his photograph taken in 1948 is of the three Bell Labs physicist-engineers, John Bardeen, william Shockley, and Walter Brattain, who invented the first transistor. ( Photo courtesy of AT&T Bell Laboratories. Their work built on the research of many before them, and much had to be done before the transistor and the solid-state devices that followed could become practical engineering tools, but in retrospect it is clear that the transistor gave the engineer the key to a whole new electronic world. Courtesy of the IEEE Center for the History of Electrical Engineering. The drain-to-source conductance can now be expressed as The reciprocal quantity is the drain-to-source resistance Tas as given by Tas=1/gas and ra(on)=1/ga(on),so 1-(√/√v+) (on) (√Vs+/Vp+9) AsVs→0,h→(on), and as V6s→V,→∞. This latter condition corresponds to the channel being inched off in its entirety all the way from source to drain. This is like having a big block of insulator(i.e, the depletion region) between source and drain. When VGs=0, Tas is reduced to its minimum value of ras(on), c 2000 by CRC Press LLC
© 2000 by CRC Press LLC The drain-to-source conductance can now be expressed as The reciprocal quantity is the drain-to-source resistance rds as given by rds = 1/gds, and rds(on) = 1/gds(on), so As VGS Æ 0, rds Æ rds(on), and as VGS Æ VP ,rdsÆ •. This latter condition corresponds to the channel being pinched off in its entirety all the way from source to drain. This is like having a big block of insulator (i.e., the depletion region) between source and drain. When VGS = 0, rds is reduced to its minimum value of rds(on), g g V V V ds ds GS P P = -+ + ( ) - + ( ) (on) 1 1 f f f f r r V V V ds ds P GS P = - + ( ) -+ + ( ) (on) 1 1 f f f f Their work built on the research of many before them, and much had to be done before the transistor and the solid-state devices that followed could become practical engineering tools, but in retrospect it is clear that the transistor gave the engineer the key to a whole new electronic world. (Courtesy of the IEEE Center for the History of Electrical Engineering.) This photograph taken in 1948 is of the three Bell Labs physicist-engineers, John Bardeen, William Shockley, and Walter Brattain, who invented the first transistor. (Photo courtesy of AT&T Bell Laboratories.)
which for most JFETs is in the 20-to 400-52 range. At the other extreme, when Vas> Vp, the drain-to-source current Ips is reduced to a very small value, generally down into the low nanoampere or even picoampere range The corresponding value of r,, is not really infinite but is very large, generally well up into the gigaohm (1000 MQ2)range. Thus by variation of VGs, the drain-to-source resistance can be varied over a very wide range. As long as the gate-to-channel junction is reverse-biased, the gate current will be very small, generally down in the low nanoampere or even picoampere range, so the gate as a control electrode draws very little current. Since Vp is generally in the 2-to 5-V range for most JFETS, the Vps values required to operate the JFET in the VVR range are generally <0.1 V In Fig. 24.23 the VVR region of the JFET Ios vs. Vps characteristics is shown VVR Applications Applications of VvRs include automatic gain control (AGC) circuits, electronic attenuators, electronically variable filters, and oscillator amplitude control circuits When using a JFEt as a VVR, it is necessary to limit Vps to values that are small compared to Vp to maintain good linearity. In addition Vas should preferably not exceed 0.8 Vp for good linearity, control, and stability. This limitation corresponds to an ras resistance ratio of about 10: 1 Vp can produce a large change in ra. Thus unit-to-unit variations in Vp as well as changes in Vp with temperature can result in large changes in Ta, as VGs approaches Vp The drain-to-source resistance ra, will have a temperature coefficient(TC) due to two causes: (1)the variation of the channel resistivity with temperature and(2)the temperature variation of Vp. The TC of the channel resistivity positive, whereas the TC of Vp is negative due to the negative TC of the contact potential d. The positive TC of the channel resistivity will contribute to a positive TC of Tas. The negative TC of Vp will contribute to a negative TC of Tas. At small values of VGs, the dominant contribution to the TC is the positive TC of the channel resistivity, so tas will have a positive TC As VGs gets larger, the negative TC contribution of Vp becomes increasingly important and there will be a value of VGs at which the net tc of ra is zero, and above this value of Vgs the tc will be egative. The TC of Ta,(on) is typically +0. 3%/C for n-channel JFETs and +0.7%/ C for P-channel JFETs. For example, for a typical ]FET with an ras(on)=500 Q2 at 25"C and Vp=2.6V, the zero TC point will occur at VG 2.0 V. Any JFET can be used as a VVR, although there are JFETs that are specifically made for this application A simple example of a VVR application is the electronic gain control circuit of Fig. 2424. The voltage gain is given by Ay=1+(ra/ras). If, willb mple, Re=19 k2 and t, (on)=1 kQ, then the maximum gain Avmax =1+[Rdlras(on)]=20. As Vos approaches Vp, ras will increase and become very large such that Tas >>Rp, so that Ay will decrease to a minimum value of close to unity. Thus the gain can be varied over a 20: 1 ratio. Note that Vos= Vin, so to minimize distortion ne input signal amplitude should be small compared to Vp FIGURE 2424 Electron Defining Terms Active region: The region of FET operation in which the channel is pinched