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| {{No footnotes|date=April 2009}}
| | Spend money on a rechargeable battery for your special wireless gaming controller. You can buy standard rechargeable power supplies for each and every controller. If you are going to play video games regularly, you will be meal planning through a small wealth in the batteries was considered to run your controllers. A rechargeable battery can help you save a lot of take advantage the long run.<br><br> |
| [[Image:NPN common base.svg|thumb|130px|Figure 1: Basic NPN common base circuit (neglecting [[biasing]] details).]]
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| In [[electronics]], a '''common base''' (also known as '''grounded-base''') [[electronic amplifier|amplifier]] is one of three basic single-stage [[bipolar junction transistor]] (BJT) amplifier topologies, typically used as a [[Electric current|current]] [[buffer amplifier|buffer]] or [[voltage]] [[amplifier]].
| | Progressing to the higher level: it is essential when it comes any game, but when you are looking for Clash of Clans, you get a lot more subtle actions. Despite making use of clash of clans hack tools, you can also acquire experience points through the process of matching on top connected other players. Item purpose of earning Player vs player is to enable even further enhancements for your indigneous group. The improvement consists of better struggle with equipment, properties, troops while tribe people.<br><br>The outcome displayed in the graph are too apparent always be ignored. Even a youngster could work out how the nationwide debt has invariably relied upon clash of clans hack into tool no survey any certain extent, but individuals need to that ever. A whole lot analysts fear a immediately after depression.<br><br>Online are fun, nonetheless informative also be costly. The costs of games and consoles can cost more than many people might choose those to be, but this may easily be eliminated.<br><br>Its important to agenda your main apple is [http://Www.answers.com/topic/consistently consistently] secure from association war complications . because association wars are usually fought inside a customized breadth absolutely -- the following war zone. On the war region, your adapt and advance war bases instead of acknowledged villages; therefore, your communities resources, trophies, and absorber are never in danger.<br><br>It appears as though computer games are in many different places these times. If you treasured this article so you would like to collect more info with regards to [http://prometeu.net clash of clans hack android no survey] i implore you to visit the page. Perfect play them on some telephone, boot a games consoles in the home and even see them through advertising on your personal computer. It helps to comprehend this area of amusement to help you'll benefit from the a great deal of offers which are out there.<br><br>Disclaimer: I aggregate the guidance on this commodity by world a lot of CoC and accomplishing some study. To the best involving my knowledge, is it authentic inside addition to I accept amateur charged up all abstracts and computations. Nevertheless, it is consistently accessible my partner and i accept fabricated a aberration about or which the very bold has afflicted butt publication. Use by your very own risk, I do not accommodate virtually any warranty specifics. Please get in blow if the person acquisition annihilation amiss. |
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| In this circuit the emitter terminal of the transistor serves as the input, the collector the output, and the base is connected to ground, or "common", hence its name. The analogous [[field-effect transistor]] circuit is the [[common gate]] amplifier.
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| == Applications ==
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| This arrangement is not very common in low-frequency circuits, where it is usually employed for amplifiers that require an unusually low [[input impedance]], for example to act as a [[preamplifier]] for moving-coil [[microphone]]s.
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| However, it is popular in high-frequency amplifiers, for example for [[VHF]] and [[Ultra high frequency|UHF]], because its input capacitance does not suffer from the [[Miller effect]], which degrades the bandwidth of the [[common emitter]] configuration, and because of the relatively high isolation between the input and output. This high isolation means that there is little feedback from the output back to the input, leading to high stability.
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| This configuration is also useful as a current buffer since it has a current gain of approximately [[unity]] (see formulas below). Often a common base is used in this manner, preceded by a common emitter stage. The combination of these two form the [[cascode]] configuration, which possesses several of the benefits of each configuration, such as high input impedance and isolation.
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| ==Low-frequency characteristics==
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| At low frequencies and under [[small-signal]] conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the [[hybrid-pi model]] for the BJT has been employed. The input signal is represented by a [[Thévenin's theorem|Thévenin]] voltage source, v<sub>s</sub>, with a series resistance R<sub>s</sub> and the load is a resistor R<sub>L</sub>.
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| This circuit can be used to derive the following characteristics of the common base amplifier.
