Base Biasing Process of Common Emitter Amplifier
One of the most commonly used polarization circuits for a transistor circuit is the self-polarization of the emitter-polarization circuit, in which one or more polarization resistances are used to adjust the first DC values for the three transistor currents. The two most common forms of bipolar transistor polarity are: Beta Dependent and Beta Independent. Transistor polarity voltages depend largely on transistor beta (β), so polarity set for a transistor may not necessarily be the same for another transistor, as the beta values may be different. Transistor polarity can be achieved using a single feedback resistance or using a simple voltage dividing network to ensure the required polarity voltage. The following are five examples of transistor base polarization configuration from a single source (Vcc).
Fixed Base Guiding a Transistor
The circuit shown is called the "fixed basic polarization circuit", since the base current of transistors remains constant for the Vcc values given IB, and therefore the working point of the transistors must also remain constant. These two resistance polarization networks are used to create the first working zone of the transistor using a constant current polarization. This type of transistor polarization regulation is also beta-dependent polarization, since the stable state operating condition is a function of the beta β value of transistors, so the polarization point will vary widely for transistors of the same type. transistors will not be exactly the same. The emitter diode of the transistor is pressed forward by applying the necessary positive basic polaration voltage through the current limitation resistance RB. Assuming a standard bipolar transistor, the forward base emitter voltage drop will be 0.7V. Then the value of rb is simply: (VCC – VBE)/IB, where IB is defined as IC/β. With this single resistance type polarizing arrangement, polarizing voltages and currents do not remain constant during transistor operation and can vary greatly. In addition, the operating temperature of the transistor can adversely affect the working point.
Collector Feedback That Leans on a Transistor
This self-polarization collector feedback configuration is another beta-dependent polarization method that requires two resistances to ensure the DC polarization required for the transistor. The collector-to-base feedback configuration ensures that the transistor is always pouched in the active zone, regardless of beta (β). Dc basic pre-redevector voltage is derived from collector voltage VC, thus ensuring good stability. In this circuit, the basic pre-refining resistance, RB, feed voltage rail is connected to transistor collector C instead of Vcc. Now, if the collector current increases, the collector voltage decreases, reducing the base drive and thus automatically reducing the collector current to keep the Q-point of the transistors constant. Therefore, this collector feedback trend method produces negative feedback around the transistor, since resistance from the output terminal to the input terminal is a direct feedback via RB. Since the polarization voltage is derived from the voltage drop on the load resistance (RL), if the load current increases, a larger voltage drop along the RL and a corresponding reduced collector voltage will be VC. This effect causes a corresponding decrease in the underlying current, which in turn is the IB that normalizes the IC. When the transistor collector current decreases, a reverse reaction will occur. Then this method of polarization is called self-polarization with transistor stability using this type of feedback polarization network, which is usually good for most amplifier designs.
Double Feedback Transistor Polaration
Adding additional resistance to the base deflection network of the previous configuration increases the flow through the basic routing resistors, further improving stability compared to variations in Beta (β). The current flowing from RB1 is usually adjusted to about 10% of the collector's current, equal to ic. Obviously, it must also be larger than the basic current required for β with a minimum Beta value. One of the advantages of this type of self-polarization configuration is that two resistances provide both automatic polarization and Rε feedback at the same time.
Transmitter Feedback Transistor Polaration
This type of transistor polarization configuration, often referred to as self-emitting polarization, uses both emitter and base collector feedback to further stabilize collector current. This is due to the rb1 and RE resistors, as well as the fact that the base-emitter connection of the transistor is all connected effectively in series with the feed voltage, VCC. The disadvantage of this absorbent feedback configuration is that it reduces output gain due to the base resistance connection. Collector voltage determines the current flowing from the feedback resistance, RB1 produces what is called "degenerative feedback". The current flowing from the emitter, IE (a combination of IC + IB), causes a voltage drop along the RE to appear in such a direction, which reverses the base-emitter connection. Therefore, if the emitter current increases due to the increase in collector current, the voltage drop increases in I*RE. Since the reverse polarity of this voltage makes the base-emitter connection polarized, IB is automatically reduced. Therefore, the emitter current increases less than it would have done without a self-preventing resistance. In general, resistance values, voltage falling over emitter resistance RE is set to be approximately 10% of VCC and the current flowing from resistance RB1 is 10% of collector current IC. Therefore, this type of transistor pre-configuring configuration gives the best result at relatively low power supply voltages.
Voltage Divider Transistor Polaration
Here, the common emitter transistor configuration is polarized using a voltage dividing network to improve stability. The name of this prerequisition configuration comes from the fact that two resistance RB1 and RB2 create a voltage or potentially divisive network throughout the feed with center point connections connected to the transistor base terminal, as shown. This voltage divider polarization configuration is the most widely used transistor polarization method. The emitter diode of the transistor is pressed forward by the voltage value developed throughout the RB2 resistance. In addition, voltage divider network polarity makes the transistor circuit independent of changes in beta, since the polarity voltages set in the transistor base, emitter and collector terminals do not depend on external circuit values. Resistance To calculate the voltage developed throughout RB2 and therefore the voltage applied to the base terminal, we use the voltage divider formula for resistances in the series. In general, the voltage drop on RB2 resistance is much less than RB1 resistance. Clearly, the base voltage of transistors by soil, VB, will equal the voltage on RB2. The amount of polarity current flowing through resistance RB2 is usually adjusted to 10 times the value of the required base current IB, so that the voltage is high enough to have no effect on the dividing current or changes in Beta. The purpose of the Transistor Tendency is to create a known sedentary working point or Q point for the bipolar transistor to work efficiently and produce an unspoiled output signal. The correct DC polarization of the transistor also establishes the first AC working zone with practical polarization circuits using a two or four resistant pre-redefinition networks. In bipolar transistor circuits, point Q is represented by NPN transistors (VCE, IC) or PNP transistors (VEC, IC). The stability of the base pre-reconstrained network and therefore the Q point are generally evaluated as a function of both Beta (β) and temperature of the collector current. Here, we briefly looked at five different configurations to "polarize a transistor" using resistant networks. However, we can also polarized a transistor using silicon diodes, zener diodes or active networks, all connected to the transistor base terminal. If desired, we can correctly deflect the transistor from a dual-voltage power source.