Analog CMOS/VLSI Circuits

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Independent study report on analog electronic circuits for Master's in electrical engineering. Covers amplifiers and different types of transistor devices. Includes schematics of some of these devices.

Text of Analog CMOS/VLSI Circuits

Analog CMOS/VLSI Circuits

Independent StudyIsi Oamen

CWID# 894816099

EGEE 599

Dr. Young D. Kwon

Fall 2011Introduction:

In modern electronics, VLSI technology plays an important part in everyday applications. The potential to decrease chip size while simultaneously improving speed is forever improving, as can be seen with the computers and cellular phones of today. The movement from analog to digital circuitry led us to replace vacuum tubes with solid-state silicon devices. Due to the immense decrease in size and manufacturing cost, Metal-Oxide Semiconductors (MOS) and Bipolar Junction Transistors (BJT) took the lead to be the popular choice in the industry. The idea is that they can act as switches or voltage/current amplifiers, which can improve power efficiency as well as performance.Although these digital devices have been the primary choice for corporations and the consumer in general, it is important to note that it is necessary for many real world systems to process an input that originates from a continuously changing source. Because there is no electronic substitute for what is generated in nature, analog input devices are required because they allow signals to be handled at the same rate that they are generated. For this reason, a combination of analog and digital technology is the best solution for those particular situations. This is commonly referred to as mixed-signal electronic design.

Mixed-signal circuits are involved in the operation of many devices, which include temperature sensors, radar systems, and amplifiers. The manner in which many of these devices work is that they receive an analog or continuous input and then convert it to a digital signal. This digital signal can then be utilized in a multitude of ways, whether as a digital readout or as an input of another device. One example of this process can be viewed in the block diagram in Figure C-1.

Figure C-1: Temperature Sensor

This temperature sensor works in the following manner: first the analog input is received by a sensor diode, which outputs a particular voltage based on the ambient temperature. This voltage is then passed through an analog-to-digital converter which changes the voltage to a particular binary value. At this point the binary value is sent to a value register. A value register in essence acts as a table which contains all of the possible binary values and their corresponding outputs. This output can then be sent to a digital readout or any device in which it can be used as needed. This same concept is similarly used in pressure and altitude sensors, light sensors, and seismometers. Another example of a mixed signal system is a Doppler radar/navigation system, seen in Figure C-2.

Figure C-2: Doppler Radar/Navigation SystemThe way a Doppler Navigation System works is that it first accepts an analog waveform through an antenna. The antenna sends this signal to a receiver which measures the frequency or performs any other necessary calculations. In the event the signal is going the other direction, it is sent out as an analog value by a transmitter to the antenna. The incoming measurement is passed along as a voltage which is then converted by the signal data converter into a digital value. That value is then displayed on the unit. As you can see, the process works in a similar fashion to that of the temperature sensor: an analog input is converted to a digital output and displayed to a device. One type of analog circuit that does not work in the same way is an audio amplifier, displayed in Figure C-3.

Figure C-3: Audio Amplifier System

In this scenario, the microphone contains a transducer which converts the analog audio input to a voltage, which is sent to the pre-amp. The pre-amp increases the waveform to a higher voltage so that there is more signal to work with. At this point the tone and volume controls are accessed to modify the high and low frequencies, as well as to increase the amplitude. The purpose of the power amplifier is to strengthen the signal by increasing the amount of current that is transferred to the speakers or any particular audio component. In turn, the component converts the signal back to sound.

This system differs from the other two because it maintains an analog signal throughout the process, and simply deals with the amplification of voltages and currents. For purposes of staying within the scope of the study, I will focus mainly on these types of analog circuits. Before that can be done, I must first explain the operational characteristics of the transistor and MOS process.nMOS Transistor:

The nMOS transistor is the most basic of all the MOS transistors. MOS stands for Metal Oxide Semiconductor, which means there is a metal gate, an oxide insulator, and silicon semiconductor. Polysilicon is more often used today than metal for the gate. The n represents n-type silicon, which is the material that composes the source and drain. It is doped with a material to give it more negatively charged ions. When the device conducts the channel also becomes n-type. A diagram of an nMOS transistor is shown below in Figure C-4.

