CLASS 12&13 JFET PARAMETERS AND BIASING

Transcription

1 CLASS 12&13 JFET PARAMETERS AN BIASING

2 The family of drain characteristic curves shows that when GS becomes more negative, Sp (or S(sat) )andi S become smaller. I is dependent on the width of the channel. The width of the channel is dependent on the depletion region. The depletion region is dependent on the GS. Hence, GS is controlling the value of I.Thisisthereason why the JFET is known as a voltage controlled device. Pinched-off occurs when S = S(sat). I I SS I S(-1) I S(-2) I S(-3) I S(-4) GS = p Sp(-4) Sp(-1) Sp(-3) Sp(-2) S GS(off) NORLAILI MOH NOH 2010/2011 2

3 uring pinch-off: GS + R R S(sat) I S = = A p'( GS ) Ap'( GS ) p When GS becomes more negative, S(sat) reduces and R A p'( GS ) increases. R A p '( increases as the GS ) depletion region increases. Hence, I S decreases. GG NORLAILI MOH NOH 2010/2011 3

4 TRANSFER CHARACTERISTIC I = I SS when GS =0 and I = 0 when GS = GS(off) =- p. I SS and GS(off) are the JFET parameters which are available in the JFET data sheet. Another important JFET parameter is the forward transconductance, g m. I I SS g m = ΔI / Δ GS at a fixed S and the S has to be in the saturation/fixedcurrent/pinch-off region. - GS(off) 0 GS I I SS ΔI - GS(off) GS 0 Δ GS NORLAILI MOH NOH 2010/2011 4

5 g m at GS = 0 is known as g m0. GS g m = g m0 1 GS ( off ) g m0 = 2I SS GS ( off ) Y fs In the data sheet g m0 is represented by. Important parameters of the JFET 1. R IN I GSS GG G p n- channel Besides GS(off), I SS and g m0, another parameter of the JFET is R IN. R GS IN = I S GSS p NORLAILI MOH NOH 2010/2011 5

6 I GSS is the G-S current when the -S is short circuited. I GSS is given in the data sheet. Since I GSS is from the flow of minority carriers, I GSS increases with the increment of temperature, T, at a fixed GS. I GSS isthe G reverse current at a known GS. As I GSS increases with T, R IN will be reduced as R GS. IN = I GSS IGSS The value of I GSS is very small making the value of R IN to be very large. GG Hence, the input impedance, Z i, is very large. G p n- channel p S NORLAILI MOH NOH 2010/2011 6

7 2. r d The next parameter of the JFET is the drain resistor, r d. r d is the ac resistance between and S when the JFET is operating in the saturation region. r d istheinverseoftheslope of the drain characteristic in the saturation region. r d is also the output tresistance. it r d = ΔΔ I S GS fixed 1 Y os = r d Y os is the output admittance. Y os is provided in the data sheet. When r d increases, Y os decreases. Y os is typically in the range of 10 to 50 μmho (or μs). ) Therefore, r d is in the range of 20 to 100 kω. NORLAILI MOH NOH 2010/2011 7

8 The GS from0to GS(off) controls the drain current that flows when the device is in its saturation/pinch-off region. When the JFET is biased in its saturation/pinched-off region, the equation that relates the drain current, I,with GS is represented by the following: I =I SS 1- GS GS(off) 2 This equation contributes to why the JFET is known as square law device. I h l p-channel n-channel I SS I I SS 0 0 GS(OFF) GS(OFF) GS GS NORLAILI MOH NOH 2010/2011 8

9 The transfer function can be obtained/drawn from the drain characteristic as shown in the diagram below. The drain current can be calculated by using I =I SS 1-2 GS GS(off) provided GS(off) and I SS are known. I (ma) ISS =12 GS =0-1 - GS () GS(off) p S () GS(off) NORLAILI MOH NOH 2010/2011 9

10 JFET BIASING 4 types of biasing circuit will be studied in this course: (i). Fixed biasingi (ii).self biasing (iii).mid-point biasing (iv).oltage division biasing Load line and Q-point for JFET I I SS I Q S(sat) Load line (determined from circuit) = p SQ GS =0-1 S ( GSQ ) GS(off) A load line connects 2 points on the drain characteristic. One point is on the I axis and the other is on the S axis. For a known GS, the intercept point between the load line and the drain curve can determine the operating/quiescent (Q) point and consequently, the I Q and SQ. NORLAILI MOH NOH 2010/

