NPN Common Emitter Amplifiers. The common emitter configuration lends itself to voltage amplification and is the most common configuration for transistor amplifiers. Common Emitter Amplifier and Transistor Amplifiers. We also saw that a family of curves known commonly as the Output Characteristic Curves, relate the transistors Collector current ( Ic ), to the Collector voltage ( Vce ) for different values of the transistors Base current ( Ib ). All types of transistor amplifiers operate using AC signal inputs which alternate between a positive value and a negative value so some way of “presetting” the amplifier circuit to operate between these two maximum or peak values is required. This is achieved using a process known as Biasing. Biasing is very important in amplifier design as it establishes the correct operating point of the transistor amplifier ready to receive signals, thereby reducing any distortion to the output signal. Related Products: Amplifier IC Development Boards and Kits . This is in fact the DC operating point of the amplifier and its position may be established at any point along the load line by a suitable biasing arrangement. The best possible position for this Q- point is as close to the center position of the load line as reasonably possible, thereby producing a Class A type amplifier operation, ie. Consider the Common Emitter Amplifier circuit shown below. Related Products: RF Amplifier Module . This type of biasing arrangement uses two resistors as a potential divider network across the supply with their center point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits. This method of biasing the transistor greatly reduces the effects of varying Beta, ( . The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, R1, R2 and the power supply voltage Vcc as shown with the current flowing through both resistors.
ESE319 Introduction to Microelectronics 2008 Kenneth R. Lopresti 2006) update 29Sep08 KRL 1 Common Emitter BJT Amplifier Design Current Mirror Design. Common-Emitter Amplifier. How can one design a BJT amplifier only after one simple characteristic experiment? Maybe students are all brilliant or this subject is already covered in a. Design of a Common-Emitter BJT Ampli. Using a Q2N2222 BJT, design a common-emitter ampli Electronics Tutorial about the Common Emitter Amplifier and Transistor Amplifier Circuits including its Load. Introduction to the Amplifier; Common Emitter Amplifier; Common Source JFET. Completed Common Emitter. Then the total resistance RT will be equal to R1 + R2 giving the current as i = Vcc/RT. The voltage level generated at the junction of resistors R1 and R2 holds the Base voltage (Vb) constant at a value below the supply voltage. Then the potential divider network used in the common emitter amplifier circuit divides the supply voltage in proportion to the resistance. This bias reference voltage can be easily calculated using the simple voltage divider formula below: Bias Voltage. The same supply voltage, (Vcc) also determines the maximum Collector current, Ic when the transistor is switched fully “ON” (saturation), Vce = 0. The Base current Ib for the transistor is found from the Collector current, Ic and the DC current gain Beta, . Beta has no units as it is a fixed ratio of the two currents, Ic and Ib so a small change in the Base current will cause a large change in the Collector current. One final point about Beta. Transistors of the same type and part number will have large variations in their Beta value for example, the BC1. NPN Bipolar transistor has a DC current gain Beta value of between 1. Beta is a characteristic of their construction and not their operation. Bjt Common Emitter Amplifier Pdf FilesAs the Base/Emitter junction is forward- biased, the Emitter voltage, Ve will be one junction voltage drop different to the Base voltage. If the voltage across the Emitter resistor is known then the Emitter current, Ie can be easily calculated using Ohm’s Law. The Collector current, Ic can be approximated, since it is almost the same value as the Emitter current. Common Emitter Amplifier Example No. A common emitter amplifier circuit has a load resistance, RL of 1. Calculate the maximum Collector current (Ic) flowing through the load resistor when the transistor is switched fully “ON” (saturation), assume Vce = 0. Also find the value of the Emitter resistor, RE if it has a voltage drop of 1v across it. Calculate the values of all the other circuit resistors assuming an NPN silicon transistor. This then establishes point “A” on the Collector current vertical axis of the characteristics curves and occurs when Vce = 0. When the transistor is switched fully “OFF”, their is no voltage drop across either resistor RE or RL as no current is flowing through them. Then the voltage drop across the transistor, Vce is equal to the supply voltage, Vcc. This establishes point “B” on the horizontal axis of the characteristics curves. Generally, the quiescent Q- point of the amplifier is with zero input signal applied to the Base, so the Collector sits about half- way along the load line between zero volts and the supply voltage, (Vcc/2). Therefore, the Collector current at the Q- point of the amplifier will be given as: This static DC load line produces a straight line equation whose slope is given as: - 1/(RL + RE) and