The emitter-base junction must be biased forward, and the collector-base junction must be biased backward. These bias conditions must be present for a transistor amplifier to function normally. When these biases are absent or insufficient, high output levels may destroy the transistor.
In practice, transistors need some bias in order to operate reliably. Base currents will flow through any unbiased junctions, which will decrease the gain of the transistor. Also, if there is no voltage across the collector-emitter junction, it will not conduct current and thus prevent damage to the device.
Bias circuits are used in radio transmitters and receivers to provide appropriate voltages to ensure that transistors function properly. For example, power transistors need to have their base-emitter junctions biased forward so that current can flow from the emitter to the base. This allows the transistor to handle power levels. In addition, magnetic-transmitters need to have their collectors biased toward positive voltage so that enough current flows through them to maintain signal strength.
When designing amplifiers, engineers should keep in mind the type of transistor being used. Different types of transistors require different bias conditions to work properly.
The two transistor junctions must be forward and reverse biased in order for the transistor amplifier to function properly. When both transistors are off, there is no current flowing through either of them. When one transistor is on and the other is off, only that single transistor will conduct current. The amplifier does not work when both transistors are on or off.
The easiest way to understand how the amplifier works is with an example. Suppose we want to amplify the signal from this microphone so we can hear it over the TV. The following diagram shows what would happen if we did nothing at all to amplify the signal from this microphone. All we did was take the output from the microphone and connect it directly to the input of our amplifier:
Now let's say we wanted to amplify the signal from this microphone so we could hear it over the TV.
As you can see, this would cause problems because even though the microphone is turned off, it still produces noise which gets amplified along with the signal from the TV.
To function, an NPN transistor requires both reverse and forward bias. The forward bias is determined by the relationship between the emitter voltage and the emitter. The collector voltage is linked to the collector voltage, and the reverse bias is connected to the collector voltage. When there is no voltage applied to the base, it will be in the off state. But if some voltage is applied to the base, the transistor will be turned on. That's why we need to apply a positive or negative voltage to the base of the transistor for it to turn on.
An NPN transistor will only conduct current in one direction. This means that you cannot have two of these transistors hooked up to a circuit board such that one acts as a pull-down resistor and the other acts as a pull-up resistor. If both transistors are on, then there would be no way to determine which direction current should flow through the circuit.
The type of bias required by an NPN transistor depends on what kind of connection is made to its collector. There are three main types of connections: open circuit, load, and tie.
In an open circuit condition, the collector is not connected to anything. It will therefore be at a high potential relative to the emitter, which will cause the transistor to be off.
A load connection means that something is connected to the collector.
The transistor can amplify voltage when the base-emitter junction is forward biased and the base-collector junction is reverse biased because the collector-to-emitter voltage is larger than the base-to-emitter voltage and is also between the cutoff and saturation states. This condition is called active mode operation or linear mode operation.
When the base-emitter junction is reversed biased, no current flows into or out of this terminal so there is no amplification of voltage possible. However, there may be some leakage current into or out of this terminal which would be undesirable in an amplifier application. This state is called passive mode operation or nonlinear mode operation.
In general, if there is any possibility that the base-emitter junction will be reversed biased under operating conditions, it should be taken into account when designing a bipolar transistor amplifier. If this possibility cannot be avoided, then a protective diode connected from the base to earth must be included in the circuit.
The purpose of this article is to inform you about the effects of reverse bias on transistors used in amplifiers. This knowledge will help you design more stable amplifiers.
This article describes how to properly bias bipolar transistors. As an example, consider the Common Emitter Amplifier (CEA) arrangement to be explored. The CEA is one of three basic bipolar transistor designs used to create a signal amplifier. The other two are the Transistor-Transistor Logic (TTL) and Metal-Oxide-Semiconductor (MOS) circuits. A CEA has two bipolar transistors with their bases connected together and to the negative input of the amplifier. The collectors of the two transistors are also connected together and to the positive output of the amplifier. A CEA produces a very high output voltage when its input is driven close to VEE (typically -15 volts). This article describes how to adjust the bias of a CEA so that it will operate in its most efficient region.
Bipolar transistors need to have their base-emitter junctions biased at or near zero potential in order for them to work well within circuit design constraints. Otherwise, there would be no way to tell which transistor was being activated by the signal driving its base terminal. For example, if both transistors were turned on at once, then there would be a short between their collector terminals and both output voltages would rise toward VEE. Since this does not make sense, we must ensure that only one transistor is active at any time.