![]() ![]() This sounds amazing since a change of the output of the circuit has an effect on the input. ![]() The idea is that this base current decrement decreases also the collector current! Finally, this base voltage increment has as a result the decrement of the voltage across the base resistor R B, which eventually decreases the current of the base I B. The voltage drop across R E is increased (emitter voltage) which eventually increases the base voltage. If the collector current is increased due to a temperature increment, the emitter current is also increased, thus the current through R E is also increased. This method never worked as it should, so it is not used anymore. In a transistor circuit with fixed bias, a resistor was added at the emitter. This is the first method that was historically used to fix the problem of the unstable current gain discussed previously. That is because the Q point operates from cut-off to hard saturation, and even large current gain changes have little or no effect at the output.Įmitter feedback bias (Fixed bias with emitter resistor) On the other hand, due to the fact that this method is very simple and cost-effective, it is widely used when the transistor operates as a load switch, for example as a relay or LED driver. That is why this biasing method is not used for transistor amplifiers. This means that the collector current will become 38.7mA, and the output voltage will also become 7.7 Volts! A whole volt higher than before. An increment by 15% is a realistic and rather small value. As we've discussed in earlier pages, this will increase the current gain. The output of this circuit is taken from the collector resistor RC: V RC = I C x R C = 6.7 Volts Suppose for example that this is a silicon transistor and operates as a B-class amplifier with current gain 300, R B=80 Kohms, R C=200 Ohms and V CC = 10 volts: The problem with this method is that the collector current is very sensitive is slight current gain changes. Since this is a common emitter circuit, we use the h fe: Now we can calculate the collector current using the appropriate hybrid parameter. ![]() So, by selecting the proper base resistor R B, we can define the required base voltage V B and base current I B. From the second law of Kirchhoff, we have: The base current I B is controlled by the base resistor R B. This is the most rarely used biasing method with transistor amplifiers, but it is widely used when the transistor operates as a switch. In this chapter, we will use a Common Emitter NPN transistor amplifier to analyze the various biasing methods, but each method can be used for other connections as well. We will analyze this method in detail, but first we need to discuss the other biasing methods. The most efficient and commonly used biasing method for transistor amplifiers, it the voltage divider bias (VDB). A correctly placed Q offers maximum amplification without signal distortion or clipping. For now, you need to know that this point will determine how the transistor will operate (amplifier or switch). We will discuss in details the quiescence point within the next chapters. For transistors, biasing means to set the proper voltage and current of the transistor base, thus setting the operating point, also known as quiescence point (Q). Biasing in general means to establish predetermined voltages and currents at specific points of a circuit, so that the circuit components will operate normally. After selecting the proper connection, the one that is most suitable for your application, you must select a biasing method. ![]()
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