LAB IX. LOW FREQUENCY CHARACTERISTICS OF JFETS

Transcription

1 LAB X. LOW FREQUENCY CHARACTERSTCS OF JFETS 1. OBJECTVE n this lab, you will study the -V characteristics and small-signal model of Junction Field Effect Transistors (JFET).. OVERVEW n this lab, we will study the -V characteristics of an n-channel JFET to calculate a few equivalent-circuit arameters in order to make a small-signal model of the JFET. You will then comare the exerimental results with the theoretical results of the equations found in the lab manual. nformation essential to your understanding of this lab: 1. Theoretical background of the JFET (Streetman 6.) Materials necessary for this Exeriment: 1. Standard testing station. One JFET (art: N5485) 3. 1kΩ resistor Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 1

2 3. BACKGROUND NFORMATON 3.1 CHART OF SYMBOLS Here is a chart of symbols used in this lab manual. This list is not all inclusive; however, it does contain the most common symbols and their units. Table 1. Chart of the symbols used in this lab. Symbol Symbol Name Units i total drain to source current ma DC drain to source current ma i ds AC drain to source current ma saturation current w/ V G = 0 ma V inch off Voltage V v total drain to source voltage V V DC drain to source Voltage V v ds AC drain to source Voltage V v total gate to source Voltage V V DC gate to source Voltage V v gs AC gate to source Voltage V g m transconductance A/V Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age

3 3. CHART OF EQUATONS All of the equations from the background ortion of the manual are shown in the table below. Table. Chart of the equations used in this lab. Equation Name Formula 1 Saturation Drain to Source current in a N-tye JFET D Sat. 1 v V for negative V Transconductance at the oerating oint g m i v V v. const v const. 3 Equation for Transconductance at the oerating oint using known variables v g m 1 V V 4 Total Drain to Source current i AT ( t) v 1 V (0) gs v V gs v V gs cos( wt) v V gs cos(wt) 5 Shift in DC oerating oint due to AC gate Voltage v gs V Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 3

4 3.3 THE -V CHARACTERSTCS OF A JFET n the JFET, the transistor action is determined by the flow of majority carriers between the source and the drain. n the low drain-source bias region, the current flow is controlled by a voltage alied to the gate terminal that consists of a reversed biased n junction. The gate voltage modulates the width of the reverse biased n junction deletion layer. The change in the crosssectional area of the current ath under the gate modulates the current flow. For a fixed source-drain voltage and with increasing gate bias the width of the deletion layer at the drain end of the channel decreases. The most imortant oerating region of the JFET occurs at larger drain-source bias levels. There the combination of the alied gate voltage and the drain to source voltages are sufficiently large so the deletion width extends fully across the channel, inching it off at the drain end of the channel. The current flow is now limited by the current flow in the non-inched off region of the channel, and when the carriers reach the inched off end they are raidly collected by the reverse bias of the inched off region. Analysis of the device geometry shows that in the inched off region the current flow is determined by the value of the gate voltage, and is relatively indeendent of the drain-source voltage. This is the ractical region for oerating the JFET as an amlifier. The DC behavior of a JFET is secified most comletely by the outut characteristics, i versus v, with v as a arameter, as shown in Figure 1, and the inut-outut characteristic, D i versus v D( Sat.), as shown in Figure. However, such detailed information is not always sulied by the device manufacturer, as is the case for the N5485 N-channel JFET used in this lab. n such circumstances the circuit designer must measure the device characteristics or use the limited information sulied by the manufacturer, consisting usually of the aroximate values of and V. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 4

5 [A ] [V] Figure 1. N5485 JFET V characteristics. D( Sa t) [m Sloe - gm V [V] 0V Figure. nut-outut characteristic (id(sat.) vs. v) of a JFET. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 5

6 When used as a small signal amlifier the JFET will be oerating in the inchedoff mode, v vv 0, and its DC behavior can be aroximately described by the following equation v V. D Sat 1 for negative V (1) Hence once you can extract information such as JFET in a circuit. Often the values of and V and V, you can use the for a given tye of transistor vary over wide ranges and the values sulied by the device manufacturer reresent only the average and extreme values of these arameters. Moreover the device may not closely obey the relationshi given by Eq. (1). n this exeriment the DC characteristics of the transistor are measured in order to obtain sufficient information to use the device in an amlifier circuit and also to determine how closely Eq. (1) reresents the actual device behavior. 3.4 Small Signal Models The small signal equivalent circuit of a JFET oerating in the inched-off mode is shown in Figure 3. The transconductance is g m and is equal to the sloe of the transfer curve in Figure which is given by: g m i i D D ( ) v. ( ) const v const. v v () The Equation () can be rearranged as: v g m 1 V V (3) Figure 3. The small signal model of a JFET in the inched off mode of oeration. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 6

