I think you mean a common emitter amplifier, which is an amplifier of voltage. Emitter-follower or common collector amplifiers are used to match impedances, or to amplify power or current. The emitter-follower is a type of common emitter circuit that has a resistor between the emitter and ground. The output signal is taken from the point between the emitter and its resistor.
A: When a signal is not amplified but simply taken from an emitter the reason is that the emitter will provide better current capabilities. The voltage gain of an emitter follower is theoretically 1.
In practice, due to losses, it is marginally less than one. The Class C, Common Collector Emitter Follower is used to amplify the available current from a voltage driving circuit that might be disturbed by the load impedance.
If the actual voltage value is important, the emitter follower is often teamed up with an opamp which sets the emitter voltage based on the input voltage. Common drain is also called an emitter follower. It has a power gain but the voltage gain is unity. Common emitter produces a voltage gain. The transistor monitors continuously Vdiff and adjusts its emitter voltage almost equal less VBEO to the input voltage by passing the according collector current through the emitter resistor RE.
As a result, the output follows the input voltage variations and hence name emitter follower. Ninan V. Colector resistance in an emitter follower circuit serves to place a limit on how much current can be supplied by the transistor.
Often, the resistor is sized so that a short circuit in the load does not cause the transistor to fail. Approximately unity. Limit current through emitter, Often the resistance is the load itself. So the restatnce limits current on what otherwise would act as a voltage follower. A darlington emitter follower is two transistors operating as one.
Both collectors are tied together. The emitter of the first is connected to the base of the second. The darlington exhibits very high gain. If hFe, for instance, on each transistor was nominally , the effective hFe would be nominally squared, or The emitter follower is a common collector configuration where the emitter tracks the base, offset by the base-emitter forward junction voltage.
In a darlington, there are two junctions in series, so the effective junction voltage is about 1. Voltage gain of the darlington, like any emitter follower, is one, or unity. Current gain, however, is limited only by the power supply, the transistor, and effective hFe. In the darlington configuration, current gain is effectively considered to be large enough to be thought of as "infinite". See an analog design textbook for some specific examples but the gist of emitter follower's purpose in life is to transform a high output resistance signal source to achieve a signal with lower output resistance.
Log in. Electronics Engineering. This result is identical to that for a common emitter amplifier with an emitter resistor. The input resistance to a common collector amplifier is large for typical values of the load resistance Re. The voltage gain is the ratio of output voltage to input voltage.
If the input voltage is again taken to be the voltage at the input to the transistor, Vb. The above equation is somewhat less than unity. The approximation equation of voltage gain is given by. This ratio can be directly derived from the voltage gain Av, and a voltage division between the source resistance Rs and the amplifier input resistance Ri.
After substitutions of appropriate equations, the overall voltage gain is given by. The output resistance is defined as the Thevenin resistance at the output of the amplifier looking back into the amplifier.
The circuit is shown below, the AC equivalent circuit to calculate the output resistance. If a voltage v is applied to the output terminals, the base current is found to be. The output resistance for a common collector transistor amplifier is typically small. This article discusses the working of the common emitter amplifier circuit and its applications.
By reading the above information you have got an idea about this concept. Furthermore, any queries regarding this article or if you want to implement Electrical and electronics projects for engineering students , please feel free to comment in the below section.
The common collector transistor circuit configuration gains its name from the fact that the collector circuit is common to both input and output circuits, the base being associated with only the input, then the emitter with the output only. The other name for the common collector is emitter follower.
This name is derived from the fact that the emitter voltage "follows" that of the base circuit - the circuit has unit voltage gain. The emitter follower transistor amplifier has a very straightforward circuit. The base is connected to the previous stage, and often this may be directly connected as this can save on additional bias resistors which lower the input impedance and hence increase the loading to the previous stage.
Looking at the circuit it can be seen that although the emitter voltage follows that of the base, in DC terms it is actually less than that of the base by a voltage equal to the PN junction drop between the base and emitter. Typically this is 0. The table below gives a summary of the major characteristics of the common collector, emitter follower transistor amplifier.
One key aspect of the characteristic is the input impedance. As it is normally used as a buffer amplifier, this is the key parameter. The simplest way of connecting an emitter follower is to directly couple the input as shown below. Often the collector of the previous stage will be at approximately the mid rail voltage, and this means that it can be directly coupled to the buffer stage. It is not always possible to directly couple the emitter follower, common collector buffer.
When this is the case, it is necessary to add a few additional electronic components: coupling capacitors and bias resistors to the circuit. The emitter follower can be designed and electronic component values determined using the design flow below as a basis:. The emitter follower circuit is particularly useful for applications where a input high impedance is required. Offering a high input impedance and low output impedance it is does not load circuits that may only have a small output capability, or those circuits like oscillators that need a high impedance load to ensure the optimum stability, etc.
Collector may need decoupling: On some occasions an emitter follower circuit can oscillate, especially if long leads are present.
This can happen when high frequency responses are needed and high frequency transistors are used.
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