Download EXPERIMENT 8: MOSFET – Common

EXPERIMENT 8
MOSFET – Common-Source Amplifier
2N7000 - NMOS
Drain (D)
Gate
(G)
Source (S)
NMOS CIRCUIT SYMBOL
MOSFETs vs BJTs
BJTs
• Three different currents
in the device: IC, IB and
IE
• Consume a lot of power
• Large size device
MOSFETs
• Mostly widely used
today
• Low power
• Very small device (nm)
• Simple manufacturing
process
• Only 1 current, ID
• Common Source is similar to Common
Emitter (BJT)
• The voltage/current gain is negative value
PART A
MOSFET DC ANALYSIS
• Construct the circuit of Fig. 8.1 but DO NOT CONNECT the
capacitor yet and record the measured values of the resistors.
• Using the DMM, measure the values of VGS, VDS and ID.
G
D
S
VDS
VGS = 2.17 V
VDS = 5.42 V
To measure drain current,
ID
ID = 1.388 mA
• Using the threshold voltage, VTN = 2.1 V,
please confirm that the transistor is in
saturation.
(VDS > VDSsat and VDSsat = VGS - VTN)
• VDSsat = VGS – VTN = 2.17 – 2.1 = 0.07 V
• So is your measured VDS > VDS sat?
• From the measured value of ID, deduce the
value of the conduction parameter Kn using
the drain current in saturation formula
I D  K n VGS  VTN 
2
Kn = 1.388 mA / (2.17 – 2.1)2
Kn = 0.283 A/V2
PART B
COMMON SOURCE AMPLIFIER (AC ANALYSIS)
• By using the same circuit configuration –
connecting the capacitor this time,
• Set the function generator to 100 mV
amplitude, 20 kHz sinusoidal wave. Connect
it to vin.
• Connect Channel 1 of the oscilloscope to vin
and Channel 2 of the oscilloscope to vout.
• Measure the peak-to-peak value for both
input and output voltages on Channel 2 of
the oscilloscope.
• Calculate the value of the gain, vo / vin
vo / vin ≈ - (34 to 37)