The three configurations of a transistor PDF

Preview

Summary

class notes...

Description

The three configurations of a transistor

Introduction with classical models A transistor amplifier can be coupled in a total of three different configurations, namely common emitter (CE), common collector (CC) and common base (CB). The common collector is also known as the "emitter follower" (EF). The classical way to represent it is shown in figure 132:

(a) Common emitter

(b) Common collector

(c) Common base

Figure 132: Classical representation of transistor configurations

199

The original definition of identifying the configurations is as follows: The pin which is common to both input and output is the common one. When inspecting figure 132, we can see this rule clearly. In a practical model like the one in figure 133, we can apply this rule and clearly see that it is a common emitter.

Figure 133: A very simple common emitter

However, in a more complicated model like the one in figure 134, it is no longer that easy to tell the configuration, since all three pins are "floating" and no one is directly coupled to the input and output.

Figure 134: A configuration which is not so easy to identify

200

To solve this identification problem, there is a very simple rule: Determine to which pin the input is coupled, determine which pin feeds the load, then, the remainder pin is the common one ……………………………………(129) Applying rule (129) to figure 134, we can see that the input is connected to the base; the collector feeds the output, so, it is a common emitter configuration. This rule is very effective and we can now identify the configuration in a very complicated circuit. In the same way, in a field-effect transistor amplifier, there is also speech of common source, common drain and common gate configurations. Use your imagination to apply all the preceding mentioned rules to a JFET and a MOSFET.

11.2 Practical models and their properties Each of the three different configurations has its own unique properties with respect to Av, Ai, zin, Zout and phase shift ø. It is important for a designer to know it well, since he/she must decide which configuration to use under which circumstances. The models shown in figure 132 do not really work; they are theoretical demonstration models only. Let us look at practical, working models, and discuss each one's properties. You should not memorise these properties, but should learn to understand the working of the circuits in such a way that you can figure out the properties. 11.2.1 The practical common emitter A practical, working common emitter amplifier is shown in figure 135. Make sure that it is a common emitter: Yes, base in, collector out, the remainder is the emitter, thus it is a common emitter.

201

Figure 135: Practical, working common emitter (CE) We have based all the theory and designs so far in this book on the CE, so I think we have enough experience to list the following properties for a CE without hesitation: Voltage gain = moderate (to high, sometimes). Current gain = moderate. Input impedance = moderate (to low, sometimes). Output impedance = moderate to high. Phase shift (ø) = 180°. The common emitter is the most popular configuration in analogue electronics, based on all the "moderate" properties. We especially use it in audio preamplification. The final output stage of a power amplifier will be dealt with later on.

202

11.2.2 The practical common collector A practical, working common collector/emitter follower amplifier is shown in figure 136. Inspect the circuit. Base in, emitter out, collector is remaining, yes, it is a common emitter.

Figure 136: Practical, working common collector (CC) From the background we gathered while drawing all the node graphs in chapter 5, it must be clear that the emitter "follows" the base dynamically; exactly– so, the voltage gain is 1, or approaching 1 if the losses in the circuit are considered. The output current is the emitter current – which is the largest current in the transistor, so, we can say that the current gain is high. Since the expression for input impedance includes the term beta times emitter-resistor, the input impedance must be high. Furthermore, we can apply (58) in reverse, then, the output impedance of the amplifier in figure 136 is probably:

203

............... depending on how accurately you want to work. The first one is acceptable for design work. We can see that the output impedance is a low value, because of the large β-term below the line in equation (130). Again, the emitter "follows" the base, so, it is clear that the phase shift is equal to 0°. The common collector is very useful as an output stage, especially because of its z in, zo properties. See the chapters on power amplifiers later, and also refer to chapter 10. The third stage, or output stage, will always have a common collector. 11.2.3 The practical common base A practical, working common base amplifier is shown in figure 137. Examine the circuit. Emitter in, collector out, base remaining, yes, it is indeed a common base configuration.

Figure 137: Practical, working common base (CB) amplifier The presence of capacitor C3 is important, to prevent the base from “following” the emitter dynamically, just as the emitter was prevented to follow the base in figure 22, chapter 2.

204

By revising equation number (115), we can roughly say that the voltage gain resembles:

........ where Rg can be a very small value or tend to zero, depending on circumstances. It is clear that the voltage gain is high to very high, especially if R g tends to zero. The input current is the emitter current and the output current is the collector current, so, the current gain is 1, or approaching 1, if we consider the little losses in the baseemitter junction. The input impedance is…………..

....... which is clearly a very small value.

....... and that can be a moderate to a high value. The phase shift between output and input in a common base can be sometimes a knotty issue. The novice may say, but, the collector and the emitter of a transistor are always 180o out of phase. However, take note of this explanation (referring to figure 137): When a positive-going signal comes from the signal source, it makes the emitter a little bit more positive, with the base still held at a fixed value by C3. Thus, the base-emitter voltage decreases, decreasing the collector current, therefore, the output potential at the collector goes relatively positive. Can you see, the collector and emitter "moved" in the same direction, hence ø= 0°. This last finding is the true one, namely ø= 0°.

205

If we refer to zin and zout, it is dear that the common base is not a suitable amplifier for audio purposes. However, the common base finds its niche in the radio frequency (RF) world. The reason is as follows: In an RF amplifier, the term "current gain-bandwidth product" (CGBP) is of importance, and it is important to realise that CGBP is a constant for a specific amplifier. Now, it is true for only the CB that whatever you design the voltage gain to be, the current gain is only 1, so the bandwidth is an absolute maximum, and a high bandwidth is also associated with high frequency. When designing a CB amplifier, R1, R2, R3 and R4 are designed as taught before. To calculate C1 and C2, use zin and zo as before (equations (132) & (133)). When calculating C3, do it as follows:

11.3 Summary We can now summarise all the important properties in table 7. A good designer knows table 7 by heart; however, do not memorise it. By understanding the information given in this chapter, and by using enough common sense, any designer should be able to compose it. Table 7: Summary of the properties of the three different transistor configurations Configuration...