off at the drain end open at the source end such that the drain-to-source current Ips approximately saturates. The condition for this is that vas < v l and vps >ve The active region is also known as the saturated region Ohmic, nonsaturated, or triode region: The three terms all refer to the region of ]FET operation in which a conducting channel exists all the way between source and drain. In this region the drain current varies with both Vo and Vps Drain saturation current, Ipss The drain-to-source current flow through the ]FET under the conditions that VGs=0 andvos>vp such that the jFET is operating in the active or saturated regio Pinch-off voltage, V,: The voltage that when applied across the gate-to-channel pn junction will cause the conducting channel between drain and source to become pinched off. This is also represented as VGs(off) Related Topic 28. 1 Large Signal Analysis c 2000 by CRC Press LLC
© 2000 by CRC Press LLC which for most JFETs is in the 20- to 400-W range. At the other extreme, when VGS > VP , the drain-to-source current IDS is reduced to a very small value, generally down into the low nanoampere or even picoampere range. The corresponding value of rds is not really infinite but is very large, generally well up into the gigaohm (1000 MW) range. Thus by variation of VG S , the drain-to-source resistance can be varied over a very wide range. As long as the gate-to-channel junction is reverse-biased, the gate current will be very small, generally down in the low nanoampere or even picoampere range, so the gate as a control electrode draws very little current. Since VP is generally in the 2- to 5-V range for most JFETs, the VDS values required to operate the JFET in the VVR range are generally <0.1 V. In Fig. 24.23 the VVR region of the JFET IDS vs. VDS characteristics is shown. VVR Applications Applications of VVRs include automatic gain control (AGC) circuits, electronic attenuators, electronically variable filters, and oscillator amplitude control circuits. When using a JFET as a VVR, it is necessary to limit VDS to values that are small compared to VP to maintain good linearity. In addition VGS should preferably not exceed 0.8 VP for good linearity, control, and stability. This limitation corresponds to an rds resistance ratio of about 10:1. As VGS approaches VP , a small change in VP can produce a large change in rds. Thus unit-to-unit variations in VP as well as changes in VP with temperature can result in large changes in rds as VGS approaches VP . The drain-to-source resistance rds will have a temperature coefficient (TC) due to two causes: (1) the variation of the channel resistivity with temperature and (2) the temperature variation ofVP . The TC of the channel resistivity is positive, whereas the TC of VP is negative due to the negative TC of the contact potential f. The positive TC of the channel resistivity will contribute to a positive TC of rds. The negative TC of VP will contribute to a negative TC of rds. At small values of VGS , the dominant contribution to the TC is the positive TC of the channel resistivity, so rds will have a positive TC.As VGS gets larger, the negative TC contribution of VP becomes increasingly important, and there will be a value of VGS at which the net TC of rds is zero, and above this value of VGS the TC will be negative. The TC of rds(on) is typically +0.3%/°C for n-channel JFETs and +0.7%/°C for p-channel JFETs. For example, for a typical JFET with an rds(on) = 500 W at 25°C and VP = 2.6 V, the zero TC point will occur at VGS = 2.0 V. Any JFET can be used as a VVR, although there are JFETs that are specifically made for this application. A simple example of a VVR application is the electronic gain control circuit of Fig. 24.24. The voltage gain is given by AV = 1 + (RF/rds). If, for example, RF = 19 kW and rds(on) = 1 kW, then the maximum gain will be AVmax = 1 + [RF/rds(on)] = 20. As VGS approaches VP , rds will increase and become very large such that rds >> RF, so that AV will decrease to a minimum value of close to unity. Thus the gain can be varied over a 20:1 ratio. Note that VDS @ Vin, so to minimize distortion the input signal amplitude should be small compared to VP . Defining Terms Active region: The region of JFET operation in which the channel is pinched off at the drain end but still open at the source end such that the drain-to-source current IDS approximately saturates. The condition for this is that *VGS * < *VP * and *VDS * > *VP *. The active region is also known as the saturated region. Ohmic, nonsaturated, or triode region: The three terms all refer to the region of JFET operation in which a conducting channel exists all the way between source and drain. In this region the drain current varies with both VGS and VDS . Drain saturation current,IDSS: The drain-to-source current flow through the JFET under the conditions that VGS = 0 and *VDS * > *VP * such that the JFET is operating in the active or saturated region. Pinch-off voltage, VP : The voltage that when applied across the gate-to-channel pn junction will cause the conducting channel between drain and source to become pinched off. This is also represented as VGS (off). Related Topic 28.1 Large Signal Analysis FIGURE 24.24 Electronic gain control
