This project details the design, simulation, and implementation of a low-power audio amplifier circuit utilizing Bipolar Junction Transistors (BJTs). The system is designed to operate on a single 1.5V battery, making it highly suitable for portable hearing aids and headphone drivers. The design prioritizes minimal component count, energy efficiency, and high gain (>40dB) while maintaining low distortion.
- Low Voltage Operation: Optimized for a single 1.5V AA battery supply.
- Multi-Stage Amplification: Utilizes a pre-amplifier, intermediate gain stage, and output driver for maximum signal fidelity.
- High Gain: Achieves a total voltage gain exceeding 40dB.
- Low Noise: Integrated decoupling and bypass networks to minimize power supply noise.
- Cost-Effective: Total prototype estimated cost is approximately ₹142 INR.
The circuit architecture consists of four main functional blocks:
The input signal from the electret microphone is conditioned and amplified by transistor Q1 (BC547).
-
Biasing: A voltage divider network sets the base voltage (
$V_{B1}$ ) to ensure operation in the active region:$$V_{B1} = V_{CC} \times \frac{R_2}{R_1 + R_2}$$ -
Gain: The voltage gain (
$A_v$ ) for this stage is defined by the collector and emitter resistances:$$A_v = -\frac{R_C}{R_E}$$
Transistor Q2 provides secondary amplification. DC components are blocked between stages using coupling capacitors to preserve the operating point of each transistor.
This stage utilizes a phase splitter (Q3) to drive the final power transistor (Q4 - BC337).
-
Impedance Matching: The output impedance (
$Z_{out}$ ) is matched to the headphone load ($Z_{load}$ ) to maximize power transfer:$$Z_{out} = Z_{load}$$ -
Filtering: The output coupling capacitor (
$C_5$ ) forms a high-pass filter with the load to determine the cutoff frequency:$$f_c = \frac{1}{2\pi R_{load}C_5}$$
The circuit was simulated using LTspice and Proteus Design Suite.
| Parameter | Specification | Observed Result |
|---|---|---|
| Supply Voltage | 1.5V DC | Stable Operation |
| Frequency Response | 20Hz - 20kHz | Consistent Gain |
| Voltage Gain | > 40dB | ~40dB |
| THD | < 5% | < 5% (Clear Audio) |
Figure 1: Input signal vs. Amplified Output showing >40dB gain.
The prototype was constructed for accessibility and low cost.
| Component | Value/Part Number | Quantity | Description | Approx Cost (INR) |
|---|---|---|---|---|
| Transistor | BC547 | 3 | NPN General Purpose (Q1, Q2, Q3) | 30 |
| Transistor | BC337 | 1 | NPN Power Driver (Q4) | 10 |
| Resistor | 10kΩ | 2 | 1/4W Carbon Film | 2 |
| Resistor | 1MΩ | 1 | 1/4W Carbon Film | 1 |
| Resistor | 5.7kΩ | 1 | 1/4W Carbon Film | 1 |
| Resistor | 100kΩ | 1 | 1/4W Carbon Film | 1 |
| Resistor | 3.9kΩ | 1 | 1/4W Carbon Film | 1 |
| Resistor | 1.5kΩ | 1 | 1/4W Carbon Film | 1 |
| Resistor | 1kΩ | 2 | 1/4W Carbon Film | 2 |
| Resistor | 32Ω | 1 | Load Resistor (Headphone Impedance) | - |
| Capacitor | 0.1µF | 2 | Ceramic Disc | 10 |
| Capacitor | 1µF | 1 | Electrolytic | 5 |
| Capacitor | 10µF | 1 | Electrolytic | 5 |
| Capacitor | 470µF | 2 | Electrolytic | 10 |
| Capacitor | 470pF | 1 | Ceramic Disc | 5 |
| Source | 1.5V | 1 | AA Battery | 10 |
| Input | Mic | 1 | Electret Microphone | 20 |
| TOTAL | ₹114 |
- Prerequisite: Ensure you have LTspice XVII installed.
- Clone this repository.
- Open LTspice and navigate to
File > Open. - Select the
.ascfile located in the/srcfolder. - Click the Run icon (running person) on the toolbar.
- View Results: Click on the Microphone Input node and the Headphone Output node to compare the amplification.
- Kavya G
- Nidhishree
- Hemashree GN
- Madeena bi
Dept. of Electronics and Communication, The National Institute of Engineering, Mysore.