12/1/2023 0 Comments Bipolar transistor circuitsOne of the most widely used combination-bias systems is the voltage-divider type shown in figure below. Sometimes degeneration is desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose.Ī combination of fixed and self-bias can be used to improve stability and at the same time overcome some of the disadvantages of the other two biasing methods. This process of returning a part of the output back to its input is known as DEGENERATION or NEGATIVE FEEDBACK. This is because the collector and base signals for this particular amplifier configuration are 180 degrees out of phase (opposite in polarity) and the part of the collector signal that is fed back to the base cancels some of the input signal. Moderate changes in ambient temperature are expected (2) it reduces amplification since the signal on the collector also affects the base voltage. Self-bias has two small drawbacks: (1) It is only partially effective and, therefore, is only used where The exact opposite effect is produced when the collector current decreases.Ī basic transistor amplifier with self-bias The decrease in base current will oppose the original increase in collector current and tend to stabilize it. This drop in Vc will be fed back to the base and will result in a decrease in the base current. Now, if an increase of temperature causes an increase in collector current, the collector voltage (Vc) will fall because of the increase of voltage produced across the load resistor (Rl). By tying the collector to the base in this manner, feedback voltage can be fed from the collector to the base to develop forward bias. This reaction to temperature is undesirable because it affects amplifier gain (the number of times of amplification) and could result in distortion, as you will see later in this discussion.Ī better method of biasing is obtained by inserting the bias resistor directly between the base and collector, as shown in figure below. This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. Unfortunately, this simple arrangement is quite thermally unstable. As you recall, it consisted basically of a resistor (Rb) connected between the collector supply voltage and the base. The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS, was used in the previous two figures. Although there are numerous biasing methods, only three basic types will be considered. As mentioned earlier, various biasing methods can be used to accomplish both of these functions. Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its dc operating point (the no signal values of collector voltage and collector current). This is accomplished by selecting the proper circuit-biasing conditions and ensuring these conditions are maintained despite variations in ambient (surrounding) temperature, which cause changes in amplification and even distortion (an unwanted change in a signal). 2N222 Small signal general purpose amplifier and switching transistors like the 2N2222 and 2N3904 are commonly available in a variety of package types such as the TO-18 metal cased package (6), and the cheaper plastic TO-92 version (7) for through-hole mounting on printed circuit boards, as well as in surface mount SOT-23 versions (8).One of the basic problems with transistor amplifiers is establishing and maintaining the proper values of quiescent current and voltage in the circuit. BD135/BD136 Complementary (NPN/PNP) pair of low-medium power audio output transistors in a SOT-32 package. 2N6487 General purpose NPN output transistor with a power rating up to 75W in a TO-220 package. 2N2219 NPN silicon transistor in a metal cased TO-39 package, designed for use as a high speed switch or for amplification at frequencies from DC (0Hz) up to UHF at about 500MHz. Can be used as one half of a complementary push-pull output pair with the PNP MJ2955 transistor. 2N3055 NPN Silicon Power transistor (115W) designed for switching and amplifier applications. High Voltage (1500V) high power (50W) NPN fast switching transistor in an ISOWATT218 package, originally designed for use in analogue TV timebases but also used in switched mode power supplies. 3.1.2 Typical Bipolar Junction Transistors
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