The necessary values of the bias are as follows:
IG = -8.55 pA, ID = 1.085 mA, UGS = - 1.085 V and UD = 10.19 V
Activity:
Analyse the values of the bias parameters above. Think over and answer the following
questions
 How can the value of UGS be negative while the voltage level of the power supply is
positive in the circuit?
 What is the right direction of the gate current if IG = -8,55 pA? Is this direction
corresponding to the direction of the arrow drawn on the gate? What is really there
between the gate and the source of the field effect transistor?
 Is the bias point of the field effect transistor in the saturation region?
 What is your opinion about the value of UD from the point of view of the output signal
level?
Simulate the voltage amplification rate of the circuit on 1 kHz frequency. How does the
voltage amplification rate change if the source capacitor C S is missing?
The set up parameters of the AC Sweep Analysis can be as follows:
After carrying out the simulation, the following Bode diagram shows the result:
The value of the output signal is 4.12 V. Because of the value of the input signal level which
is 100 mV, the voltage amplification rate of the circuit is A U = 41.2 (32.3 dB). The lack of the
source capacitor results in the following change:
with the capacitor CS without the capacitor CS
data:
voltage
amplification
rate
41.2
5.77
voltage
amplification
rate ( in dB)
32.3
15.2
Show the shape of the output signal. Does the shape of the output signal have any distortion
when the level of the input signal is 100 mV? (A larger output signal level can be received
with the capacitor C S.) The set up window of the transient analysis can be seen below:
The length of five periods takes 5 ms if the frequency of the signal is 1 kHz. (This value is the
Final Time in the window.)
After carrying out the simulation, the distortion of the output signal can not be seen when the
level of the input signal is 100 mV. So it has been good to choose this value as an input signal
level.
Increasing the value of the input signal level step by step (100-150-200-250-300 mV), the
distortion of the output signal can be seen clearly:
Change the values of all three capacitors with the help of the command Param.
Use the commands CTRL+G and Param. Now choose one of the three capacitors. Let it be
the source capacitor CS. Simulate what effect can the change of the value of this capacitor
have on the value of the amplification rate of the circuit at the low frequency range. During
the simulation the values of the other two capacitors should be set up to 100 µF. The
breakdown frequencies caused by them fall under 0.1 Hz, and they do not disturb the effect of
the source capacitor CS in the frequency range investigated between 0.1 Hz and 10 kHz.
The set up parameters of the CS are given in the window below:
As it can be seen in the Bode diagram below, there are two breakdown frequencies developed
because of the source capacitor CS. Both of these two breakdown frequencies can be shifted
down in the frequency range (to the left on the diagram) by increasing the value of the source
capacitor CS.
The low frequency range of the voltage amplification rate can be amended by choosing a
larger value for the capacitor CS. The breakdown frequencies under 0.03 Hz that can be seen
on the diagram belong to the input and output capacitors Cin and Cout.
Fix the values of the input capacitor Cin and the source capacitor CS as large as 100 µF and
10000 µF. Change the value of the output capacitor as follows:
The output capacitor Cout causes only one breakdown frequency at the low frequency range,
as it can be seen in the following Bode diagram:
Even the effects of the capacitors Cin and CS with fixed values can be seen in the frequency
range at about 0.1 Hz, because new breakdown frequencies have appeared.
The effect of the input capacitor Cin is similar to the effect of the output one.
1.b.
Take out the capacitor CS from the previous circuit. (The circuit remains a common source
amplifier in the future, too, but the source will not connect with the ground in the ac small
signal equivalent circuit.)
Describe the output signal shape with the help of the transient analysis. Increase the input
signal level and show the distortion of the output signal shape. Simulate the effects of the
input and output capacitors on the voltage amplification rate of the circuit at the low
frequency range.
1.c.
Assemble the following common drain amplifier using an n-channel MOS-FET as an active
device.
Evaluate the bias point, the voltage amplification rate and the output signal shape of the
amplifier based on the knowledge studied before.
1.d.
Assemble a common gate amplifier alone or with the help of your tutor. Evaluate the bias
point, the voltage amplification rate and the output signal shape of the amplifier based on the
knowledge studied before.