FPGA Based Digital World - Analog Electronic Device

Analog Electronic Device

Active and Passive Electric Component

Active and passive components are two types of electric components in the circuit.

An active electric component is the electric device that supplies energy to a circuit. An active component is powered by source of electricity, and is capable of

- producing a power gain;

- amplifying the power of the signal;

- controlling the flow of electricity.

Active components are energy donors, and requires external source of power for their operations.

Examples of active components are

- Voltage source;

- Current source;

- Transistors(BJT, FETs, etc.): capable of amplifying the power of the signal

- Diodes.

A passive electric component is the electric device that receives energy, which it can either dissipate, abosorb or store.

Passive components are energy acceptors, and do not require external source for operation. Examples of passive components are

- Resistor: dissipate energy as heat;

- Inductor: store energy in magnetic field;

- Capacitor: store energy in electric field;

- Transformer: although it's able to raise the voltage or current of a circuit, yet the power is unchanged.

The active components will be discussed in the website, while the passive components will not.

Another concept is the reactive component which is defined as the component of the voltage across the circuit that is in quadrature with the current and that produces no power in an alternating current circuit(called also wattless component).

Semiconductor: From PN Juntion to Diode and Transistors

By dopping process, the semiconductors could form two type of extrinsic materials: p-type and n-type materials. In an n-type material, the electron is the majority carrier and the hole is the minority carries as there're relatively more 'free' electrons, while in a p-type material, it's vice versa.

While joining an n-type and a p-type material together, a PN junction is created, acting as the basic structure of a semiconductor diode. A diode behaves similar to a one-way mechanical switch that only allows the current to flow forward.

When combining two PN junctions back to back, a NPN or PNP bipolar junction transistor(BJT) is formed.

In a NPN BJT, the current from the collector to the emitter(IE, IC) could be controlled by the current from the base to the emitter(IB). With this property, different configurations are avaible to amplify the current or voltage, forming basic amplifying circuits.

Another method to form a transistor with two PN junctions is to connect the n-type channel to a top and a bottom terminals by ohmic contact, forming a JFET.

The current between the drain and source could be controlled by the voltage between the gate the source. Based on this, amplifying circuits built with JFETs are available in different configurations.

Similar FETs are MOSFET and MESFET.

For high frequency applications, HBT(Heterojunction Bipolar Transistor) and HEMT( High Electron Mobility Transistor) are available.

Analog Amplification: From BJT&FET to Operational Amplifier, then to FPAA

An analog signal can be viewed as a signal having continuous values within a specified range, and an analog voltage amplifier(referred as voltage amplifier below) is the most common amplifier, with both its input and output being voltages.

A voltage amplifier can be modeled as below[1].

The above amplifier can be built with different components:

- Discrete component, e.g., BJT(Bipolar Junction Transistor) or FET(Field-EffectTransistor);

- Monolithic operational amplifier;

- Monolithic Field Programmable Analog Array (FPAA).

A simple way to build a voltage amplifier with BJT is to employ the common-emitter configuration, as below.

Similiarly, a FET is a three-terminal device with one terminal capable of controlling the current betweenthe other two. The primary difference between a BJT and FET is that a BJT transistor is a current-controlled device while a FET transistor is a voltage-controlled device. Three types of FETS are available: JFET(junction field-effect transistor), MOSFET(metal-oxide-semicondutor field-effect trasistor) and MESFET(metal-semiconductor field-effect transistor).

A similiar N-channel JFET amplifier with voltage-divider configuration is shown below.

To improve the gain and performance of an amplifier, an monolithic operational amplifier was introduced which can be configured to different amplifiers. A basic operational amplifier has two inputs(differential) and one output.A simplified structure of an operational amplifier is shown below.

And a voltage amplifier built with operational amplifer is below.

To be more flexible, monolithic Field Programmable Analog Arrays (FPAA)was introduced to build ANALOG reconfigurable circuits.As analog signals can be divided into discrete time and continuous time signals, the FPAA can be divided into continuous time, discrete time and mixed analog reconfigurable circuits.The continuous time class of reconfigurable circuits contains devices based on Operational Transconductance Amplifiers (OTA) , current conveyors and also a wide class of so called Externally Linear(ELIN) circuits – e.g. log domain, square root domain and others. The discrete time circuits are mainly based on the Switched Capacitance (SC) principle or Switched Current (SI) principle[1][3].

The base of SC is the relation of a capacitor's impdedance to the frequency of the switched current, or the 'capacitor eqation'(I=CdV/dt or dQ = CdV).To simulate different impdedance values of a resistor or inductance, changing the switching frequency is one of the method[1].

Configurable SC-based filters include LTC1060(AD company), MF10(TI company)[1],etc.

A typical FPAA is the discrete time FPAA of 'dpASP' from anadigm company,with its structure shown below. The corresponding SW can be used to config the circuit to different analog ciruits,e.g., filters, voltage amplifers.

FPAA has been applied in many fields, not only amplifiers.

Audio Applications: AGC, tremble effect, infinite limiter, etc.[3]

Biomedical Signal Processing: QRS dector, etc. [3]

References

1 Sergio Franco, Design with Operational Amplifiers and Analog Integral Circuits, fourth edition

2 Robert L.Boylestad and Louis Nashelsky, Electronic Devices and Circuit Theory, 11th Edition

3.Andrzej Malcher and Piotr Falkowski, Analog Reconfigurable Circuits, INTL JOURNAL OF ELECTRONICS AND TELECOMMUNICATIONS, 2014, VOL. 60, NO. 1


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