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CALIFORNIA STATE UNIVERSITY OF LONG BEACH ET - 341 Laboratory Exp. 34: JFET Applications Group: Channraingsey Yek RaamSundararaj Instructor: William Lopez Date: February 21, 2014 Introduction: In this lab, we will practice some of the main applications of the field effect transistor. One main application is the analog switch that acts like a switch, which allows us to block or transmit an ac signal. We will also explore other applications such as voltage-variable resistance, voltage gain controller, and JFET chopper circuit. Procedure: We began with analog switch. As mentioned, this analog switch allows us to build circuits that either transmit an ac signal or block it from the output terminals from a push of a button. In this experiment, we try to transmit an ac signal 1 kHz of 100 mVpp. We see that it can fluctuate the signal from 100 mVpp to zero volt within seconds. XSC1 Ext T rig + _ B A + XFG1 _ + _ R1 10kΩ V1 10 V R3 100kΩ Q1 S1 Key = Space R2 100kΩ J310 Figure 1. Analog Switch Circuit When the VGS is zero, the JFET operates in the ohmic region with a resistance of RDS = 152.5Ω (value vary for each JFET). This value obtains by connecting the gate and the source lead, and measure with an ohmeter from drain to source. Chopper circuit as shown in figure 2 is a continouse square wave that continously switches on and off really fast. XSC1 XFG1 R1 Ext T rig + 10kΩ Q2 _ C1 XFG2 1µF Figure 2. JFET Chopper Circuit + D1 1N914 R2 100kΩ J310 B A R3 100kΩ _ + _ Figure 3. Multisim Chopper Circuit The Set Rise/Fall Time of the square wave generator must set to the 25 μsec to get a clean signal output shown on the oscilloscope in Figure 3. Voltage-Variable Resistance Application VGG Vin (mV) Vout (mV) RDS (kΩ) -2.8V 100 10 1.11 -2.9V 100 20 2.5 -3V 100 32 4.70 -3.1V 100 48 9.23 -3.2V -3.3V 100 100 65 87 18.57 66.92 -3.4V 100 92 115 -3.5V 100 96 240 As we can see from the above table, the value RDS changes when the gate voltage VGG changes. Conclusion and Results: The Analog Switch circuit allows us to build circuits that either emit an AC signal or block it from the output terminals. When a JFET is used in the voltage variable resistance circuit, we can change the value of rds(on) by changing the VGS. In the AGC circuit, we can build amplifiers whose voltage gain can be controlled by the AGC voltage.