Two voltage sources have been used in all circuits discussed thus far. Forward and reverse bias in a PNP transistor amplifier circuit. The emitter junction bias is provided by battery 1, and the collector junction bias is provided by battery 2.įigure 5. I E is for emitter current, I C for collector current, and I B for base current. The currents for each circuit are labeled. The same type of bias is needed for operation of a PNP transistor amplifier, Figure 5. Forward and reverse bias in an NPN transistor amplifier circuit. Notice the forward bias in the emitter junction and reverse bias in the collector junction.įigure 4. Figure 4 shows a complete NPN transistor circuit. Reverse bias for collector junction in an NPN transistor NPN Transistor Amplifier Workingīoth forward and reverse bias are needed for the operation of a transistor amplifier. Forward bias for emitter junction in an NPN transistor.įigure 3. Reverse bias is needed in the collector junction of an NPN transistor, Figure 3.įigure 2. The voltage applied to these elements with the correct polarity will create current flow. The other is between the collector and the base and is commonly referred to as the collector junction.įor proper transistor operation, energy from the internal power source (battery or power supply) is needed to overcome these junction resistances.Ī bias voltage must exist between emitter and base of an NPN transistor, Figure 2. One junction is between the emitter and the base and is called the emitter junction. Amplifier Biasingįor amplifiers to operate properly, they must be correctly biased.īiasing means setting up the correct dc operating voltages between input leads of a transistor. These circuits produce outputs that vary or are linear.Īmplifier devices such as transistors or ICs can also be used to switch current on or off, depending on how they are biased in the circuit.įigure 1.
A small amount of current or voltage can control a larger amount of voltage or current. The gain is the ratio between the strength of the output (current, voltage, or wattage) and the strength of the input (current, voltage, or wattage), Figure 1.Īmplifier circuits can be thought of as control circuits.
The amount of amplification in a circuit is known as gain. Amplification can be accomplished using vacuum tubes or semiconductor devices such as transistors or integrated circuits. In electronics, amplifiers have been used since the early twentieth century.