SCHOOL OF ELECTRONICS ENGINEERING
Program :Electronics and Communication
Analog Electronic Circuits laboratory Course code: 22EECP202
EXPERIMENT NO: 5
5.1 Title of the experiment: To designCurrent Mirror circuit using MOSFET [ALD1115PAL]
5.2 Aim or objective: To design and study the current mirror circuit using MOSFET
( as a Current Source).
5.3 List of equipment required to conduct the experiment with their specifications:
SI.NO Components/Devices/Equip Specifications Quantity
ments/
1. MOSFET ALD1115PAL 2
2. Variable Power Supply 0-30V/2A 2
3. DMM (Ammeters) 0-10mA 2
4. Resistors R1=1kohm,0.25W 1 each
R2 = Pot of 1M
5. Wires - assorted
5.4 Theoretical background:
Current sources play a major role in the design of Integrated circuit amplifiers. Constant
current sources are generated using a technique called Current Mirror. The constant current
SCHOOL OF ELECTRONICS ENGINEERING
Program :Electronics and Communication
Analog Electronic Circuits laboratory Course code: 22EECP202
source is used both in biasing and as an active load. On an IC chip with a number of amplifier
stages, a constant dc current (called a reference current) is generated at one location and is then
replicated at various other locations for biasing the various amplifier stages through a process
called as Current Steering.
The heart of the circuit is transistor Q1, the drain of which is shorted to its gate ,there by
forcing it to operate in the saturation mode. The drain current (ID1) of Q1 is supplied by VDD
through resistor R, which in most cases would be outside IC chip, since the gate currents are
Zero. The current through R is considered to be the reference current of the current source and is
denoted by IREF.
The current source discussed here is the basic current mirror. Specifically two important
parameters are required for evaluating any current mirror. The accuracy of the current transfer
ratio of the mirror and the output resistance of the current source. Here we will be evaluating the
accuracy of the current transfer ratio of the Current mirror. Consider the circuit shown in Fig 1,
The saturation current ID for Q1 and Q2 MOSFET are as follows,
ID1 = ½ Kn’(W/L)1(VGS-Vtn)2 (1)
ID1 = IREF = (VDD – VGS) / R (2)
I0= ID2= ½ Kn’(W/L)2 (VGS-Vtn)2 (3)
I0 / IREF = (W/L)2 / (W/L)1 (4)
Neglecting IB,
current transfer ratio=Iref/Io = 1 (5)
SCHOOL OF ELECTRONICS ENGINEERING
Program :Electronics and Communication
Analog Electronic Circuits laboratory Course code: 22EECP202
5.5 Design:
Fig 1: Current Mirror
5.6 Step by step procedure to carry out experiment:
Connect the circuit as shown in Fig 1.
Set the VDD to 5 V voltage using DC power supply.
Measure the reference current and output current with ammeter .
Determine the current ratio.
5.7 Table of observations:
a) When supply voltage is varied
SCHOOL OF ELECTRONICS ENGINEERING
Program :Electronics and Communication
Analog Electronic Circuits laboratory Course code: 22EECP202
When R2= 1kΩ
SI.NO Input voltage Refrence Output Cuttent ratio
current ( Iref) cuttent(Io)
(Vi in volts) (Iref/Io)
1.
2.
3.
b) When resistance is varied
When Vdd = 5V
SI.NO Refrence Output cuttent Cuttent ratio
current (Iref)
(Io) (Iref/Io)
1.
2.
3.
From datasheet:
Transconductance = Gfs = 1.8 mmho for VDS=5V, IDS=10mA
Drain source ON resistance = RDS (on) = 350-500Ω for VGS=5V, VDS=0.1V
Drain source Breakdown Voltage = 10V for IDS=1uA, VGS=0V
Input Capacitance = CISS= 1-3 pF
SCHOOL OF ELECTRONICS ENGINEERING
Program :Electronics and Communication
Analog Electronic Circuits laboratory Course code: 22EECP202
5.8 Specimen Calculation:
i. With Iref= 0.025mA ; Io= 0.024mA ; Current ratio=1.0416
ii. With Iref= 0.008mA ; Io= 0.008mA ; Current ratio=1
5.9 Nature of graph: NA
5.10 Conclusion of the experiment:
Results & Discussion: Experimental set up for current mirror is realized and following
observations are obtained:
i. With Iref= 0.025mA ; Io= 0.024mA ; Current ratio=1.0416
ii. With Iref= 0.008mA ; Io= 0.008mA ; Current ratio=1
Conclusion: It is observed that
The Current transfer ratio will be Unity when Iref = Io , if both the MOSFETS are
identical.
The Current transfer ratio will be non-Unity If two MOSFETS are not identical, when Iref
≠ Io
