Signal clipped at collector due to lack of dc base bias. See class c commonbase 750 mw rf power amplifier ch 9. Three amplifier configurations are the commonemitter, the commonbase, and the commoncollector. Bjt amplifiers 6 chapter outline 61 amplifier operation 62 transistor ac models 63 the commonemitter amplifier 64 the commoncollector amplifier 65 the commonbase amplifier 66 multistage amplifiers 67 the differential amplifier 68 troubleshooting device application chapter objectives describe amplifier operation discuss transistor models. This is mainly because the input impedance is low as it is connected to a forwardbiased pnjunction, while the output impedance is high as it is taken from a reversebiased pnjunction.
Frequency response of a commonemitter bjt amplifier. Describe and analyze the operation of commoncollector amplifiers. Ee 105 fall 2000 page 1 week 10 npn bjt amplifier stages. Designing audio power amplifiers bob cordell new york chicago san francisco lisbon london madrid mexico city milan new delhi san juan seoul singapore sydney toronto. The collector is connected tied to a dc power supply. Designing a commoncollector amplifier background in the previous lab, you designed a commonemitter ce amplifier. In the bjt, this effect means that the effective width of the base is less than w b. This is the case when the common collector is used as a \current ampli er.
The common emitter amplifier circuit has a resistor in its collector circuit. There are additionally three capacitors but they do not play a role in the basic transistor amplifier design which. The circuit analysis can be done by dc and ac analysis. The analogous fet circuit is the commonsource amplifier, and the analogous tube circuit is the commoncathode amplifier. Note the distinction between cs the configuration and c s the capacitor dont let this confuse you. It is quite easy to adopt preferred component values in the common emitter amplifier design. Capacitors c b and c c are used to block the amplifier dc bias point from the input and output ac coupling.
Simulation of circuit performance with multisim and comparison with measured values. In this circuit the base terminal of the transistor is the input, the collector is the output, and the emitter is common to both. More transistor amplifiers imperial college london. Commonemitter amplifiers are also used in radio frequency transceiver circuits.
Department of eecs university of california, berkeley eecs 105fall 2003, lecture 17 prof. The current flowing through this resistor produces the voltage output of the amplifier. The schematic of a typical commonemitter amplifier is shown in figure 1. The common emitter bjt amplifier is one of three basic singlestage bipolarjunctiontransistor. Study aids, multisim, and lt spice files for this chapter are available at. The common emitter amplifier is widely used and the electronic circuit design for it is relatively straightforward there are a few straightforward calculations which can be combined with a simple design flow to give a surefire result. Emitter degeneration by inserting a resistor in series with the emitter, we degenerate the ce stage. In common emitter amplifier configuration, the emitter of a bjt is common to both the input and output signal as shown below. Further, by choosing an appropriate biasing point, one may make the device suitable for either amplification or for switching i. The commonemitter ce configuration has the emitter as the common terminal, or ground, to an ac signal.
Even before that, a much simpler circuit investigation would be more beneficial to understand the commonemitter amplifier. Since an integrated circuit is constructed primarily from dozens to even millions of transistors formed from a single, thin silicon crystal, it might be interesting and instructive to. The input of this amplifier is taken from the base terminal, the output is collected from the collector terminal and the emitter terminal is common for both the terminals. Common emitter amplifier the common emitter npn amplifier circuit is shown in the figure below. Frequency response of commonemitter ampli er theory. It has a emitterdegeneration bias with a voltage divider. It provides large voltage gain typically tens to hundreds and provides moderate input and output impedance. As you discovered, the input impedance r in of the ce amplifier is moderatetohigh on the order of a few k.
The magnitude of the forward bias voltage should be such that it should keep the emitterbase junction always in the forward. A generic commonemitter ce amplifier circuit is shown to the right this is a modified representation of figure 5. Commonbase transistor amplifiers are socalled because the input and. The voltage gain, a, of the common emitter amplifier can be expressed as the ratio of load resistor r l to the small signal emitter resistance r e. The ce configuration has the emitter as the common terminal, or ground, to an ac signal. The common emitter amplifier circuit shown uses a voltage divider network to bias the transistors base and the common emitter configuration is a very popular way of designing bipolar transistor amplifier circuits. With proper biasing, we can use it as a current amplifierca, voltage amplifierva, transconductance anplifiertca as well as transresistance amplifiertra. We are going to be concentrating on the bjt implementation of the differential pair as emittercoupled, commonemitter. What are the application of common emitter amplifier. To an ac signal this looks like the collector is connected to ground. Improved common emitter amplifier the common emitter amplifier in fig. The basic transistor amplifier circuit is indicated below. To work properly in an amplifier circuit a bipolar junction transistor bjt must be properly biased and operate in the active mode.
