![]() ![]() The common-emitter circuit is the only configuration that provides a phase reversal. Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees out of phase. The collector current that flows through the high resistance reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of amplification. This increases forward bias and allows more current carriers to be released from the emitter, which results in an increase in collector current and a decrease in collector voltage (making Vc less negative or swing in a positive direction). During the negative alternation of the input signal, the base is driven more negative with respect to the emitter. This action decreases forward bias which reduces collector current (Ic) and increases collector voltage (making Vc more negative). As the input signal swings positive, it also causes the base to swing positive with respect to the emitter. When a transistor is connected in a common-emitter configuration, the input signal is injected between the base and emitter, which is a low resistance, low-current circuit. Since you have already covered what you now know to be a common-emitter amplifier, let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in figure above (view A). ![]() Since the input signal is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, then the emitter is the element common to both input and output. The common emitter also has a somewhat low input resistance (500 ohms - 1500 ohms), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kilohms - 50 kilohms or more), because the output is taken off the reverse-biased junction. The common-emitter configuration (CE) shown in figure above (view A) is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain. Therefore, by applying these three simple steps to the circuit in the previous figure, we can conclude that this circuit is more than just a basic transistor amplifier. The remaining element is the common element, and gives the configuration its name. Identify the element (emitter, base, or collector) from which the output signal is taken.ģ. Identify the element (emitter, base, or collector) to which the input signal is applied.Ģ. An easy way to identify a specific transistor configuration is to follow three simple steps:ġ. Because the common element is often grounded, these configurations are frequently referred to as grounded emitter, grounded base, and grounded collector.Įach configuration, as you will see later, has particular characteristics that make it suitable for specific applications. The term common is used to denote the element that is common to both input and output circuits. A transistor may be connected in any one of three basic configurations (see figure above): common emitter (CE), common base (CB), and common collector (CC).
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