Lab 11:  JFET:  Characteristics and Applications

In this lab you will build 8 different types JFET circuits. You will then use the circuit as an AM transmitter.

 

In order to do this lab you need to understand the characteristics of JFETs.

 

Reading Ch 4.3.3+ lecture notes.

Lab credit:  Hayes & Horowitz, Student Manual for the Art of Electronics, Cambridge Univ. Press, 1989.

Components:  
The following circuits use the 2N5485 n-channel JFET in the TO-92 package.

               

PART 1) FET Characteristics I_D vs V_GS. In this part you will measure I_DSS, V_GS,off, and I_D vs V_GS. You will compare your results to the model for the JFET in the active region (AKA saturated region). The active region model is appropriate in this case because V_GS = 15V is large enough to keep the JFET out of the ohmic region (see the I_D vs V_DS graphs).

Construct the circuit below. Be sure to choose the DMM with the most sensitive current scale to use as the ammeter. You will need to measure current down to 0.01 μA. The ammeter measures I_D. The voltmeter measures V_G which is the same as V_GS.

Q 1) 2pts. Why is V_G the same as V_GS in this circuit?

First check that your circuit works. Use the yellow adjustment tool to operate the potentiometer (variable resistor). At one extreme, V_GS = 0 and I_D will be a maximum. (The specification sheet for the 2N5485 says that I_DSS will fall in the range 4–10 mA.) As you rotate the pot (short for potentiometer) in the other direction, V_GS becomes more negative and I_D decreases. At some point I_D is reduced to zero. Decreasing V_GS beyond this point has no further effect.

For these measurements, as you should for any measurements, always use the most sensitive scale available to perform the measurement. For example, do not use the 2 mA scale to measure a 0.1 mA current if a 200 μA scale is available.

Measure I_DSS. Adjust the pot so that V_GS = 0. Record the current, this is I_DSS.

Measure V_GS,off. Rotate the pot so that the current is decreasing. When the current just reaches zero on the 20 μA scale, record V_GS (use a more sensitive scale if it is available). This is V_GS,off. (The specification sheet for the 2N5485 says that V_GS,off will fall in the range −0.5 to −4 V.)

Measure I_D vs V_GS. Make a table in you lab notebook. Make your measurements in the following way. Beginning at V_GS,off, increase the current until it reaches approximately 0.3 μA and then record I_D and V_GS. Continue increasing the current recording data at the approximate I_D values of 0.3, 1, 3, 10, 30, 100, 300, 1000, and 3000 μA. Do not waste time setting I_D exactly to these values, getting within ~30% of the target value is close enough. What is important is that you measure both the I_D and V_GS values accurately. You will notice that above 500 μA the voltage points are farther apart. Go back and take a few more data points to fill these gaps. Select voltages within the gaps so that no two points at separated by more than 0.5 V. Add these to you table. Your table should also include I_DSS and V_GS,off.

Repeat these measurements using you second JFET. Manufacturing variations cause the I_DS and V_GS,off to vary.

Q 2) 4pts. What are I_DSS and V_GS,off for each of your JFETs?

Graph 1) 25pts. Graph your data using excel as we did in previous labs. You will graph your experimental data and compare you results to the model. In the plot use open symbols for the experimental points and solid lines for the values calculated from the model. The model is the equation for I_D in the active region. The model uses the parameters I_DSS and V_GS,off that you measured for each JFET. Each JFET must be modeled separately because these parameters will be different. Use different symbols for each set of experimental points and a different line type (e.g. solid or dashed) for the model for each JFET.

Q 3) 2pts. How closely does your experimental data fit the model?

Before breaking apart this test circuit, you need to make some specific measurements on your JFET for the next part. Measure the V_GS where I_D = 1.0, 0.5, and 0.1 mA. Do this for each JFET. It’s a fast easy measurement because your have the circuit set up already.

PART 2) FET Current Sources. In part 1 you used an independent voltage source to control V_GS. A simple current source can be made from a FET and a source resistor R_S. The voltage drop across R_S caused by the drain current I_D causes the source voltage V_S to rise above ground. Grounding V_G, this makes V_GS negative and acts to reduce I_D. The connection between I_D and V_GS is a simple form of negative feedback. Feedback is the connection between a quantity (I_D) and a control (V_GS) that acts on that quantity. Negative feedback means that the action of the control (V_GS) in response to any change in the quantity (I_D), acts to return the quantity (I_D) to its equilibrium value. In other words, negative feedback causes the FET to hold I_D constant. The value of I_D is determined by R_S and the I_D vs V_GS graph.

Make a current source from one of your FETs. You previously measured V_GS for I_D = 0.1, 0.5, and 1.0 mA. Use these to calculate the value of resistance R_S to make V_GS = I_DR_S for each current. Assemble the circuit below using the standard resister values closest to your calculated values of R_S.

Q 4) 5pts. Calculate your three resistor values. Show your work. Make a table showing V_GS, I_D, and R_S.

For each of your resistor values, measure the actual resistance. Place the resistor into the circuit as R_S. Adjust the pot to give the maximum current; V_D should then be nearly 15 V. The pot is the load in this circuit, the device through which you would like to hold the current constant. Rotate the pot to increase its resistance. What happens to I_D? What happens to V_D?

