EE 3111 – Lab 6
Analog Switch
Analog Switch vs. Digital Switch
• Not much difference between analog and digital switches
 Construction is identical, operation is different
• Switches cut-off or direct current from one circuit to another
 If directing between digital circuits it is a digital switch
 If between analog circuits it is an analog switch
• Digital switches handle DC signals, generally “high and low”
• Analog switches are designed to pass or block AC signals
• Uses of analog switches include:
• Power relays and regulators
• PWM signals for motors
• Multiplexors
Digital switch
Analog switch
Analog Switch
• Often more than one transistor is used
• Bidirectional switch allows current to flow in both directions
(and blocks in both directions)
• Multiplexor chooses output from several inputs
• JFETs typically allow current in both directions so two aren’t
needed for bidirectional operation like with MOSFET
Bidirectional MOSFET switch
(transmission gate)
Multiplexor
Images from http://www.electronics-tutorials.ws
Analog Switch
• Our use today is perhaps closest to use in an Adjustable Gain
Amplifier
• We see the JFET used to select different effective resistances, if
used in feedback of an Op-Amp, it would allow you to tune the
gain with different control voltages.
𝑉𝑜𝑢𝑡
𝑅𝑓
= − 𝑉3
𝑅𝑖
R2
J2N5485
J2
0
V2
2k
4
2
-
R3
VOS1
OUT
3
+ 7
OS2
V+
0
Images from http://www.electronics-tutorials.ws
1
6
8
Analog Switch
• We are operating with low VDS
• JFET is in the “ohmic region”, and acts
like a resistor between Drain and Source
• RFET is determined by VGS
Images from http://www.electronics-tutorials.ws
Procedure
• IGNORE ORIGINAL LAB 6 REPORT INSTRUCTIONS! FOLLOW
UPDATED PROCEDURE DOCUMENT
• FET_Resistance.vi uses 2 source meters
• One connected to +Gate/-Source
• One connected +Drain/-Source
• Select GPIBO from the pulldown menu
• Top SM is #24, bottom SM is #25
• You are supposed to find the right range of VGS and VDS - start
with what you got on your prelab and work from there
• You can build the correct resistance out of the following resistors:
100, 150, 220, 1K, 1.5K, 1.6K, 2K
• For AC Analysis, input goes to Drain – use sin wave
• Control goes to Gate – use square wave
• Sync from the generator with the slower signal to scope!