One through the emitter resistance, R E to ground and the other through r’e and the base terminal to ground. We have seen above that the input is connected to the emitter and the output taken from the collector.īetween the input and ground terminal there are two possible parallel resistive paths. One of the interesting characteristics of the common base amplifier circuit is the ratio of its input and output impedances giving rise to what is known as the amplifiers Resistance Gain, the fundamental property which makes amplification possible. Thus the input and output waveforms are “in-phase” with each other showing that the common base amplifier is non-inverting amplifier configuration. Note that the amplifiers power gain is about the same as its voltage gain.Īs the voltage gain of the common base amplifier is dependant on the ratio of these two resistive values, it therefore follows that there is no phase inversion between the emitter and the collector. However, a practical common base amplifier circuit would be unlikely to use a load resistor greater than about 20kΩ with typical values of voltage gain range from about 100 to 2000 depending on the value of R C. Likewise, the higher the value of load resistance the greater the amplifiers voltage gain. The volt gain, A V for a collector load resistance of 10kΩ would be: 10,000/25 = 400, and the more current which flows through the junction, the lower becomes its dynamic resistance and the higher the voltage gain. So if for example, 1mA of current is flowing through the emitter-base junction, its dynamic impedance would be 25mV/1mA = 25Ω. The electrical relationship between the three transistor currents can be shown to give the expressions for alpha, α and Beta, β as shown. Thus the CB amplifier attenuates the current, with typical values of alpha ranging from between 0.980 to 0.995. In a BJT amplifier the emitter current is always greater than the collector current as I E = I B + I C, the current gain ( α) of the amplifier must therefore be less than one (unity) as I C is always less than I E by the value of I B. The current gain for a CB configuration is called Alpha, ( α ). For a common base amplifier configuration, current gain, A i is given as i OUT/i IN which itself is determined by the formula I C/I E. Since the emitter current, I E is also the input current, any changes to the input current will create a corresponding change in the collector current, I C. Then we can see from the basic common base configuration that the input variables relate to the emitter current I E and the base-emitter voltage, V BE, while the output variables relate to the collector current I C and the collector-base voltage, V CB. Common Base Amplifier using an NPN Transistor That is the base-emitter junction is forward-biased.Ĭonsider the basic common base amplifier configuration below. Thus the emitter current is also the input current, and the collector current is also the output current, but as the transistor is a three layer, two pn-junction device, it must be correctly biased for it to work as a common base amplifier. The common base configuration is less common as an amplifier than compared to the more popular common emitter, (CE) or common collector, (CC) configurations but is still used due to its unique input/output characteristics.įor the common base configuration to operate as an amplifier, the input signal is applied to the emitter terminal and the output is taken from the collector terminal. The Common Base Amplifier is another type of bipolar junction transistor, (BJT) configuration where the base terminal of the transistor is a common terminal to both the input and output signals, hence its name common base (CB).
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