EGR220 Than Lab Design Project

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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

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- 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

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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.