**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

_{}

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

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 a independent voltage
source to control

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*_{D}*R*_{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

**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

**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

**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

**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 two
function generators for this part. Connect one function generator to the

**Q 18) 4pts.** Sketch the
modulated waveform you observe on the scope.