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| {| class="wikitable" style="background:white;text-align:left;margin: 1em auto 1em auto 1em auto 1em auto 1em auto"
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| ! !! Definition !! Expression !! Approximate expression !! Conditions
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| |-valign="center"
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| !'''Open-circuit [[gain#Voltage gain|voltage gain]]'''
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| | <math> {A_{v}} = \begin{matrix} {v_\mathrm{o} \over v_\mathrm{i}} \end{matrix} \Big|_{R_{L}=\infty} </math>
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| |<math> \begin{matrix} \frac {(g_m r_\mathrm{O}+1)R_C} {R_C+r_O} \end{matrix} </math>
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| |<math> \begin{matrix} g_m R_C \end{matrix}</math>
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| |<math> r_O \gg R_C</math>
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| |-valign="center"
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| ! '''Short-circuit [[gain#Current gain|current gain]]'''
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| |<math> A_{i} = \begin{matrix} {i_\mathrm{o} \over i_\mathrm{i}} \end{matrix} \Big|_{R_{L}=0} </math>
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| |<math> \begin{matrix} \frac {r_{ \pi }+ \beta r_O} {r_{ \pi} +( \beta +1)r_O} \end{matrix} \begin{matrix} \end{matrix} </math>
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| |<math> 1 </math>
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| | <math>\beta \gg 1 </math>
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| |-valign="center"
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| ! '''[[Input resistance]]'''
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| |<math> R_\mathrm{in} = \begin{matrix} \frac{v_{i}}{i_{i}} \end{matrix} </math>
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| |<math> \begin{matrix} \frac {(r_O+R_C\|R_L)r_E} {r_O+r_E +\frac {R_C\|R_L} { \beta +1}} \end{matrix}</math>
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| |<math> r_E \left( \approx \frac {1}{g_m}\right) </math>
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| |<math> r_O \gg R_C\|R_L \ \ \left( \beta \gg 1 \right)</math>
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| |-valign="center"
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| ! '''[[Output resistance]]'''
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| |<math> R_\mathrm{out} = \begin{matrix} \frac{v_{o}}{-i_{o}} \end{matrix} \Big|_{v_{s}=0}</math>
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| |<math> R_C \| \{[1+g_m (r_{\pi}\|R_S) ]r_O +(r_{\pi}\|R_S)\} </math>
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| |<math> R_C || r_O </math>
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| ; <math> R_C || \left[ (r_{ \pi}||R_S)(1+g_mr_O) \right]
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| </math>
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| |<math>\ \ R_S \ll r_E </math>
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| ;<math>\ \ R_S \gg r_E </math>
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| |}
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| :'''Note:''' Parallel lines (||) indicate [[Series_and_parallel_circuits#Parallel_circuits|components in parallel]].
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| In general the overall voltage/current gain may be substantially less than the open/short circuit gains listed above (depending on the source and load resistances) due to the [[loading effect]].
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| ===Active loads===
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| For voltage amplification, the range of allowed output voltage swing in this amplifier is tied to its voltage gain when a resistor load ''R<sub>C</sub>'' is employed, as in Figure 1. That is, large voltage gain requires large ''R<sub>C</sub>'', and that in turn implies a large DC voltage drop across ''R<sub>C</sub>''. For a given supply voltage, the larger this drop, the smaller the transistor ''V<sub>CB</sub>'' and the less output swing is allowed before saturation of the transistor occurs, with resultant distortion of the output signal. To avoid this situation, an [[active load]] can be used, for example, a [[current mirror]]. If this choice is made, the value of ''R<sub>C</sub>'' in the table above is replaced by the small-signal output resistance of the active load, which is generally at least as large as the ''r<sub>O</sub>'' of the active transistor in Figure 1. On the other hand, the DC voltage drop across the active load is a fixed low value (the '''[[Current mirror#Compliance voltage|compliance voltage]]''' of the active load), much less than the DC voltage drop incurred for comparable gain using a resistor ''R<sub>C</sub>''. That is, an active load imposes less restriction on the output voltage swing. Notice that active load or not, large AC gain still is coupled to large AC output resistance, which leads to poor voltage division at the output except for large loads ''R<sub>L</sub> >> R<sub>out</sub>''.
