Lesson 6: Single Transistor Amplifiers.



This lesson ties in closely with lesson 2 in the electronics chapter where it talks about voltages in a series resistance circuit, and also with lesson 4 in the equipment chapter where it talks aboutSingle transistor A Class amplifier. A Class amplifiers. It will help you to understand this chapter if you take a moment to revise those two before continuing.

As described in the previous chapter, a transistor can be thought of as a voltage controlled resistor. If a resistor (called the Load Resistor) is connected to the collector of the transistor, and a power source (eg battery) is connected between the emitter and the other end of the resistor, as shown in the diagram, then the voltage at the collector will be set by the ratio of the "resistances" of the load resistor and transistor. The resistance of the transistor is in turn controlled by the voltage applied at the base, or more correctly, between the base and emitter. When the base voltage is low the resistance between the collector and emitter is high when compared with the resistance of the load resistor, so the collector voltage will be high. The opposite happens when the base voltage is high causing the transistor to appear low resistance compared to the load resistor and therefore the collector voltage is low. When the range of base voltages are plotted against the collector voltages a curve is created called the Charactistic Curve for that particular transistor.

Audio signals come in the form of alternating current analogous to sound waves, and therefore when applied to the base of a transistor will only have the half of the wave with the polarity that matches the transistor's base amplified. As seen in chapter 4 of the equipment section, the solution to this is to add an extra voltage source (which can be obtained from the battery supply at the top of the load resistor). The definition of a Class A amplifier is to set the bias voltage at the midpoint of the characteristic curve where the line is the straightest and will give the most accurate fidelity from the input to the output.

Another important thing about an amplifier like this is that it is heavily dependant on very small voltages, so any stray voltage coming in the input or output can upset the balance of voltages around the circuit and cause it to malfunction, or operate poorly. To get around this we use a capacitor to stop the DC before the input and after the output. You can see them in the circuit diagram. It is not uncommon for a simple amplifier like this to have an amplification factor of 10, so putting the output of one into the input of a second will amplify the input of the first stage 100 times by the time it comes out of the second stage.


We're now ready to move on to the next lesson. Click here to go to the lesson about using transistors as switches, or click here to return to the Electronics for Sound Techs index page.