Name

Date Class

.t Experiment 8 FET Amplifiers

ReadingBoylestad and Nashelsky, Electronic Devtces and Circuit Theory, Ninth Edition,Chapter 8

Key ObjectivesPart 1: Calculate and measure dc and ac parameters for a common-source

amplifier.Calculate and measure dc and ac parameters for two common-drainamplifiers.

Part3: Calculate and measure dc and ac parameters for a cascade amplifier.

Components NeededPart 1: The Common-source JFET Amplifier

Resistors: one 620 f), one 1.0 kf), one 3.3 kf), one 10 ke), one 100 kO,one 1.0 Mf)

One 2N5458 n-channel JFETCapacitors: one 0.1 pF, one 1.0 pF, one 10 pF

Part2: The Common-Drain JFET AmplifierResistors: one of each: 470 Q,1.0 kO, 10 kf), 100 kQ, 1.0 Mf)Two 2N5458 n-channel JFETOne 1 kf) potentiometerCapacitors: one 0.1 pF, one 1.0 pF, one 10 pF

Part 3: A Cascade AmplifierResistors: one of each: 180 O, 2.7 k{r,3.9 kO, 5.1 kO, 27 k{r,56 kQ, 1.0 MOCapacitors: one of each: 0.1 pF, 1.0 pF, 10 pFTransistors: one 2N3904 npn transistor, one 2N5458 z-channel JFET

61

Part 1: The Common-Source JFET1. Measure and record the values of the

resistors listed in Table 8-1.

2. Construct the common-source (CS)amplifier shown in Figure 8-1. Set thesignal generator for a 500 mVoo sinewave at 1.0 kHz. Checkthe amplitudeand frequency with your oscilloscope.

AmplifierTable 8-1

&1.01()

vo,+15 V

1.0 pF2N5458

V,:500 mV*1.0 kHz

R.1.0 MO

_

Figure 8-1

Measure the dc voltages listed in Table 8-2 and compute 1p. Set the functiongenerator for a 500 mVpp sine wave and measure the ac quantities listed. Comparethe input and output ac voltage by viewing Vinatd %,t simultaneously. Measurethe voltage gain and note the phase difference (0o or 180') between the input andoutput signal. Enter all data from this step in Table 8-2.

c210I pF

3.

Table 8-2 Parameters for CS

lnput voltage, VinOutput voltage, Vo,,Voltage gain, Au

62

4. change the source resistor from 1.0 ko to a smaller value. If you are using thesame JFET as in Experiment 7, use the source resistor that you selected for theself-bias case (Part 1). Otherwise, use a 620 O resistor for Rs. you should obseryea slight increase in gain with the smaller resistor, despite the fact that it isbypassed. can you explain this gain increase? (Hint.:consrder g-.)Observation:

5. Now change the load resistor from l0 ko to 100 ko. Does the gain change?Explain your observation:

Troubleshooting6. Assume each of the faults listed in Table g_3 is

circuit of Figure 8-1. predict the outcome withfault in the circuit and test your prediction.

Table 8-3

in the common-source amplifierthe fault in place. Then put the

Conclusion: Part 1

Questions: Part 1I . If the operating point is changed in the circuit of Figure 8- 1 because of a different

source resistor, is there any effect on the input or output impedance? Explain.

2. Compare the amplifier in Figure 8-1 with the CE amplifier in Experiment 5(Figure 5-1). Which is better suited to amplifii a signal from a rorrr.. that has a100 kf) Thevenin resistance? Explain your answer.

C2 is openSource and drain reversedZpp drops to +l2Y

63

Part 2: The Common-Drain JFET Amplifier1' A self-biased common-drain (CD) circuit rt shown in Figure g-2. Connect thecircuit and measure the dc voltage at the drain, source, aid gate and compute 1p.observe the input and output ac voltage with tile oscilioscope. Measure thevoltage gain and note the phase. Enterthe data in Table g-4.

Adding Current-source Bias2' The voltage gain is much less than 1.0 due to the transconductance, g-. This canbe visualized as an internal resistance equal to the reciprocat org..-fiiis internalresistance (U s; is analogous to r" of a bipolar transistor, but is Iarger for a given

2N5458

V,:2.0 Yw1.0 kHz

cu.rent. It forms a voltage divider with Rs as shown in Figure g-3. To it,rL4BE urvrus.f wrln l(s as snown m llgure g_3. To improve thegain, the JFET current source (with its high internal resisiince) can be added as

+Voo+15 V

shown in Figure 8-4..

C-hange the previous self-biased circuit to include the current-sourcebiasing shown in Figne 8-4. This circuit does not rrave "oupting capacitors (anadvantage for the low-frequency response) but the source resistor now includes a

variable resistor to adjust the dc offset. Notice that the output is taken at the drainof Qz. Start with Rsz set near the center of its resistance.

64

C:VI / Omt\--r

Figure 8-2

Table 8-4 Parameters for CD Self-Biased

Gatevoltage, V6Source voltage, ZgDrain voltage, Vj

lnput voltaga, Vin

Voltage gain, A,

f

*V,,

v,--2.0 vpp1.0 kHz

Figure 8-3 llg* and Rs form avoltage divider.

- /.t

-15 v

Figure 8-4 Q2is a curent source that hashigh drain-source resistance.

3. DC couple your oscilloscope and view the output. Adjust Rsz for no dc offset inthe output. Measure and record the dc and ac quantities listed in Table 8-5 for thecommon-drain, current-source amplifier. Notice the improvement in the gain fromthe circuit in Figure 8-2.

Table 8-5 Parameters for CD Current-Source Biased

Ql gatevoltage, V6Q1 source voltage, 7gQl drainvoltage, VeQ2 gate voltage, VsQ2 source voltage, Z5Qz drainvoltage, VpDrain current,lpInput voltaga, Vin

Voltage gain, A,

65

4. With no dc offset, the load resistor can be directly connected to the output. Testthe effect of a 10 kQ load on the amplifier. Then test the signal clippiipping point byincreasing the input signal from the generator.

Observations:

5. Try switching the two FETs. Does the DC offset need to be readjusted?Observations:

Conclusion: Part2

Questions: Part21. Why was the gain much better with current-source biasing than self-bias?

2. Estimate the input and ou@ut resistance of the amplifier in Figure 8-4 based onthe observations you made in step 4.

Part 3: A Cascade Amplifier1. Measure and record the values of the

resistors listed in Table 8-6.

2. Construct the amplifier shown inFigure 8-5.

Table 8-6

66

a,2N3904

RC5.1 ld)

R",180 f)Rs23.9l()

V

V,:300 mVpp1.0 kHz

Questions: Part 31. What are the advantagescircuit as was done in the

Rs 1.0 pF2.7 K>

of mixing a FETcircuit in Figure

R227 kO

c310 pF

J. Calculate the dc and ac parameters listed in Tables 8-7 and 8-8 for the amplifier.Assume the gain of the common-drain stage is approximately 0.75 (actuaf valuedepends on g-). (Use 0.75 to compute the ac base vortag

",

vt). Measure theparameters listed and verify that the amplifier works as expected. Enter data inTables 8-7 and 8-8.

Conclusion: Part 3

and a bipolar transistor in the same8-5?

2.

Multisim

Figure 8-5

Table 8-7 Table 8-8

What is the purpose of .Rsr?

67

Mu

WMultisim

Itisim SimulationOpen the Multisim file Exporiment O8_cascade. The circuit has a problem with it(designed not to be obvious!). See if you can find the problem using the scope andmeter. After correcting the problem, compa.re the simulation with your results.

68