References R Mauro, Engineering Electronics, Englewood Cliffs, N J: Prentice-Hall, 1989, pp. 199-260 J. Millman and A. Grabel, Microelectronics, 2nd ed, New York: McGraw-Hill, 1987, Pp. 133-167, 425-429 F. H. Mitchell, Jr and F.H. Mitchell, Sr, Introduction to Electronics Design, 2nd ed, Englewood Cliffs, NJ Prentice-Hall, 1992, Pp. 275-328 CJ Savant, MS. Roden, and G L. Carpenter, Electronic Design, 2nd ed, Menlo Park, Calif. Beni Cummings, 1991, pp. 171-208 A.S. Sedra and K C. Smith, Microelectronic Circuits, 3rd ed, Philadelphia: Saunders, 1991, Pp. 322-361 24.2 Bipolar Transistors Joseph Watson Modern amplifiers abound in the form of integrated circuits(ICs), which contain transistors, diodes, and other structures diffused into single-crystal dice. As an introduction to these ICs, it is convenient to examine sing transistor amplifiers, which in fact are also widely used in their own right as discrete circuits-and indeed much more complicated discrete signal-conditioning circuits are frequently found following sensors of various sorts. There are two basic forms of transistor, the bipolar family and the field-effect family, and both appear in ICs. They differ in their modes of operation but may be incorporated into circuits in quite similar ways. To nderstand elementary circuits, there is no need to become too familiar with the physics of transistors, but some basic facts about their electrical properties must be known Consider the bipolar transistor, of which there are two types, npn and pnp. Electrically, they differ only in terms of current direction and voltage polarity. Figure 2425(a)illustrates the idealized structure of an npn transistor, and diagram(b)implies that it corresponds to a pair of diodes with three leads. This representation does not convey sufficient information about the actual operation of the transistor, but it does make the point that the flow of conventional current(positive to negative)is easy from the base to the emitter, since it pass through a forward-biased diode, but difficult from the collector to the base, because flow is prevented by a reverse Figure 2425(c) gives the standard symbol for the npn transistor, and diagram(d)defines the direction of current flow and the voltage polarities observed when the device is in operation. Finally, diagram(e)shows that for the pnp transistor, all these directions are reversed and the polarities are inverted. For a transistor, there is a main current flow between the collector and the emitter, and a very much smaller current flow between the base and the emitter. So, the following relations may be written: Ie=Ic+ IB (Note that the arrow on the transistor symbol defines the emitter and the direction of current flow-out for the npn device, and in for the pnp ) Also (24.2) Collector Base VcE Emitter c (d) FIGURE 24.25 The bipolar transistor. (a)to(d)mpn transistor;(e)pmp transistor. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC References R. Mauro, Engineering Electronics, Englewood Cliffs, N.J.: Prentice-Hall, 1989, pp. 199–260. J. Millman and A. Grabel, Microelectronics, 2nd ed., New York: McGraw-Hill, 1987, pp. 133–167, 425–429. F. H. Mitchell, Jr. and F.H. Mitchell, Sr., Introduction to Electronics Design, 2nd ed., Englewood Cliffs, N.J.: Prentice-Hall, 1992, pp. 275–328. C.J. Savant, M.S. Roden, and G.L. Carpenter, Electronic Design, 2nd ed., Menlo Park, Calif.: BenjaminCummings, 1991, pp. 171–208. A.S. Sedra and K.C. Smith, Microelectronic Circuits, 3rd ed., Philadelphia: Saunders, 1991, pp. 322–361. 24.2 Bipolar Transistors Joseph Watson Modern amplifiers abound in the form of integrated circuits (ICs), which contain transistors, diodes, and other structures diffused into single-crystal dice. As an introduction to these ICs, it is convenient to examine singtransistor amplifiers, which in fact are also widely used in their own right as discrete circuits — and indeed much more complicated discrete signal-conditioning circuits are frequently found following sensors of various sorts. There are two basic forms of transistor, the bipolar family and the field-effect family, and both appear in ICs. They differ in their modes of operation but may be incorporated into circuits in quite similar ways. To understand elementary circuits, there is no need to become too familiar with the physics of transistors, but some basic facts about their electrical properties must be known. Consider the bipolar transistor, of which there are two types, npn and pnp. Electrically, they differ only in terms of current direction and voltage polarity. Figure 24.25(a) illustrates the idealized structure of an npn transistor, and diagram (b) implies that it corresponds to a pair of diodes with three leads. This representation does not convey sufficient information about the actual operation of the transistor, but it does make the point that the flow of conventional current (positive to negative) is easy from the base to the emitter, since it passes through a forward-biased diode, but difficult from the collector to the base, because flow is prevented by a reversebiased diode. Figure 24.25(c) gives the standard symbol for the npn transistor, and diagram (d) defines the direction of current flow and the voltage polarities observed when the device is in operation. Finally, diagram (e) shows that for the pnp transistor, all these directions are reversed and the polarities are inverted. For a transistor, there is a main current flow between the collector and the emitter, and a very much smaller current flow between the base and the emitter. So, the following relations may be written: IE = IC + IB (24.1) (Note that the arrow on the transistor symbol defines the emitter and the direction of current flow—out for the npn device, and in for the pnp.) Also IC /IB = hFE (24.2) FIGURE 24.25 The bipolar transistor. (a) to (d) npn transistor; (e) pnp transistor