4/27/2023 0 Comments Transistor diagram![]() ![]() In other words, the load voltage will always be about 0.7 volts less than the input voltage for all conditions where the transistor is conducting. Given the voltage polarities across the base-emitter PN junction and the load resistor, we see that these must add together to equal the input voltage, following Kirchhoff’s Voltage Law. This 0.7volt drop is largely irrespective of the actual magnitude of base current thus, we can regard it as being constant:Įmitter follower: Emitter voltage follows base voltage (minus a 0.7 V VBE drop.) If this junction is forward-biased (the transistor conducting current in either its active or saturated modes), it will have a voltage drop of approximately 0.7 volts, assuming silicon construction. Referring to the diode current source transistor model in figure below, we see that the base current must go through the base-emitter PN junction, which is equivalent to a normal rectifying diode. ![]() It is simple to understand why the output voltage of a common-collector amplifier is always nearly equal to the input voltage. This holds true for transistors of any β value, and for load resistors of any resistance value. Examined from the perspective of output voltage change for a given amount of input voltage change, this amplifier has a voltage gain of almost unity (1), or 0 dB. This is the unique quality of the common-collector amplifier: an output voltage that is nearly equal to the input voltage. Moreover, a close examination reveals that the output voltage is nearly identical to the input voltage, lagging behind by about 0.7 volts. Unlike the common-emitter amplifier from the previous section, the common-collector produces an output voltage direct rather than inverse proportion to the rising input voltage.Īs the input voltage increases, so do the output voltage. The netlist is in the figure below.Ĭommon collector: Output equals input less a 0.7 V V BE drop. Let’s proceed immediately to a SPICE analysis of this amplifier circuit, and you will be able to immediately see what is unique about this amplifier. However, this is not necessarily what sets it apart from other amplifier designs. This presumption is indeed correct: the current gain for a common-collector amplifier is quite large, larger than any other transistor amplifier configuration. Since the emitter lead of a transistor is the one handling the most current (the sum of base and collector currents, since base and collector currents always mesh together to form the emitter current), it would be reasonable to presume that this amplifier will have a very large current gain. It should be apparent that the load resistor in the common-collector amplifier circuit receives both the base and collector currents, being placed in series with the emitter. Output is from emitter-collector circuit. It is called the common-collector configuration because (ignoring the power supply battery) both the signal source and the load share the collector lead as a common connection point as in the figure below.Ĭommon collector: Input is applied to base and collector. Called the common-collector configuration, its schematic diagram is shown in the figure below.Ĭommon collector amplifier has collector common to both input and output. Our next transistor configuration to study is a bit simpler for gain calculations.
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