evident especially in Q2 (Av = 1.4) due to its load resistance being the base to emitter junction voltage of Q3.

Stabilizing the amplifier was challenging, requiring a 0.01uF capacitor across the base to collector of Q3.

Despite this, the base drive signal remains distorted, indicating a low level oscillation.

The base circuit of Q3 may exhibit a negative resistance effect at low voltages potentially useful for creating an oscillator.

Considering other devices like germanium or MOSFET transistors for low voltage applications could be interesting as germanium transistors are believed to perform better at low voltages.

However, caution is advised regarding ultrasonic noise, it is crucial to check the AC output voltage before using headphones to avoid potential health risks associated with undetected oscillations.

Formulas:

The low voltage audio amplifier in the circuit above is made to run off of a single 1.5V supply.

Using a common emitter amplifier stage and an emitter follower output stage, it has a two transistor arrangement.

Some general formulas that may be used to transistor amplifiers are as follows, while exact calculations for this circuit would require particular transistor specifications and load conditions:

Transistor Current Gain (Beta or hfe)

I_c = β * I_b

where,

• Ic is collector current
• β (or hfe) is current gain of the transistor
• Ib is base current

Voltage Gain (Av)

A_v = -R_c / R_e

where,

• v is voltage gain
• Rc is collector resistor
• Re is emitter resistor

Power Gain (Ap)

A_p = A_v * A_i

where,

• Ap is power gain
• Av is voltage gain
• Ai is current gain

Output Power

P_o = (V_p-p)² / (8 * R_l)

where,

• Po is output power
• Vp-p is peak to peak output voltage
• Rl is load resistance speaker impedance

Note:

A broad foundation for comprehending the behavior of transistor amplifiers is provided by these formulae.

Detailed study utilizing transistor models and circuit simulation tools is frequently needed for accurate computations and optimization.

How to Build:

To build a Simple 1.5V Audio Amplifier Circuit you need to follow the below mentioned connections steps:

Transistor Selection:

• Choose suitable NPN, PNP and NPN transistors for the amplifier.
• Ensure they can handle the power requirements and are suitable for class A operation.

Circuit Design:

• Use the provided schematic to design the amplifier circuit.
• Ensure proper biasing and coupling for class A operation.

Component Selection:

• Select resistors and capacitors according to the circuit design and transistor specifications.
• Use high quality components for better performance.

Assembly:

• Place the transistors, resistors and capacitors on a breadboard or PCB according to the circuit diagram.
• Connect the components using wires or PCB traces ensuring proper connections and polarity.

Testing:

• Connect the power source 1.5V to the amplifier circuit.
• Connect the speaker to the output of the amplifier.
• Test the amplifier with a low level input signal and adjust the biasing if necessary to ensure class A operation.

Enclosure:

• Place the amplifier circuit in a suitable enclosure to protect it and improve the overall aesthetics.
• Ensure proper ventilation to prevent overheating.

Final Testing:

• Test the amplifier with different audio sources and load conditions to ensure it performs as expected.
• Use an oscilloscope or multimeter to check for distortion and adjust the circuit if necessary.

Usage:

• Once the amplifier is built and tested it can be used to amplify audio signals from various sources.

Note:

• Building electronic circuits requires knowledge of electronics and soldering skills.
• If you are not familiar with these, it is advisable to seek help from someone experienced or consult detailed guides and tutorials.

Conclusion:

A 1.5V audio amplifier circuit is a simple and efficient way to amplify audio signals using a low voltage power supply.

It is suitable for applications where higher voltages are not available or practical such as in battery powered devices.

By using basic components and a single transistor, this type of amplifier can provide adequate amplification for various audio sources while remaining energy efficient.

References:

Using a 1.5V battery to increase the voltage from a headphone jack so it can switch a transistor at lower volumes