that it crosses the vertical Ic axis at a point equal to Vcc/(RL + RE). The actual position of the Q- point on the DC load line is determined by the mean value of Ib. As the Collector current, Ic of the transistor is also equal to the DC gain of the transistor (Beta), times the Base current (. Resistors, R1 and R2 can now be chosen to give a suitable quiescent Base current of 4. The current flowing through the potential divider circuit has to be large compared to the actual Base current, Ib, so that the voltage divider network is not loaded by the Base current flow. A general rule of thumb is a value of at least 1. Ib flowing through the resistor R2. Transistor Base/Emitter voltage, Vbe is fixed at 0. V (silicon transistor) then this gives the value of R2 as: If the current flowing through resistor R2 is 1. Base current, then the current flowing through resistor R1 in the divider network must be 1. Base current. The voltage across resistor R1 is equal to Vcc – 1. VRE + 0. 7 for silicon transistor) which is equal to 1. V, therefore R1 can be calculated as: The value of the Emitter resistor, RE can be easily calculated using Ohm’s Law. The current flowing through RE is a combination of the Base current, Ib and the Collector current Ic and is given as: Resistor, RE is connected between the Emitter and ground and we said previously that it has a voltage of 1 volt across it. Then the value of RE is given as: So, for our example above, the preferred values of the resistors chosen to give a tolerance of 5% (E2. Then, our original Common Emitter Amplifier circuit above can be rewritten to include the values of the components that we have just calculated above. Completed Common Emitter Circuit. Coupling Capacitors. In Common Emitter Amplifier circuits, capacitors C1 and C2 are used as Coupling Capacitors to separate the AC signals from the DC biasing voltage. This ensures that the bias condition set up for the circuit to operate correctly is not effected by any additional amplifier stages, as the capacitors will only pass AC signals and block any DC component. The output AC signal is then superimposed on the biasing of the following stages. Also a bypass capacitor, CE is included in the Emitter leg circuit. This capacitor is an open circuit component for DC bias meaning that the biasing currents and voltages are not affected by the addition of the capacitor maintaining a good Q- point stability. However, this bypass capacitor short circuits the Emitter resistor at high frequency signals and only RL plus a very small internal resistance acts as the transistors load increasing the voltage gain to its maximum. Generally, the value of the bypass capacitor, CE is chosen to provide a reactance of at most, 1/1. RE at the lowest operating signal frequency. Output Characteristics Curves. Ok, so far so good. We can now construct a series of curves that show the Collector current, Ic against the Collector/Emitter voltage, Vce with different values of Base current, Ib for our simple common emitter amplifier circuit. These curves are known as the “Output Characteristic Curves” and are used to show how the transistor will operate over its dynamic range. A static or DC load line is drawn onto the curves for the load resistor RL of 1. Likewise when the transistor is fully “ON” and saturated the Collector current is determined by the load resistor, RL and this is point A on the line. We calculated before from the DC gain of the transistor that the Base current required for the mean position of the transistor was 4. We could quite easily make life easy for ourselves and round off this value to 5. We need to find the maximum and minimum peak swings of Base current that will result in a proportional change to the Collector current, Ic without any distortion to the output signal. As the load line cuts through the different Base current values on the DC characteristics curves we can find the peak swings of Base current that are equally spaced along the load line. These values are marked as points N and M on the line, giving a minimum and a maximum Base current of 2. This then gives us a theoretical maximum input signal to the Base terminal of 6. It can be seen that the Collector- emitter voltage is in anti- phase (- 1. As the Base current Ib changes in a positive direction from 5. In other words the output signal is 1. Common Emitter Voltage Gain. The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the input voltage to the change in the amplifiers output voltage. But voltage gain is also equal to the ratio of the signal resistance in the Collector to the signal resistance in the Emitter and is given as: We mentioned earlier that as the signal frequency increases the bypass capacitor, CE starts to short out the Emitter resistor. Then at high frequencies RE. The transistors semiconductor material offers an internal resistance to the flow of current through it and is generally represented by a small resistor symbol shown inside the main transistor symbol. Transistor data sheets tell us that for a small signal bipolar transistors this internal resistance is the product of 2. V. At high frequency, the bypass capacitor shorts out the Emitter resistor leaving only the internal resistance Re in the Emitter leg resulting in a high gain.
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