Figure C-4: nMOS Transistor Process

The reason nMOS is simple is because it requires less steps in manufacturing and utilizes much less space than other types of transistors. The silicon substrate, which is generally p-type, is doped with two n-wells to become source and drain. Field oxide is grown across the substrate; a polysilicon gate and much thinner gate oxide are also applied. Metal contacts are added to the source, gate and drain.When zero voltage is applied to the gate, there is no current flowing through the channel. As the voltage is increased positively, negatively charged ions are attracted to the surface of the p-type material while the positively charged ions are moved away. Once the gate voltage reaches a level that collects enough negative ions, the surface of channel becomes n-type and conducts between the source and drain. This point is called the threshold voltage. Eventually as the drain voltage increases the current will approach a maximum point and no longer be linear. When that situation occurs the mode is called saturation, and all of this is characteristic of an enhancement type nMOS transistor. This can be seen in Figure C-5(a).

The other type of nMOS transistor is one which contains a small amount of doping at the surface of the substrate, creating an n-type channel. This channel is already conducting when there is zero voltage applied to the gate. The only way to eliminate the channel is to apply a negative gate voltage, which is depleting the channel. This is called a depletion type nMOS transistor, which is shown in Figure C-5(b).

Figure C-5: Enhancement and Depletion Mode nMOS Characteristic

There are a few equations that will come in useful when deciding which mode the transistor is in, as well as the amount of current flowing through the drain, or Id. The three modes are cutoff, triode (or linear), and saturation. The equations along with their constraints are below.

Id = 0

Vgs < Vto (cut-off)

Id = Cox(W/L)Vds*(Vgs Vto Vds/2)

Vds < Vgs Vto (triode)

Id = Cox/2(W/L)*(Vgs Vto)

Vds > Vgs Vto (saturation)

Where

carrier mobilityCox gate oxide capacitance

W, L channel length/width

Vgs gate-source voltageVds drain-source voltageVto threshold voltageTo understand how these transistors would operate together in a circuit, Figure C-6 shows a depletion-load nMOS inverter and its drain current and output voltage characteristic.

Figure C-6: Depletion-load nMOS Inverter

Due to the negative threshold voltage and the gate being shorted to the source (Vgs1 = 0), Transistor 1 is always on. Beginning with Vi at 0V, Transistor 2 is off. This means the output voltage across the load is at maximum value or Vdd. As Vi is increased, Transistor 2 eventually reaches its threshold voltage and begins to conduct. This draws current away from the load and Vo approaches 0V. The second graph shows the effects of increasing Vdd as well as Vgs2 (Vi). Because the depletion transistor acts as a resistor in the circuit, an increase in Vdd also increases Id. A higher value of Vgs allows more current to flow through Transistor 2 which also raises Id. When this condition is true, these equations can be used:

Id = uCox(W2/L2)(Vo)(Vi Vt2 Vo/2)Vo = (Vi Vt2) [(Vi Vt2) - (W1L2/L1W2)*(-Vt1)]

Now that it is better understood how the components of analog circuits work, I can better explain the amplifier and its operation.

Operational Amplifier:

One of the most popular devices in analog electronics is the operational amplifier (op-amp). They are very versatile units that have many applications. The low cost of manufacturing makes them a good candidate for large scale production, and this also allows them to be used for circuits that contain a system where many are needed. A typical op-amp has three main stages in its function, which include: differential amplifier, voltage amplifier and output amplifier. These stages can be seen in Figure C-7.

Figure C-7: Op-Amp Stages of Operation

The differential amplifier stage receives two input voltages and then outputs the difference in voltages multiplied by a gain factor, which is usually unity. A single output can be generated by combining it with a source follower or current mirror. The voltage gain amplifier uses the output of the first stage and increases the voltage. After this happens, the output amplifier increases the current output with a low impedance and unity voltage gain.Generally an ideal op-amp will have a few theoretical characteristics such as infinite input res