Whites, EE 322 Lecture 21 Page 1 of 8 Lecture 21: Junction Fiel Effect Transistors. Source Follower Amplifier As mentione in Lecture 16, there are two major families of transistors. We ve worke with BJTs in the past few lectures. The secon transistor type we will consier is the fiel effect transistor (FET). In the NorCal 40A, we use iscrete junction fiel effect transistors (JFETs). (Recall that in ICs, metal oxie semiconuctor FETs are usually use.) Four important points concerning the JFET are: 1. Probably the simplest transistor, 2. Very large input impeance (but MOSFETs even larger), 3. Virtually obsolete compare to the MOSFET, 4. Now use mainly in iscrete circuit esign switches, amplifiers, etc. In the NorCal 40A, JFETs are use in the: 1. Buffer Amplifier (as a buffer amplifier uh!), 2. Variable Frequency Oscillator, VFO (as the gain element in the oscillator), 3. Automatic Gain Control, AGC (as a voltage controlle variable resistance). 2006 Keith W. Whites
Whites, EE 322 Lecture 21 Page 2 of 8 Physical Behavior of Junction Fiel Effect Transistors As with BJTs, there are n an p type JFETs. We use only ntype in the Norcal 40A (the J309). Channel g (Gate) v gs (Drain) I s (Source) Drain Gate Source p Depletion Region n n Depletion Region p Notice that the gate is reverse biase wrt the source, which is itself often connecte to the ptype boy. By making V gs more negative, the gate pn junction evelops a larger epletion region. This has the effect of narrowing the channel an, consequently, ecreasing I (the rain current). Eventually, when Vgs = Vc ( < 0) the channel becomes close (or pinche off ). V c is calle the cutoff voltage an is less than zero. (For the J309, Vc 2.5 V.) Mathematically, the rain current is expresse as
Whites, EE 322 Lecture 21 Page 3 of 8 Vgs I = Iss 1 [A] (9.74) Vc I ss is the rain to source current with the gate shorte to the source. This characteristic curve is shown in Fig. 9.15: 2 I I V = Iss 1 V gs c 2 I ss Fowar biase gate ioe: not operational. V c 0 V gs Channel cutoff Channel is maximum The slope of this rain current versus V gs is calle the transconuctance g m of the JFET: I gm = [S] (9.75) V The significance of g m to a JFET is analogous to β for a BJT. Substituting (9.74) into (9.75) an performing the ifferentiation gives I V ss gs gm = 2 1 [S] (9.76) Vc Vc This curve is simply a straight line, as shown in Fig 9.16: gs
Whites, EE 322 Lecture 21 Page 4 of 8 g m g m I V ss gs = 2 1 Vc Vc 2I ss V c V c 0 V gs Interestingly, we see that g m actually changes as V gs changes. The β for BJTs i not have such epenence. Later in Section 11.4 (an Probs. 26 an 27), we will harness this behavior of JFETs to make a nice oscillator! Small Signal Moel of the JFET The low frequency, small signal moel for an ntype JFET is shown in Fig. 9.17a: g m v gs r o g v gs s The gate is open circuite, which moels the extremely large input impeance of properly biase JFETs (remember that the
Whites, EE 322 Lecture 21 Page 5 of 8 input can be a reversebiase pn junction). This input impeance is easily greater than 1 MΩ. In the moel above, r o is the output resistance of the JFET. It s often neglecte in paper analysis of these circuits. (However, in Prob. 27, r = r = 5 kω is use in seconorer calculations.) o Source Follower FET Amplifier A JFET source follower amplifier is very similar to the BJT emitter follower (Fig. 9.17b): V v i R v The FET source follower (1) has a very large input impeance, (2) is very simple to bias, an (3) has Gv 1. The source follower is use in the Buffer Amplifier (Q5) in the NorCal 40A. The Buffer Amplifier isolates the Transmit Mixer from the Driver Amplifier. This isolation keeps changes in the input impeance to the Driver Amplifier from affecting the Transmit Mixer.
Whites, EE 322 Lecture 21 Page 6 of 8 Surprisingly, R alone is all that s neee to set the bias of the source follower, provie the gate is c groune (but, of course, not ac groune!). As an example of a c groune gate, consier the Buffer Amplifier in the NorCal 40A. Notice that R10 an L6 provie a c path to groun so there is no c current. Also, notice that ac signals at the gate are not groune. The c source voltage in the above circuit is Vs = IbR where I b is the rain to source bias current. Then, Vgs = Vg Vs = 0 IbR= IbR (9.77) This is simply an equation for a straight line. This straight line is the loa line an we can use it together with the JFET characteristic equation (9.74) to etermine the c bias point for V gs an I s. Referring to Fig. 9.18: Loa line I R I ss I Vgs = Iss 1 Vc 2 1 I b c bias pt V c V b 0 V gs
Whites, EE 322 Lecture 21 Page 7 of 8 The intersection of the loa line with the JFET characteristic curve gives a graphical solution for the c bias point of the source follower amplifier: V gs = V b an I = I b. Once the source follower has been properly biase, the ac output impeance an the voltage gain can be easily etermine. Using the smallsignal equivalent circuit moel for the source follower (Fig. 9.17c): g m v gs g v gs s v i R v From this circuit we see that v= g v R (9.78) where gs g s i m gs v = v v = v v (9.79) Substituting (9.79) into (9.78) we fin v= g R v v = g Rv g Rv ( ) or ( ) m i m i m 1 gmr v= gmrvi Now, solving for the ratio of output to input voltage, we fin v gmr Gv = v 1 g R i m
Whites, EE 322 Lecture 21 Page 8 of 8 1 or G v = (9.82) 1 1 gmr where G v is the voltage gain. The JFET transconuctance is usually quite small. But if we 1 choose R such that gmr 1 (i.e., R g m ), then Gv 1 which is typical for a sourcefollower JFET amplifier.