Then, our original Common Emitter Amplifier circuit above can be rewritten to include the values of the components that we have just calculated above. So, for our example above, the preferred values of the resistors chosen to give a tolerance of 5% (E24) are: Thus the value of the Emitter resistor, R E is calculated as: Resistor, R E is connected between the transistor’s Emitter terminal and ground, and we said previously that there is a voltage drop of 1 volt across it. The current flowing through R E is a combination of the Base current, Ib and the Collector current Ic and is given as: The value of the Emitter resistor, R E can be easily calculated using Ohm’s Law. Thus the voltage across resistor R1 is equal to Vcc – 1.7v (V RE + 0.7 for silicon transistor) which is equal to 10.3V, therefore R1 can be calculated as: If the current flowing through resistor R2 is 10 times the value of the Base current, then the current flowing through resistor R1 in the divider network must be 11 times the value of the Base current. Transistor Base/Emitter voltage, Vbe is fixed at 0.7V (silicon transistor) then this gives the value of R2 as: 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.Ī general rule of thumb is a value of at least 10 times Ib flowing through the resistor R2. Resistors, R1 and R2 can now be chosen to give a suitable quiescent Base current of 45.8μA or 46μA rounded off to the nearest integer. Instead of using a separate Base bias supply, it is usual to provide the Base Bias Voltage from the main supply rail (Vcc) through a dropping resistor, R1. The actual position of the Q-point on the DC load line is determined by the mean value of Ib.Īs the Collector current, Ic of the transistor is also equal to the DC gain of the transistor (Beta), times the Base current ( β*Ib), if we assume a Beta ( β) value for the transistor of say 100, (one hundred is a reasonable average value for low power signal transistor) the Base current Ib flowing into the transistor will be given as: This static DC load line produces a straight line equation whose slope is given as: -1/(R L + R E) and that it crosses the vertical Ic axis at a point equal to Vcc/(R L + R E). The Collector current, Ic can be approximated, since it is almost the same value as the Emitter current. If the voltage across the Emitter resistor is known then the Emitter current, Ie can be easily calculated using Ohm’s Law. This is because Beta ( β ) is an inherent characteristic of the transistor’s construction and not of its operation.Īs the Base/Emitter junction is forward-biased, the Emitter voltage, Ve will be one junction voltage drop different to the Base voltage. So one BC107 may have a Beta value of 110, while another one may have a Beta value of 450, but they are both BC107 npn transistors. For example, the BC107 NPN Bipolar transistor has a DC current gain Beta value of between 110 and 450 (data sheet value). Transistors of the same type and part number will have large variations in their Beta value. Beta (h FE) 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. Beta is an electrical parameter built into the transistor during manufacture. The aim of any small signal amplifier is to amplify all of the input signal with the minimum amount of distortion possible to the output signal, in other words, the output signal must be an exact reproduction of the input signal but only bigger (amplified).Ī transistor’s Beta value, sometimes referred to as h FE on datasheets, defines the transistor’s forward current gain in the common emitter configuration. This can be achieved using a process known as Biasing.īiasing 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.Īlso, the use of a static or DC load line drawn onto the output characteristics curves of an amplifier allows us to see all the possible operating points of the transistor from fully “ON” to fully “OFF”, and to which the quiescent operating point or Q-point of the amplifier can be found. Then some way of “presetting” a common emitter amplifier circuit configuration is required so that the transistor can operate between these two maximum or peak values. Transistor amplifier’s amplify an AC input signals that alternates between some positive value and a corresponding negative value.
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