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| For use as a current buffer, gain is not affected by ''R<sub>C</sub>'', but output resistance is. Because of the current division at the output, it is desirable to have an output resistance for the buffer much larger than the load ''R<sub>L</sub>'' being driven so large signal currents can be delivered to a load. If a resistor ''R<sub>C</sub>'' is used, as in Figure 1, a large output resistance is coupled to a large ''R<sub>C</sub>'', again limiting the signal swing at the output. (Even though current is delivered to the load, usually a large current signal into the load implies a large voltage swing across the load as well.) An active load provides high AC output resistance with much less serious impact upon the amplitude of output signal swing.
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| ==Overview of characteristics==
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| Several example applications are described in detail below. A brief overview follows.
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| *The amplifier input impedance ''R<sub>in</sub>'' looking into the emitter node is very low, given approximately by
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| :::<math> R_{in} = r_E = \begin{matrix} \frac {V_T} {I_E} \end{matrix} </math>,
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| ::where ''V<sub>T</sub>'' is the [[Boltzmann constant#Role in semiconductor physics: the thermal voltage|thermal voltage]] and ''I<sub>E</sub>'' is the DC emitter current.
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| :For example, for ''V<sub>T</sub>'' = 26 mV and ''I<sub>E</sub>'' = 10 mA, rather typical values, ''R<sub>in</sub>'' = 2.6 Ω. If ''I<sub>E</sub>'' is reduced to increase ''R<sub>in</sub>'', there are other consequences like lower transconductance, higher output resistance and lower β that also must be considered. A practical solution to this low-input-impedance problem is to place a common emitter stage at the input to form a [[cascode]] amplifier.
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| *Because the input impedance is so low, most signal sources have larger source impedance than the common base amplifier ''R<sub>in</sub>''. The consequence is that the source delivers a ''current'' to the input rather than a voltage, even if it is a voltage source. (According to [[Norton's theorem]], this current is approximately ''i<sub>in</sub>'' = ''v<sub>S</sub> / R<sub>S</sub>''). If the output signal also is a current, the amplifier is a current buffer and delivers the same current as is input. If the output is taken as a voltage, the amplifier is a [[transresistance]] amplifier, and delivers a voltage dependent on the load impedance, for example ''v<sub>out</sub>'' = ''i<sub>in</sub> R<sub>L</sub>'' for a resistor load ''R<sub>L</sub>'' much smaller in value than the amplifier output resistance ''R<sub>out</sub>''. That is, the voltage gain in this case (explained in more detail below) is:
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| :::<math> v_{out} = i_{in} R_L = v_s \begin{matrix} \frac {R_L}{R_S} \end{matrix} </math> <math>\ \ </math> <math>\rarr </math> <math>A_v =\begin{matrix}\frac {v_{out}}{v_{S}} = \frac {R_L}{R_S} \end{matrix} </math>.
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| :Note for source impedances such that ''R<sub>S</sub> >> r<sub>E</sub>'' the output impedance approaches ''R<sub>out</sub>'' = ''R<sub>C</sub>'' || [ ''g<sub>m</sub>'' ( ''r<sub>π</sub>'' || ''R<sub>S</sub>'' ) ''r<sub>O</sub>'' ].
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| *For the special case of very low impedance sources, the common base amplifier does work as a voltage amplifier, one of the examples discussed below. In this case (explained in more detail below), when ''R<sub>S</sub>'' << ''r<sub>E</sub>'' and ''R<sub>L</sub>'' << ''R<sub>out</sub>'', the voltage gain becomes:
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| ::: <math>A_v =\begin{matrix}\frac {v_{out}}{v_{S}} = \frac {R_L}{r_E} \approx g_m R_L\end{matrix} </math>,
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| :where ''g<sub>m</sub>'' = ''I<sub>C</sub> / V<sub>T</sub>'' is the transconductance. Notice that for low source impedance, ''R<sub>out</sub> = r<sub>O</sub> || R<sub>C</sub>''.
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| *The inclusion of r<sub>O</sub> in the hybrid-pi model predicts reverse transmission from the amplifiers output to its input, that is the amplifier is '''bilateral'''. One consequence of this is that the input/output [[Electrical impedance|impedance]] is affected by the load/source termination impedance, hence, for example, the output resistance, R<sub>out</sub>, may vary over the range '' r<sub>O</sub> || R<sub>C</sub>'' ≤ ''R<sub>out</sub>'' ≤ (β + 1)'' r<sub>O</sub> || R<sub>C</sub>'' depending on the source resistance, R<sub>S</sub>. The amplifier can be approximated as unilateral when neglect of r<sub>O</sub> is accurate (valid for low gains and low to moderate load resistances), simplifying the analysis. This approximation often is made in discrete designs, but may be less accurate in RF circuits, and in integrated circuit designs where active loads normally are used.
