Introduction: Transistor Microphone Amplifier
This article shows you how to make a transistor microphone amplifier.
The minimum power supply for this circuit is 1.5 V. However, you will need at least 3 V if you are making an optional LED detector (transistor Q3) and want your LED to turn ON.
The signal from the microphone is amplified by transistor Q1 and Q2 before applied to Q3 transistor for detection.
You can see my circuit working in the video.
I thought of this idea after reading this article:
Components: cheap microphone - 2, general-purpose transistors - 5, 100 ohm high power resistor - 5, 1 kohm resistor - 1, 10 kohm resistor - 10, 470 uF capacitor - 10, 220 kohm resistor - 2, 470 nF capacitor - 5, matrix board, insulated wires, 1 mm metal wire, 1.5 V or 3 V power source (AAA/AA/C/D batteries), 1 Megohm to 10 Megohm resistor pack.
Tools: pliers, wire stripper
Optional components: solder, LEDs - 2, battery harness.
Optional tools: soldering iron, USB oscilloscope, multimeter.
Step 1: Design the Circuit
Calculate the maximum LED current:
IledMax = (Vs - Vled - VceSat) / Rled
= (3 V - 2 V - 0.2 V) / 100
= 0.8 V / 100 ohms
= 8 mA
Calculate the Q1 transistor collector voltage, Vc1:
Vc1 = Vs - Ic1 * Rc1 = Vs - Ib1 * Beta* Rc1
= Vs - (Vs - Vbe) / Rb1 * Beta* Rc1
= 3 V - (3 V - 0.7 V) / (2.2 * 10 ^ 6 ohms) * 100 * 10,000 ohms
= 1.95454545455 V
The biasing components are the same for the second transistor amplifier:
Vc2 = Vc1 = 1.95454545455 V
The transistor should be biased at half supply voltage 1.5 V, not 1.95454545455 V. However, it is hard to predict the current gain, Beta = Ic / Ib. Thus you will need to try different Rb1 and Rb2 resistors during circuit construction.
Calculate the minimum Q3 transistor current gain to ensure saturation:
Beta3Min = Ic3Max / Ib3Max
= Ic3Max / ((Vs - Vbe3) / (Rc2 + Ri3a))
= 10 mA / ((3 V - 0.7 V) / (10,000 ohms + 1,000 ohms))
= 10 mA / (2.3 V / 11,000 ohms)
Calculate the lower high pass filter frequency:
fl = 1 / (2*pi*(Rc+Ri)*Ci)
Ri = 10,000 ohms
= 1 / (2*pi*(10,000 ohms + 10,000 ohms)*(470*10^-9))
= 16.9313769247 Hz
Ri = 1,000 ohms (for LED detector)
= 1 / (2*pi*(10,000 ohms + 1,000 ohms)*(470*10^-9))
= 30.7843216812 Hz
Step 2: Simulations
PSpice software simulations show that the maximum LED current is only 4.5 mA. This is because the Q3 transistor is not saturating due to the inconsistencies of the Q3 transistor model and the real-life Q3 transistor that I used. The Q3 PSpice software transistor model had a very low current gain when compared to real-life Q3 transistor.
The bandwidth is about 10 kHz. This could be due to transistor stray capacitance. However, there is no guarantee that reducing Rc resistor values will increase the bandwidth because the transistor current gain could be decreasing with frequency.
Step 3: Make the Circuit
I implemented the optional power supply filter for my circuit. I omitted this filter from the circuit drawing because there is a possibility of a significant voltage drop that would reduce the LED current and LED light intensity.
Step 4: Testing
You can see my USB oscilloscope showing a waveform when I talk into the microphone.