Q 5) 4pts. When the pot is adjusted to give maximum I_D and maximum V_D, is the value of the resistance the pot contributes to the circuit a maximum or a minimum?

Q 6) 3pts. For each resistor value make a table of the actual resistor value, the maximum current, and the V_D when I_D dropped to 90% of its maximum value.

Q 7) 4pts. You have observed the behavior of I_D and V_D. Generalize your observations and describe what the FET is doing to control the current as you change the resistance of the load (the pot) from one extreme to the other.

PART 3) FET Characteristics I_D vs V_DS. In this part you will measure I_D vs V_DS. You will be able to see the active region where I_D is essentially independent of V_DS and the ohmic region where I_D depends strongly on V_DS. Construct the circuit shown below. Note that the voltmeter measures V_D which in this circuit is the same as V_DS.

           

Graph 2) 15pts. Make a table of I_D vs V_DS. Adjust the pot so that V_DS is nearly 15 V. Measure I_D and V_DS. Adjust the pot to decrease the voltage and record I_D and V_DS approximately every 2 V until V_DS is around 4 V, then take data every 1 V. When I_D has dropped ~1 mA below its maximum, begin recording data in approximately 1 mA steps. This procedure will give you an excellent graph with the smallest number of measurements. (See the above example of an excellent graph.)

Q 8) 2pts. How does I_D in the active region compare to I_DSS that you measured in part 1?

PART 4) FET Source Follower. Construct the circuit below. Connect your function generation to V_in. Set the frequency to 1 kHz and 2 V_p-p. Measure the voltage gain A_V = V_out/V_in. (Use peak-to-peak voltages to calculate the gain.)

Q 9) 3pts. What is the voltage gain you measured?

Q 10) 4pts. The voltage gain of a follower ideally would be unity. The equation for V_out shows that the gain will be less that unity unless g_m is very large. Use the equation for the source follower gain to calculate g_m. What is g_m for your JFET?

PART 5) Follower with Current-Source Load. The addition of second FET functioning as a current-source load on the upper FET corrects many of the defects of the previous FET source follower circuit. Construct the circuit below. Connect your function generation to V_in. Set the frequency to 1 kHz and 2 V_p-p. Measure the voltage gain A_V = V_out/V_in using your oscilloscope. (Use peak-to-peak voltages to calculate the gain.)

Q 11) 3pts. What is the voltage gain you measured?

Q 12) 2pts. The voltage gain of the improved circuit is much closer to unity. Using the equation for the source follower gain, what is the effective value of g_m if A_V = 1?

PART 6) The FET as a Variable Resistor. In the ohmic region the FET can be used as a voltage-controlled resistor. In the following circuit the FET forms part of a voltage divider. Construct the circuit below. Connect your function generation to V_in. Set the frequency to 1 kHz and 0.2 V_p-p. Observe V_out using your scope. Adjust the 100K pot. What happens? Measure the min and max V_out/V_in. Is the wave form distorted? Change the waveform to 1 kHz triangle waves and sketch the results at enough different attenuations to describe what is happening.

Q 13) 2pts. What are the min and max V_out/V_in?

Q 14) 6pts. Sketch the shape of the triangle waveforms at enough different attenuations to describe what is happening.

Q 15) 3pts. From your schematic above, calculate the range of V_GS spanned by the adjustment. Show how you arrived at your answer.

PART 7) Compensated Attenuator. The addition of a resistor and a capacitor improves the voltage controlled attenuator. Repeat the above measurements.

Q 16) 2pts. What are the min and max V_out/V_in?

Q 17) 5pts. Sketch the shape of the triangle waveforms at enough different attenuations to describe what is happening.

PART 8) AM Modulation: Build your own AM Radio Station. You will need to use both channels of your function generator (FG) for this part. Connect FG CH1 to the V_carrier input. This is the frequency that will be modulated. Set the carrier frequency to a 20 Vp-p sine wave with a frequency within the AM broadcast band (540-1610 kHz). This is the frequency of your station (and the frequency to which you will need to tune the AM radio). Connect FG CH2 to the V_mod input AND to scope channel 2. Select a modulation tone in the mid audio range 500-5000 Hz and 5 Vp-p amplitude. V_mod controls the amplitude of the carrier wave appearing at V_out. Observe V_out on the oscilloscope channel 1 using the 1 X scope probe.

Scope set up to view the envelope of the AM modulated wave.
Trigger menu: source =  CH2 (V_mod)
Acquire menu: acquisition = peak detect.

Adjust the 100K pot and the amplitudes of the carrier and the modulation. The modulation should not completely pinch off the carrier. Connect a wire ~ 1m long to V_out, this will serve as an antenna. Tune the AM radio through the frequency region of the carrier frequency. When you have found your station, you will hear the modulation tone. Verify that this is in fact your signal by varying the frequency of the tone. Does the tone you hear on the radio change? Adjust the frequency of the carrier. Your station should move on the radio dial. If you change the carrier frequency, you should need to retune your radio to hear your tone again.

Q 18) 4pts. Sketch the envelope of the modulated waveform (V_out) you observe on the scope.