Eletronics Lab Report - Current Mirror
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Transcript of Eletronics Lab Report - Current Mirror
EGR220 Than Lab Design Project
Page 1
Introduction and Objectives
Since common gate MOSFET amplifiers are popular
in design cascode circuit and current follower, it is
important to know the configuration of common gate
MOSFET. Moreover, in IC design, active load
(constant current source) are really useful. Therefore,
in this lab, we were instructed to measure and analyze
the common gate configurations of the MOSFET
amplifier to determine the voltage gain, input
resistance and output resistance by using active load.
The primary objectives of this lab are:
1. To analyze and understand current mirror
circuit with P-MOS
2. To be able to set up biasing with active load
3. To be able to analyze the voltage gain, input
resistance and output resistance of Common
Gate Amplifier
Equipments and Components used
In this lab, the equipments and components we used
are:- Transistor: CD4007CN (x2) Resistors: 10kΩ
potentiometer, 1KΩ resistor, a breadboard, a waveform
generator, ±20V power supply, a multi-meter, wires
and cords.
Procedures
Procedure 1: Analyzing Current Mirror
Circuit
Figure 1: Current Mirror Circuit
We connected the circuit as in figure 1. According
to lab instructions, we gave VDD = 10V. Then, we
adjusted RREF until we got Io = 1.5mA. Then, we
measured IREF. We found that IREF = 1.23, which is
less than Io. Therefore, the ratio of Io/IREF = 1.23 ≠
1. From that, we could conclude that the
transistors Q1 and Q2 are not equal.
Active Load usually refers to the constant current
source used in place of resistor. In IC (Integrated
Circuit), resistors are usually difficult to fabricate.
By using two P-MOS transistors, current mirror
circuit can be easily built, and it can be used as a
constant current source in place of resistors. This
is one of the reason why active load are useful. In
addition to that, by adjusting IREF, we can easily
control the current fed into the amplifier.
Procedure 2: DC Biasing CG Amplifier with
Active Load
Figure 2 : Biasing the CG amplifier
We set I = 1.5mA by giving Vbias = 6V and
adjusting RREF. Then, we set Vbias = 0V. When
Vbias = 0V, I = 0A. We gradually increased Vbias
until we got I = 10μA. From our measurement
VTN = 2.9V. Then, we set Vbias = 6V again, and
measured Vo. We found Vo = 3.10V. VD = Vo – IRs
= 3.1 – 1.5 = 1.6. VGS = 6 – 1.5 = 4.5V. VDS = VGS
EGR220 Than Lab Design Project
Page 2
- VTN = 1.6V. Therefore, the amplifier could
operate in saturation region.
Procedure 3: Small Signal Analysis of CG
Amplifier
Figure 3
In small signal analysis, we can ignore the bias
point. In the circuit, since Body was not
connected to Source, the body effect on small
signal would be taken into account.
gm = 2I/(VGS – VTN) = 0.00187
gmb = χgm = 0.00037
ro = VA/I = 6.67KΩ.
Therefore, input resistance (assume RL = ∞)
Rin = (ro + RL ) / (1 + (gm + gmb)ro) = ∞
GV= GVO = (1 + (gm + gmb)ro) =16.13 V/V
Rout = ro + [(1 + (gm + gmb)ro)]Rs = 22.8KΩ
Gis = Gvo (Rs/Rout) = 0.707
Figure 4: Voltage Gain
Figure 5: Voltage Gain (Closer View)
From the screen image, we found that the overall
voltage gain ≈ 10V/V. Therefore, our
measurement was close enough to our calculation.
The difference might be due to the fact that the
resistance of oscilloscope is not infinite although
it is very large enough to assume that it is infinite.
In order to measure output resistance, we removed
the current source and grounded VG and VI, and
provided VO = 1V. We measured IO, which was
0.2mA. Therefore, we calculated Rout = VO/IO =
25KΩ, which was close enough to our
calculations in the prelab.
Another thing to note is that the output signal is
not inverted. Therefore, CG amplification is non-
inverting. Although CG provides high voltage
gain, the current gain is almost unity.
When we gave input signal = 1Vpp Sine Wave,
EGR220 Than Lab Design Project
Page 3
we found the the output signal was clip off. That
was expected.
Figure 6: Output Signal was clip off
From our pre-calculation, the maximum input
signal swing was ±427mV.
Discussion We found that in common gate circuit, the input
resistance is greater than output resistance, and the
voltage gain is very high, almost 10 times. However,
the current gain is unity. That's why common gate
circuit are popular for current follower circuit and
cascode circuits. Unlike, common source, common
gate is non-inverting.
References [1] Sedra, Adel S., and Smith. Kenneth C. “Microelectronics
Circuits”. 5th. New York: Oxford University Press, 2004.