An emitter follower has a slight loss in voltage gain but makes up for it in current. The input and output of an emitter follower are the base and the emitter, respectively, and the collector is at ac zero. Emitter follower article about emitter follower by the. For instance, in the loaded circuit shown in figure 2. Hi, so, im learning emitter follower by building a circuit that is described in learning the art of electronic book in the lab 3 chapter. Electronic fundamentals ii elnc 1226 1231 page 49 the circuit below is an example of the emitter follower being used as a. This will lead to increase m the collectoremitter resistance of the transistor causing an increase in collector to emitter voltage and as a result the output voltage will be reduced. Lecture 10 emitter follower common collector amplifier. The ac voltage gain of cc amplifier is calculated as e v ee r a rr 1. The emitter follower circuit is also called a common collector circuit, because the collector is common with the power supply voltage. The voltage on the emitter resistor follows the input voltage staying around 2 percent less at all times. The common collector circuit configuration is more widely known as the emitter follower and it provides a high input impedance and a low output impedance.
The input voltage to an emitter follower is usually 9. Therefore, under normal operation the emitter follows the base with a 0. As discussed before, the voltage a circuit receives from a source depends on its input impedance as well as the internal impedance of the source, while the voltage it delivers depends on its output impedance as well as the load impedance. Bjt emitter follower voltage amplifiervoltage amplifier with r l, r o should be as small as possible what if a given amplifier configuration does not have small rwhat if a given amplifier configuration does not have small r o for example ce. Transistor emitter follower, common collector amplifier. Emitter follower at the condition of vin 0, vp should be set to give zero meter reading, vm 0. If the input voltage to an emitter follower is too large the. Emitter follower article about emitter follower by the free.
Experiment 6 transistors as amplifiers and switches. However, for a better understanding i like to give some additional explanation. May 25, 2018 theres a handy trick for reducing the voltage offset between an emitter followers input and output. One of the application in which i have personally used emitter follower circuit is a discrete voltage regulator. The emitter follower circuit is also called a common collector circuit, because the collector is. In its behavior, the emitter follower is analogous and very similar to the source follower in metaloxidesemiconductor mos circuits. The rise time of the output can be relative fast as the emitter current is limited only by beta times the base current that can be supplied by the signal source driving the base. An emitter follower can serve as a buffer for a voltage source. For that we set the quiescent point need to be set with the circuitry external to the transistor, the values of resistors rc and rb, and the dc voltage sources, vcc and vbb, have chosen accordingly. The increased zener current will increase the output voltage but only a little. Dc voltmetercircuit diagram, block diagrambasic guide. Experiment 6 transistors as amplifiers and switches our final topic of the term is an introduction to the transistor as a discrete circuit element.
In this configuration the output voltage is always a shade lower than the input base signal due to the inherent base to emitter drop. This file is licensed under the creative commons attributionshare alike 3. If beta 200 and re 150 ohm, the input impedance of the base is approximately 8. According to my understanding, a bjt in emitter follower configuration has its emitter following the base, no matter what the emitter signal is fed to. The voltage divider at left is a poor voltage source because it is so strongly affected by the value of the load resistor. However, theres a note in the book that a base resistor is needed to omit any oscillation. Emitter follower regulator, assignment help, zener voltage. Note that when the emitterfollower amplifier is fed by the low resistance voltage source output of the m1k, r s. Aug 24, 2019 now, in an emitter follower transistor configuration as shown below, the load is connected at the emitter side of the transistor, that is between the emitter and the ground rail. A circuit that uses a commoncollector transistor amplifier stage with unity voltage gain, large input resistance r i, and small output resistance r o see illustration.
Comparing this with the input resistance of the common emitter circuit, we see that the emitter follower has much higher input resistance. The ordering of the fields serves as a stepbystep guide for the design process. In the calculations we have overlooked the v be of 0. A square wave out of an emitter follower implies 18. Example find the voltage reading and % error of each reading obtained with a voltmeter on i 5. In essence it is a current amplifier with unit gain. This means that the emitter follower circuit provides an ideal buffer stage, and as a result it is used in many circuits where there is a need not to load a circuit like an oscillator or. There is usually no collector load resistor, with the collector connected directly to the supply. This name is derived from the fact that the emitter voltage follows that of the base circuit the circuit has unit voltage gain. The output resistance is the parallel combination of and the resistance of the circuit to the left of the emitter of the transistor including, which can be found as the ratio of the opencircuit voltage with and the shortcircuit. Analysis of emitter follower core impact of source resistance impact of early effect emitter follower with biasing reading. The additional emitterfollower is a fet sourcefollower with its gate input resistance typically in excess of 1 m.
