This circuit is like a megaphone for tiny sounds.
It uses just two transistors like tiny electronic switches and a 1.5V battery to boost a faint signal making it much louder.
This is a basic way to build a mini amplifier for simple projects, like listening to music a little bit better.
Circuit Working:
Parts List:
| Category | Item | Quantity | Additional Specifications |
|---|---|---|---|
| Resistors | 2.2M, 10k | 1 each | All resistors are 1/4 watt unless specified |
| Capacitors | Electrolytic 1µF 25V, 220µF 25V | 1 each | |
| Semiconductors | Transistors BC547, BC557 | 1 each | |
| Miscellaneous | ON/OFF Switch | 1 | |
| 64Ω Speaker | 1 | ||
| Battery | 1.5V | 1 |
with their quantities and relevant details, making it easy to reference and understand what is available.
This circuit demonstrates exceptional output clarity and includes several notable features.
Firstly, the two transistors are directly connected maximizing the gain or amplification.
Another significant feature is the utilization of a high impedance speaker.
A speaker functions as a linear motor deriving its pulling force from the amp turns generated by the coil.
The amp turns produce a magnetic flux that interacts with the flux from the permanent magnet driving the speaker cone.
To achieve maximum amp turns one can use a coil with fewer turns and supply a high current.
Conversely, the same amp turns can be achieved with a coil featuring numerous turns and requiring only a small current which is what this circuit accomplishes.
While an 8 ohm speaker typically delivers more volume this circuit with a 1.5V supply surprisingly delivers significant volume even with a 64 ohm speaker.
In terms of biasing the BC557 transistor is activated through the 2.2M resistor.
The voltage on the base of the BC557 drops below the 1.5V supply by approximately 0.6V activating the BC547.
The BC547 is then pulled towards the positive rail reducing the voltage across the 2.2M resistor.
Initially, this voltage difference was 0.9V 1.5V to 0.6V.
However, the BC557 struggles to elevate the BC547 transistor due to the speakers resistance.
Consequently, the voltage across the speaker remains minimal measuring around 0.05mA when the circuit is idle.
During operation, assuming the input signal can fully activate the transistor pair the amplitude will swing between 0V and 0.9V.
This 0.9V swing is then directly transmitted to the speaker.
Formulas:
Below are some general formulas for above circuit diagram for transistor amplifier design:
Voltage Gain (Av):
The ratio of the input voltage (Vi) to the output voltage (Vo) may be found using this formula.
Av = Vo / Vi
Gain in dB:
The voltage gain (Av) is expressed in decibels (dB) using this formula.
Gain (dB) = 20 * log10(Av)
Collector Current (Ic):
The current passing through the transistors collector is computed using this formula.
Ic = (Vcc – Vce) / Rc
where,
- Vcc is supply voltage, in this example, the battery voltage
- Voltage from collector to emitter (Vce)
- Rc is the resistor collector
Note:
Depending on the particular circuit layout and transistor biasing, these formulas may need to be adjusted for simple transistor amplifiers.
It is advised to utilize higher voltage sources while building a functioning audio amplifier circuit and to take into consideration the usage of transistors with appropriate biasing techniques.
How to Build:
To build a simplest two transistor amplifier circuit using 1.5V battery follow the below mentioned steps:
Transistor Connections:
- Connect the collector of the BC547 transistor to the positive terminal of the power supply 1.5V.
- Connect the emitter of the BC547 transistor to one terminal of the speaker, and connect the other terminal of speaker to ground.
- Connect base of BC547 to collector of BC557 and also to ground through 10k resistor.
- Connect the base of BC557 to the emitter of BC547 through a 2.2M resistor.
- The audio input is connected to the base of BC557 through a 1µF capacitor.
- Connect a 220µF capacitor across the positive and the negative supply lines.
- Connect a ON/OFF switch to the positive supply of a 1.5V battery.
- Connect the negative supply of 1.5V battery to ground.
- Power up the circuit and test it with a small input signal e.g. from an audio source like a smartphone.
- Adjust the input signal and observe the output from the speaker.
Note:
- This circuit demonstrates amplification of the input signal driving the speaker with sufficient power to produce sound.
- Adjustments to the circuit can be made based on the specific requirements and desired performance.
Conclusion:
The simplest two transistor amplifier circuit consists of two NPN transistors with one transistor amplifying the signal and the other providing additional amplification and impedance matching.
This basic circuit provides moderate amplification and is commonly used in simple audio amplifier applications.
References:
A 1.5V Current-Mode Operational Amplifier Using Level Shifter Technique