7 Equation (3) is evaluated at a fixed value v V. The inut terminals from the gate to the source aear as a reversed biased diode and are an effective oen circuit. The numerical value of g m can be estimated from either Eq. (3) or from Figure 1. The latter aroach will be used in this exeriment. To find g m from the characteristic curves of Figure 1, find the desired oerating oint (i D,v ) that is determined by the load resistor and the drain suly voltage v DD. Then, draw a vertical line through the v oerating oint. On this line find the voltage difference between the two nearby characteristic curves, Δv. Extraolate the two intersection oints to the y-axis and find Δi. Then use Eq. () to find g m. The outut resistance r d shunting the gmv current generator is included in the model to account for changes in the drain current due to changes in The numerical value of r d can be obtained from the sloe of the D v., V curve above saturation (as shown in Fig. 1). Use the following grahical analysis to obtain r d. Find the characteristic curve closest to the oerating oint and draw a straight line suerimosed on the saturation art of the curve. Select two convenient values of v and draw two vertical lines through these oints to where they intersect the straight curve. Circle the intersection oints. The x- axis searation gives the value Δv. Next draw horizontal lines through the circles to the y-axis. The y-axis searation gives the value of Δi D. The value of r d=(δv /Δi D) V=const. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 7

8 4. RE-LAB REORT 1. Study Figures 6-4 and 6-5 in Streetman and describe the -V characteristics of a JFET. a. Manually re-lot Figure 6-4 (do not scan it or coy it) and describe in your own words the variation of deletion regions and channel as voltage changes. Describe what inch-off is. dentify the V in the lot. b. Manually re-lot Figure 6-5 (b) and identify. n this lot, describe how to calculate g m in your own words. 5. ROCEDURE NOTE: Determine the absolute maximum ratings (oerating range) of the JFET. These can be found on the first age of the JFET s datasheets. Ste 1. Construct the circuit shown in Figure 4. Figure 4. Circuit diagram for the vs. V characteristics measurement for the N5485 JFET. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 8

9 Once the circuit has been built, oen and execute the rogram FET_V_Curve.vi using LabVEW to obtain a lot of the vs. V characteristic similar to the one shown in Figure 1. This rogram allows you to set a start voltage for V and V. t also allows you to set a ste size for each of them. FET_V_Curve.vi will start at the initial V and V voltages and then will ste the V value from its initial value to its final value. After the comuter reaches the final value of V at a fixed V then it will increment V. This rocess will continue until the final values of both V and V are reached. Set V to vary from 0.0 V to 0 V in 0.5 V stes. Let V vary from 0.0 V to -.5 V in -0.5 V incremental stes. f you accidentally ut ositive incremental values, you will destroy the transistor! f your transistor fails, you must get another JFET and re-characterize another transistor. (Save the JFET characteristics data on a ersonal account or device; any files saved on the deskto will be removed after logging off the comuter.) Examine the grah that you now have dislayed in the LabVEW window. Now examine Figure 1. Take note of the inch off locus (dotted red line) on the left art of the grah. The locus asses through the oint where the current flattens out at every value of V. After you have visualized the inch off locus for your grah, estimate the values for V and D(Sat.) at the different gate voltages and determine. Next, use Eq. (1) to calculate D(Sat.) values for V 0. Fill out the table below. Table 3. Exerimental values of V, D(Sat.), g m, and r d and theoretical values of D(Sat.). V V(exerimental) D(Sat.)(exerimental) D(Sat.)(equation) gm rd 0.0 V N/A -0.5 V -1.0 V -1.5 V -.0 V -.5 V Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 9

10 Now, lot D(Sat.) vs. V curve in Excel using the exerimental values in the table above. This lot should look similar to the Figure and it shows JFET amlifier s inut-outut (v - i D) characteristic. Next, imagine you lan to use the N5485 in an amlifier at oerating oint v = 8 V. For this oerating oint, find: 1. The transconductance g m and fill out the table above. Read 3.4 in this manual to understand how to calculate g m.. Calcuate r d based on the method described in the 3.4 in this manual and fill out the table above. 6. LAB REORT Tye a lab reort with a cover sheet containing your title, name, your lab artner s name, class, section number, date the lab was erformed and the date the reort is due. Use the following outline to draft your lab reort: ABSTRACT: Briefly describe the contents of your reort and its significant findings. DATA RESENTATON: Create a well formatted table resenting the data from Table 3. ANALYSS: 1. JFET outut characteristics ( V ) lot the vs. V characteristic. Show the inch-off locus in the lot. Make sure both axes are labeled and the grah is aroriately titled.. JFET inut-outut characteristics ( D V ) a. lot the D(Sat.) vs. V curve. lot both exerimental and theoretical D(Sat.) in the lot. Make sure both axes are labeled and the grah is aroriately titled. b. lot the vs. V characteristic again and exlain how you use this lot to find g m and r d at a ν equal to 8 V at the secified gate voltages. CONCLUSON: Describe how your analyses corresond with the exectation of theory described in Section 3. ATTACH: Signed instructor verification form. Lab X: Low Frequency Characteristics of Junction Field Effect Transistors age 10