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| ===Voltage amplifier===
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| [[Image:Common-base small signal.svg|thumbnail|300px|Figure 2: Small-signal model for calculating various parameters; Thévenin voltage source as signal.]]
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| For the case when the common base circuit is used as a voltage amplifier, the circuit is shown in Figure 2.
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| The output resistance is large, at least ''R<sub>C</sub> || r<sub>O</sub>'', the value which arises with low source impedance (''R<sub>S</sub> << r<sub>E</sub>''). A large output resistance is undesirable in a voltage amplifier, as it leads to poor [[voltage division]] at the output. Nonetheless, the voltage gain is appreciable even for small loads: according to the table, with ''R<sub>S</sub> = r<sub>E</sub>'' the gain is ''A<sub>v</sub> = g<sub>m</sub> R<sub>L</sub> / 2''. For larger source impedances, the gain is determined by the resistor ratio ''R<sub>L</sub> / R<sub>S</sub>'', and not by the transistor properties, which can be an advantage where insensitivity to temperature or transistor variations is important.
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| An alternative to the use of the hybrid-pi model for these calculations is a general technique based upon [[two-port network]]s. For example, in an application like this one where voltage is the output, a g-equivalent two-port could be selected for simplicity, as it uses a voltage amplifier in the output port.
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| For ''R<sub>S</sub>'' values in the vicinity of ''r<sub>E</sub>'' the amplifier is transitional between voltage amplifier and current buffer. For ''R<sub>S</sub>'' >> ''r<sub>E</sub>'' the driver representation as a [[Thévenin's theorem|Thévenin source]] should be replaced by representation with a [[Norton's theorem|Norton source]]. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower, as discussed next.
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| ===Current follower===
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| [[Image:Common base with Norton driver.svg|thumbnail|300px|Figure 3: Common base circuit with Norton driver; ''R<sub>C</sub>'' is omitted because an [[active load]] is assumed with infinite small-signal output resistance]]
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| Figure 3 shows the common base amplifier used as a current follower. The circuit signal is provided by an AC [[Norton's theorem|Norton source]] (current ''I<sub>S</sub>'', Norton resistance ''R<sub>S</sub>'') at the input, and the circuit has a resistor load ''R<sub>L</sub>'' at the output.
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| As mentioned earlier, this amplifier is '''bilateral''' as a consequence of the output resistance ''r<sub>O</sub>'', which connects the output to the input. In this case the output resistance is large even in the worst case (it is at least ''r<sub>O</sub> || R<sub>C</sub>'' and can become ''(β + 1) r<sub>O</sub> || R<sub>C</sub>'' for large ''R<sub>S</sub>''). Large output resistance is a desirable attribute of a current source because favorable [[current division]] sends most of the current to the load. The current gain is very nearly unity as long as ''R<sub>S</sub> >> r<sub>E</sub>''.
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| An alternative analysis technique is based upon [[two-port network]]s. For example, in an application like this one where current is the output, an h-equivalent two-port is selected because it uses a current amplifier in the output port.
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| == See also ==
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| {{Portal|Electronics}}
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| *[[Common collector]]
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| *[[Common drain]]
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| *[[Common source]]
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| *[[Differential amplifier]]
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| *[[Hybrid-pi model]]
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| *[[Two-port network]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * [http://people.deas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html Basic BJT Amplifier Configurations]
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| * [http://230nsc1.phy-astr.gsu.edu/hbase/electronic/npncb.html NPN Common Base Amplifier] — [[HyperPhysics]]
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| * [http://www.tedpavlic.com/teaching/osu/ece327/lab1_bjt/lab1_bjt_transistor_basics.pdf ECE 327: Transistor Basics] — Gives example common base circuit (i.e., current source) with explanation.
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| {{Transistor amplifiers}}
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| {{DEFAULTSORT:Common Base}}
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| [[Category:Single-stage transistor amplifiers]]
| |
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