11 1. Fixed biasing To determine the Q-point (I Q and SQ ), the relationship between the I and S needs to be known. Looking at the output loop p( (Loop 1 - drain GG characteristic): - +IR + S = 0 I - S 1 = =- S + R R R RG G R I S I S Loop 1 This expression is in the form of y = mx + c. Slope is - 1 and c =. R R NORLAILI MOH NOH 2010/

12 - +IR + S = 0 I = - =- 1 + S S R R R Slope is -1/R and c = /R.AtI =0, S =. Hence, the load line intersects the I axis at /R and the S axis at.once GSQ is known, I Q and SQ can be determined. R I I SS Load line with a slope of -1/R GS =0 I Q Q-point GS1 GSQ GS2 GS3 p SQ S GS(off) NORLAILI MOH NOH 2010/

13 2. Self-biasing In order to operate in the saturation region (hence, as an amplifier), G- S of the JFET needs to be reverse biased. To obtain this condition, the GS has to be negative for the n-channel JFET and positive for the p- channel JFET. The following topologies will enable the mentioned condition to be achieved without the need of an external voltage to be connected to the G. ue to this capability, this topology is called selfbiasing circuit. NORLAILI MOH NOH 2010/

14 In an n-channel JFET self-biasing cct: - + I R + S + I R S = 0. I R + S + I R S =. S = I R S is positive. Since I G 0, then RG 0. Hence, G =0. Since G =0and S is positive, GS = G - S = negative. So, the n-channel is properly biased as an amplifier. GS = - S = - I R S. R S = S / I. NORLAILI MOH NOH 2010/

15 In a p-channel JFET self-biasing cct: I R S + S + I R - = 0. I R S + S + I R =. S = - I R S. S is negative. Since I G 0, then RG 0. Hence, G =0. Since G = 0 and d S is negative, GS = G - S = positive. So, the p-channel is properly biased as an amplifier. GS = - S = I R S. R S = GS / I. For both n- and p-channel JFETs, R S = GS / I. NORLAILI MOH NOH 2010/

16 In the JFET, majority carriers are moving from S to. In the n-channel JFET, is positive to attract the electrons to move from S to. Conventional current flow is opposite to the flow of electron. Hence, the direction of current is from to S. In the p-channel JFET, is negative to attract the holes to move from S to. Conventional current flow is the same as the flow of holes. Hence, the direction of current is from S to. NORLAILI MOH NOH 2010/

17 etermining the operational point of the self-biasing JFET 2 ways: 1. graphical method, i.e. from the transfer and drain characteristic 2. from calculation, i.e. using: I =I SS 1- GS GS(off) 2 or using KL Example 1 etermine the R S that is needed to self bias an n-channel JFET that has the transfer characteristic curve as shown at GS = -5. GS Solution From the graph, at GS = -5, I =6.25mA. R S = GS / I = 5 / 6.25 m = 800 Ω S GS NORLAILI MOH NOH 2010/

18 Example 2 etermine S and GS. Given I = 5 ma. S GS Solution: R =1kΩ G = 0 (criteria of the self-biased circuit). 1k S = I R S = 5m x 500 = 2.5 GS = - S GS = -2.5 = I R + S + I R S R = I R = 5m x 1k = 5 RS = I R S = S = 2.5 S = = 2.5 R G I S I R S =500 Ω NORLAILI MOH NOH 2010/

19 3. Mid-point biasing The JFET is typically y biased near the mid-point of the drain characteristic curve to achieve maximum signal swing at the input. The purpose is to obtain maximum undistorted ouput signal. Steps to obtain the Q-point 1. Mid-point biasing enables maximum drain current swing between I SS and 0. I Q =I SS /2. 2. From 2 I GSQ Q =ISS 1- GS(off) and when I Q =I SS /2, GSQ = GS(off) / To set the drain voltage at mid- point, Q = /2 I I SS I Q p SQ GS =0-1 GSQ -3-4 S 4. Choose large R G to prevent loading effect. R G =1MΩ NORLAILI MOH NOH 2010/

20 Example 3 etermine the resistors to be implemented in the following circuit for mid-point biasing. The parameters for the JFET are: I SS =15mAand GS(off) =-8. Solution I ISS Q = =7.5 ma 2 GS(off) GSQ = = -8 = = = 12 =6 Q 2 2 R G = 1MΩ GSQ = - S SQ = 2.35 = I Q R S R S = SQ / I Q = 2.35 /7.5 m = 313 Ω R =( - Q )/ I Q = (12 6) /7.5 m = 800 Ω RG R I S I R S NORLAILI MOH NOH 2010/