Large output resistance is a desirable attribute of a current source because favorable current division sends most of the current to the load. A large output resistance is undesirable in a voltage amplifier, as it leads to poor voltage division at the output. If a resistor RC is used, as in Figure 1, a large output resistance is coupled to a large RC, again limiting the signal swing at the output. As current is sunk from the emitter, this provides potential difference so causing the transistor to conduct. On the other hand, the DC voltage drop across the active load has a fixed low value (the compliance voltage of the active load), much less than the DC voltage drop incurred for comparable gain using a resistor RC. The diagram below shows how a common base amplifier circuit can be implemented. The following are the characteristics of the Common Base amplifier circuit. An alternative analysis technique is based upon two-port networks. An alternative to the use of the hybrid-pi model for these calculations is a general technique based upon two-port networks. For use as a current buffer, gain is not affected by RC, but output resistance is. The BJT can be of npn type of pnp type based on the configuration of the regions. To avoid this situation, an active load can be used, for example, a current mirror. This circuit can be used to derive the following characteristics of the common base amplifier. In common base configuration, the base terminal is grounded so the common base configuration is also known as grounded base configuration. Often a common base is used in this manner, preceded by a common-emitter stage. That means the emitter terminal and common base terminal are known as input terminals whereas the collector terminal and common base terminal are known as output terminals. In this case (explained in more detail below), when, This page was last edited on 11 May 2020, at 16:03. A Bipolar Junction Transistor or simply a BJT is a three terminal semiconductor device with alternating layers of n and p type semiconductor material. For the case when the common-base circuit is used as a voltage amplifier, the circuit is shown in Figure 2. Since both electrons and holes act as current carriers, the term “Bipolar” is used. Therefore, if no current is sunk at the emitter, the transistor does not conduct. The three terminals, which correspond to the three regions of a transistor are called Emitter, Base and Collector. The Common Base Amplifier is another type of bipolar junction transistor, (BJT) configuration where the base terminal of the transistor is a common terminal to both the input and output signals, hence its name common base (CB). signal source and the load share the base of the transistor as a common connection point shown in Figure The current conducted via the collector is proportional to the voltage across the base–emitter junction, accounting for the bias, as with other configurations.. In common base configuration, emitter is the input terminal, collector is the output terminal and base terminal is connected as a common terminal for both input and output. The output resistance is large, at least RC || rO, the value which arises with low source impedance (RS ≪ rE). The input signal is represented by a Thévenin voltage source vs with a series resistance Rs and the load is a resistor RL. That is, an active load imposes less restriction on the output voltage swing. In this circuit the emitter terminal of the transistor serves as the input, the collector as the output, and the base is connected to ground, or "common", hence its name. For example, for VT = 26 mV and IE = 10 mA, rather typical values, Rin = 2.6 Ω. That is, large voltage gain requires large RC, and that in turn implies a large DC voltage drop across RC. Learn how and when to remove this template message, https://en.wikipedia.org/w/index.php?title=Common_base&oldid=956117076, Articles lacking in-text citations from April 2009, Creative Commons Attribution-ShareAlike License, Because the input impedance is so low, most signal sources have larger source impedance than the common-base amplifier, 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 the output resistance is large even in the worst case (it is at least rO || RC and can become (β + 1) rO || RC for large RS). Figure 3 shows the common base amplifier used as a current follower. Common Base Amplifier Circuit. Nonetheless, the voltage gain is appreciable even for small loads: according to the table, with RS = rE the gain is Av = gm RL / 2. Because of the current division at the output, it is desirable to have an output resistance for the buffer much larger than the load RL being driven, so large signal currents can be delivered to a load. This high isolation means that there is little feedback from the output back to the input, leading to high stability. 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. For RS >> rE the driver representation as a Thévenin source should be replaced by representation with a Norton source. We have seen that such a configuration cannot amplify currents since its current gain is approximately equal and strictly lower than 1, hence the name “current buffer/follower” often given to CBAs. In this circuit, care has to be taken such that correct impedance match is provided to the input signal. Sometime… An active load provides high AC output resistance with much less serious impact upon the amplitude of output signal swing. This configuration is also useful as a current buffer, since it has a current gain of approximately unity (see formulae below). For a given supply voltage, the larger this drop, the smaller the transistor VCB and the less output swing is allowed before saturation of the transistor occurs, with resultant distortion of the output signal. For voltage amplification, the range of allowed output voltage swing in this amplifier is tied to voltage gain when a resistor load RC is employed, as in Figure 1. However, it is popular in integrated circuits and in high-frequency amplifiers, for example for VHF and 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. If IE is reduced to increase Rin, there are other consequences like lower transconductance, higher output resistance and lo… (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.) Common base transistor electronic circuit design / configuration This circuit design uses an NPN transistor, but the configuration is equally applicable to PNP transistors but with battery polarities reversed. 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. The analogous field-effect transistor circuit is the common-gate amplifier. Several example applications are described in detail below. Basically, a BJT ca… A brief overview follows. The current gain is very nearly unity as long as RS ≫ rE. 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. 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. 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 RL ≫ Rout. Common base transistor amplifier circuit. 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. In this tutorial, we dealt with many aspects of one of the three elementary topology of amplifier known as the Common Base Amplifier (CBA). The circuit signal is provided by an AC Norton source (current IS, Norton resistance RS) at the input, and the circuit has a resistor load RL at the output. Characteristics of Common Base Amplifier Circuit. For larger source impedances, the gain is determined by the resistor ratio RL / RS, and not by the transistor properties, which can be an advantage where insensitivity to temperature or transistor variations is important. For RS values in the vicinity of rE the amplifier is transitional between voltage amplifier and current buffer. In electronics, a common-base (also known as grounded-base) amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. The common base circuit stops behaving like a voltage amplifier and behaves like a current follower, as discussed next. If this choice is made, the value of RC 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 rO of the active transistor in Figure 1. However, we have seen through theory and an example that the voltage signal can be highly amplified and its voltage gain is only limited by the power supplied in the collector branch.
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