This topology will decrease the gain of the amplifier but improve other aspects, such as linearity, and input impedance. Capacitor c e is an ac bypass capacitor used to establish a low frequency ac ground at the emitter of q 1. The common base configuration is usable at higher frequencies than common emitter or common collector. Amplifier bandwidth the gain within the bandwidth is usually constant with respect to frequencywe call this value the midband gain.
Bjt common emitter amplifier analog electronics course. Find the bias point and the amplifier parameters of the circuit below. The ac signal voltage gain is defined as a v v o v i where v o and v i. We can approximately analyse this circuit with the second form of the miller theorem. This article discusses the working of the common emitter amplifier circuit and its applications. Bjt amplifier should come before asking for designing such circuit. Lets start our discussion on common emitter amplifier ce amp, rather from. The calculations for gain, lower cutoff frequency and higher cutoff frequency are as follows. To design a common emitter transistor npn amplifier circuit. Miller capacitor c f is a small capacitance that will be used to control the high frequency 3db response.
Differential amplifiers the differential amplifier may be implemented using bjts or fets and is a commonly used building block in analog ic design. This is analogous to the commonemitter configuration of the bjt. W eff w bx debx dcb where w b is the physical thickness of the base, x deb and x dcb are the depletion region widths from the emitter and collector sides. The common emitter amplifier is a three basic single stage bipolar junction transistor and is used as a voltage amplifier. For a more elaborate circuit see class a commonbase smallsignal high gain amplifierch 9. Ecen 326 lab 1 design of a commonemitter bjt ampli. An important feature of this circuit is that an appreciable amount of current flows into the base of the transistor. Experiment 6 transistors as amplifiers and switches. Problems for amplifier section university of california. The transconductance, g m, of the transistor is a function of the collector current i c and the so called thermal voltage, ktq which can be approximated by around 25 mv or 26 mv at room temperature.
Transistors are can be configured in three different ways depending on whether the common terminal in between the input and output ports is base, collector or emitter and are named common base, common collector and common emitter, accordingly. In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both for example, it may be tied to ground reference or a power supply rail, hence its name. The circuit diagram of a commonemitter ce amplifier is shown in fig. To build and test a similar circuit, go to experiment 6 in your lab manual laboratory. Bias amplifier in highgain region note that the source resistor r s and the load resistor r l are removed for determining the bias point. This transistor amplifier has a common configuration and it is a standard format of transistor circuit whereas voltage gain is desired. Emitter resistance and the transistor emitter resistor. This report attempts to characterize the common emitter amplifier circuit by explaining the theoretical basis and simulating the circuit in the. Voltage gain a v is easy to achieve with this type of amplifier. Because the emitter is grounded, even if sometimes via a resistor, this transistor configuration is referred to as a common emitter amplifier. Commonemitter amplifier the commonemitter amplifier exhibits high voltage and current gain.
Ce amplifiers exhibit high voltage gain and high current gain. Common emitter amplifier circuit working and characteristics. The object is to solve for the smallsignal voltage gain, input resistance, and output resistance. In this amplifier, purposely, the resistor r e constitutes a common path between the output and the input of the amplifier, feedback resistor. The output resistance of the common collector ampli er in fact for all transistor ampli ers is somewhat complicated because the load can be con gured in two ways see gure. Bode plots of firstorder lowpass filters include one capacitor display the following.
The source v bb is applied to the input circuit in addition to the signal. Experiment 6 transistors as amplifiers and switches our final topic of the term is an introduction to the transistor as a discrete circuit element. Cb is used to couple the input signal to the input port of the amplifier. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. Common emitter amplifier working principle and its. In the source resistor sr configuration, the ac input is applied at c g, the ac output is taken at c d and c s is omitted. Niknejad lecture outline mos common source amp current source active load common gate amp. Resume of theory the commonemitter ce transistor amplifier configuration is widely used. The common emitter amplifier circuit in the bipolar transistor tutorial, we saw that the most common circuit configuration for an npn transistor is that of the common emitter amplifier circuit and that a family of curves known commonly as the output characteristic curves, relate the transistors collector current ic, to the output or collector voltage vce, for different values of base.
Frequency response of commonemitter ampli er theory the circuit diagram the commonemitter ce ampli er is shown in fig. Ce mode is the universal mode of operation for a bjt. This is analogous to the emitterresistor configuration of the bjt. Figure 1a shows the circuit of a common emitter ce amplifier using selfbias and load resistor r 0 capacitively coupled to the collector.
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