A buffer is a circuit that is used for impedance matching that aids the transfer of power from the source to the load. If the load resistance of an emitter follower is very large, the external ac emitter resistance equals 16. Apr 02, 2017 this lecture presents emitter follower, that is common collector amplifier using exact and approximate analysis. Common collector emitter follower transistor amplifier basics.
Comparing this with the input resistance of the commonemitter circuit, we see that the emitter follower has much higher input resistance. It is therefore important to consider these input and output impedances of an amplification circuit as well as its voltage gain. I have met a emitter follower design example in the horowitzhills book scheme is below. I am submitting herewith a thesis written by william parker finch entitled analysis of the white emitter follower amplifier. Because this feedback voltage is caused by the emitter current we can say that the common collector configuration emitter follower realizes currentcontrolled voltage feedback. In this voltmeter, voltage to be measured is firstly attenuated with range selector switch to keep the input voltage of amplifier within its level. When voltage is applied to the base it no longer sees its full voltage potential to ground it now sees a potential only 2 percent less than itself. These are the mostly used ones with a number of applications.
The basic simple emitter follower voltmeter circuit, illustrated in. I have examined the final electronic copy of this thesis for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of master of. As we discussed in our previous article, there are three transistor configurations that are used commonly for signal amplification i. In the actual case of an emitter follower it is the voltage developped at the emitter node which causes the feedback. The output voltage of an emitter follower is across the 7. The emitter follower is typically used as a buffer, which provides high input impedance and low output impedance. Therefore, the voltage at the output is the same as the voltage at the input. Theres a handy trick for reducing the voltage offset between an emitter followers input and output. Practical emitterfollower voltmeter using second transistor q. It is a mirror image, so to speak, of the input voltage. The emitter follower has a gain of less than one, typical gain of around.
In a bjt configuration when the emitter terminal is used as the output, the network is called an emitterfollower. Therefore, the potentiometer r5 is for the zero adjust. I recommend that it be accepted for nine quarter hours of credit in. It is called so because the transistor used is connected in an emitter follower configuration. And i dont understand why the resulting steps is correct. If the input voltage to an emitter follower is too large, the output voltage will be a. The emitter follower configuration is frequently used for impedance matching and to increase the current gain.
In its behavior, the emitter follower is analogous and very similar to the source follower in. The buffer, an emitter follower, pro vides the low output impedance necessary to drive 500 loads. This is an emitter follower or buffer amplifier circuit, where the output is simply equal to the input minus a diode drop about 700mv. Just fill the input fields below in given order from top to bottom. Difference between the two circuits shown is only that the one, which is a current series feedback amplifier uses another load resistor coupled to the emitter resistor using one capacitor that is the load resistor and emitter resistor are. Emitter follower voltage regulator all about circuits. Vout is taken from the emitter junction and follows the input voltage vin. Emitter follower common collector transistor amplifier basics 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. A problem suffered by the simple emitter follower can be seen when it drives a capacitive load. To design, build, and test an emitterfollower amplifier using a 2n3904 npn transistor, with an input resistance of at least 1 k. Hybrid equivalent circuit and model asutosh kar, iiit bhubaneswar. Voltage regulators,circuits,types,working principle, design.
This is called the emitter follower or voltage follower. The source follower gives a very high input impedance and nearunity voltage gain hence the alternative title of voltage follower. The ac base voltage of an emitter follower is across the a. Emitter follower or common collector amplifier circuit this tutorial will help you to build and simulate a more complex circuit. The emitter follower common collector amplifier assumptions. Common collector amplifier or emitter follower circuit. The circuit consists of an npn transistor and a zener diode. The amplifiers output is fed via c2 to the ls1r1 junction, thus providing a low impedance drive to the loudspeaker and simultaneously bootstrapping the r1 value so that the circuit gives high voltage gain. In the book, theres an emitter follower circuit to demonstrate the clipping effect and how to remedy it. If the input voltage to an emitter follower is too large. Here, commonemitter amplifier q1 uses collector load ls1r1d1rv2, and drives the q2q3 complementary emitter follower stage.
In this circuit a zener diode is placed at the base of the transistor to clamp the voltage at the base to vz of the zener. Wu,uc berkeley emitter follower core ee105spring2008 lecture10,slide3prof. The commoncollector cc or emitterfollower ef transistor amplifier configuration is used primarily for impedance matching operation. Notice, the effective output impedance of the emitter follower is a factor smaller than the circuit on the left. Voltage regulators,circuits,types,working principle. Now, in an emitter follower transistor configuration as shown below, the load is connected at the emitter side of the transistor, that is between the emitter and the ground rail. An emitter follower is often used this way, since this avoids the need to bias the transistor and avoids the coupling capacitor. Please help me with emitter follower voltage regulator. When we place the emitterfollower on the output of the ce amplifier from the class a npn commonemitter amplifier lab, we have nonzero r s, and. The background of the commonly applied design criterion divider current base current is the fact that the ratio icibbeta has very large tolerances. The advantage of this circuit is that the transistor can provide current and power gain. Emitter follower is a case of negative current feedback circuit. With a single emitter follower transistor operating in class a, you will need. Figure 6 shows a simple selfbiasing via rv1 source follower.
