By Doug Lowe
The most common way to use a transistor as an amplifier is in an electronic circuit sometimes called a common-emitter circuit because the emitter is connected to ground, which means that both the input signal and the output signal share the emitter connection.
This circuit uses a pair of resistors as a voltage divider to control exactly how much voltage is placed across the base and emitter of the transistor. The AC signal from the input is then superimposed on this bias voltage to vary the bias current. Then, the amplified output is taken from the collector and emitter. Variations in the bias current are amplified in the output current.
Recall that a voltage divider is simply a pair of resistors. The voltage across both resistors equals the sum of the voltages across each resistor individually. You can divide the voltage any way you want by picking the correct values for the resistors. If the resistors are identical, the voltage divider cuts the voltage in half. Otherwise, you can use a simple formula to determine the ratio at which the voltage is divided.
If you look at the schematic diagram, you see that there are actually two voltage dividers in the circuit. The first is the combination of resistors R1 and R2, which provide the bias voltage to the transistor’s base. The second is the combination of resistors R3 and R4, which provide the voltage for the output.
This second voltage divider is a variable voltage divider: The ratio of the resistances changes based on the bias voltage, which means the voltage at the collector varies as well. The amplification occurs because very small variations in an input signal are reflected in much larger variations in the output signal.
Let’s look at this circuit more closely:
The input arrives at the left side of the circuit in the form of a signal, which usually has both a DC and an AC component. In other words, the voltage fluctuates but never goes negative.
One side of the input is connected to ground, to which the battery’s negative terminal is also connected. The transistor’s emitter is also connected to ground (through a resistor), as is one side of the output.
The purpose of C1 is to block the DC component of the input signal. Only pure AC gets past the capacitor. Without this capacitor, any DC voltage in the input signal would be added to the bias voltage applied to the transistor, which could spoil the transistor’s ability to faithfully amplify the AC part of the input signal.
R1 and R2 form a voltage divider that determines how much DC voltage is applied to the transistor base. The AC portion of the signal that gets past C1 is combined with this DC voltage, which causes the transistor’s base current to vary with the voltage.
R3, R4, and the variable resistance of the collector-emitter circuit form a voltage divider on the output side of the amplifier. Amplification occurs because the full power supply voltage is applied across the output circuit. The varying resistance of the collector-emitter path reflects the small AC input signal on the much larger output signal.
C2 blocks the DC component of the output signal so that only pure AC is passed on to the next stage of the amplifier circuit.
The trick in designing transistor amplifiers is picking the right values for all the resistors and capacitors. Most hobbyists can get along with published circuits that you can find in kits or on the Internet. But if you really want to know how to calculate these values for yourself, you can find excellent tutorials on the subject on the Internet. Just search for common emitter and you’ll find what you’re looking for.
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