Class A audio amplifiers are relatively simple to assemble and configure. They typically use just a single transistor for amplification and deliver sufficient volume without the need for integrated circuits. However, this amplifier has one significant drawback: it consumes a lot of power in quiet mode, making it suitable for home use.
To understand how such an amplifier works, you must understand one very important detail. In transistors, the collector-emitter and drain-source junctions are resistors whose resistance depends on the bias voltage, the signal on the base or gate, and other factors.
I use an IRF3205 MOSFET transistor in the amplifier. To measure the drain-to-source resistance (RD) of the transistor, you need to apply an adjustable voltage to the transistor's gate. This can be done with a variable resistor. I connected an ohmmeter to the drain and source. The photo shows that the drain-to-source resistance is 156 ohms at a certain gate voltage. If this voltage is varied, the resistance will vary from a few milliohms to a megaohm.
Next, I built a simple signal generator. By changing the resistance of a variable resistor, a sine-wave-like signal would appear at the generator's output. You'll have to rotate the resistor's rotor with your fingers to get this signal.
A bias voltage is applied to the transistor, and the transistor is slightly off or the drain-source junction resistance (RDS) appears. I set the resistance to approximately 155 ohms. I apply a signal from the oscillator to the gate, and the RDS resistance changes depending on the signal level. The resistance fluctuates between approximately 140 and 170 ohms. What happened? The oscillator signal began modulating the gate bias voltage, and the RDS resistance began to change dynamically.
Here is a complete schematic diagram of a Class A amplifier. How does it work?
The resistor R1 sets an operating point or a fixed resistance between the drain and the source. This is determined by the current consumption and optimal sound quality. There is no signal applied to the input of the amplifier. The transistor is slightly closed, but not completely. The resistor R3 and the resistance of the transistor between drain and source (RDS) act as voltage dividers. When a weak audio signal is applied to the gate of the transistor, it modulates the bias voltage of the transistor, causing the current and voltage in the voltage divider to change synchronously (but inverted) with a weak audio signal. This amplifies a weak signal in terms of power (current + voltage), as a result of which the sound from the speaker becomes louder. Capacitor C2 allows the alternating component of a strong audio signal to pass through, but blocks direct current from the power supply, preventing it from entering the speaker.
The resistance of resistor R3 should be selected depending on the volume you want to get from the amplifier. Its resistance can be in the range of 10-50 ohms or lower. Instead of this resistor, I installed an incandescent lamp, since the filament has resistance, and the lamp can be used to adjust the amplifier.
I turned on the amplifier and set the rotor position of resistor R1 to one of its extreme positions. What do we see? The lamp is not lit, meaning the transistor is completely open. This transistor operating mode is called cutoff.
Now I turned the resistor rotor to the other extreme position. The lamp shines very brightly. The transistor is completely closed. This mode is called saturation, and in this mode there will be no signal amplification.
Now I've positioned the resistor rotor approximately in the middle. The lamp glows at half power, and the current consumption at 9V is approximately 1.3A. The transistor is not completely closed, and this operating mode is called linear. In this mode, the transistor perfectly amplifies a weak signal. The amplifier is configured, and a weak audio signal can be fed to the input. Incidentally, the lamp will flicker slightly in time with the sound.
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