If transistors q1 and q 2 are identical, vbe1 vbe2 vv v v v vv v vme e in be p be in p. Common collector amplifier or emitter follower circuit and. If the q point is at the middle of the dc load line, clipping will first occur on the 14. The emitter follower or common collector circuit is widely used in amplifier configuration of bipolar junction transistors bjts with gain of unity or less. As shown in the figure below, the collector and emitter terminals of the transistor are in series with the load. Live tutors are available 24x7 hours for helping students in their emitter follower regulator related problems. Construction is simplified by the use of a printedcircuit board, but any wiring. Now that we have established that you were thinking in the wrong area when you said one or two milliamps, you can work on things like the current gain of your emitter follower transistor to find out how. It rovides voltage gain near unity, high input and low output impedance. The circuits actual inputtooutput voltage gain is 0.
The follower circuit is often used as a buffer stage for this reason. If the input voltage increases then the output voltage will try to increase, but the transistor keeps the output voltage nearly the same. Such a circuit is also named an emitter follower voltage regulator. The generic circuit for the emitter follower common collector amplifier is given to the left below and the high frequency small signal circuit is shown below and to the right figures 10. The emitter follower circuit which was just discussed lacks to meet the requirements of the circuit current gain a and the input impedance z. H1lin this circuit r c is at ac ground at both ends. The same voltage divider with the transistor buffer at right will supply power to keep the voltage constant over its range of operation. Emitter follower and darlington amplifier are the most common examples for feedback amplifiers. H2lthe base bias can be represented by its theveninequivalent hr b r b1 eer b2 l emitter follower. An emitter follower circuit is a transistor circuit in which the voltage at the emitter follows the input voltage. We provide step by step emitter follower regulator questions answers with 100% plagiarism free content. Your author states that the load for an ef amplifier is small and often capacitive, so he has included the load capacitance, c. Hi, in a textbook the emitter follower is depicted as a voltage series negative feedback amplifier and a current series negative feedback amplifier as well.
Designing a stiff voltage source using an emitter follower. Emitterfollower configuration part 1 topics covered. Original signatures are on file with official student records. The transistor continuously monitors v diff and adjusts its emitter voltage almost equal less v beo to the input voltage by passing the according collector current through the emitter resistor r e.
The voltage gain is approximately equal to unity and output voltage is in phase with the input voltage. Bjt emitter follower configuration design calculator. Emitter diode and external ac emitter resistance d 6. If an emitter follower has re 10 ohm and re 90 ohm, the voltage gain is approximately a. In electronics, a common collector amplifier also known as an emitter follower is one of three basic singlestage bipolar junction transistor bjt amplifier topologies, typically used as a voltage buffer in this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both for example, it may be tied to ground reference or. Class a npn emitterfollower amplifier analog devices wiki. An emitter follower, or common collector, configuration usually has either a current source or resistor supplying current into the emitter. Emitter follower circuit has a prominent place in feedback amplifiers. Here, common emitter amplifier q1 uses collector load ls1r1d1rv2, and drives the q2q3 complementary emitter follower stage.
In the circuit shown below, note the relatively high input. In order to achieve some increase in the overall values of circuit current gain and input impedance, two transistors are connected as shown in the following circuit diagram. Sometimes the current gain and input impedance of emitter followers are insufficient to meet the requirement. When this happens the emitter is not able to acquire a 0v potential, and the bjt is unable to turn on with a regular 0. Negative voltage on emitter follower physics forums. Permission is granted to copy, distribute andor modify this document under the terms of the gnu free documentation license, version 1. If an emitter follower has vceq 5 v, icq 1 ma, and re 1 kohm, the maximum peaktopeak unclipped output is. Electronic instruments for measuring basic parameters. Emitter followers can form a colpitts type oscillator if there is some stray inductance present in the base drive and some capacitance on the output, oscillation can be sensitive to the stray output capacitance value your manipulation of the output wires and can cease at higher values, the best assurance of stability though is addition of base resistor as you already discovered. Similar explanation can be given for decrease in supply voltage. We prepare quality content and notes for emitter follower regulator topic under electronics devices and circuits theory and study. Emitter follower or voltage follower phys321 the transistor can be configured in a unity gain situation. Since an integrated circuit is constructed primarily from dozens to even millions of transistors formed from a single, thin silicon crystal, it. The circuit shown next uses the v be shift up of a pnp emitter follower to partially cancel the v be shift down of an npn emitter follower.
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