An Overview of Linear Integrated Circuits

Catalog

I Development of Linear Integrated Circuits

In the early 1960s, the first integrated amplifier was made from semiconductor silicon wafers. Since all the devices of integrated circuits are manufactured on a small chip at the same time, their characteristics are consistent, and the component parameters are of high accuracy. However, there are different types of linear devices in one linear circuit, making it difficult for integration, so its initial development is slow.

In 1964, lateral PNP transistors were produced, which played an important role in the development of linear integrated circuits. It uses an annular P-type diffusion region as the collector and another P-type diffusion region as the emitter. This structure can be manufactured at the same time as standard NPN transistors are produced, creating conditions for the implementation of bipolar complementary circuits. 

In 1966, the first high-performance general-purpose operational amplifier came out. It has a small size and can be used flexibly in circuits, which promoted the development of electronics.

In the late 1960s, various linear circuits began to be widely used. Then in the 1970s, high-precision digital-to-analog converters(DAC) and analog-to-digital converters(ADC) became the key devices for the application of digital technology and microprocessors in information processing and process control.

A new development in linear circuits is the use of MOS technology in the manufacture of voice frequency filters. Its principle is to use switches to alternately connect capacitors to different voltage nodes in the circuit to transfer charges, thereby generating equivalent resistance. Besides, due to the application of analog sampling techniques, MOS technology can be used to produce high-stability operational amplifiers and high-precision DACs and ADCs. The combination of these two technologies opens up broad prospects for the large-scale integration of analog information processing and communication equipment subsystems.

II Production Process and High-frequency Technology

1. Production Process

Most linear integrated systems are manufactured through standard bipolar processes. In order to obtain high-performance circuits, some modifications are added to produce components with different performances on the same chip.

● Bipolar Field-effect Compatible Technology

It's the technology for manufacturing high-performance junction field-effect transistors on bipolar chips. After the NPN transistor is formed on the chip, the ion implantation technique is used twice to form a low-concentration P-type channel and a high-concentration N + type gate region. The gate-drain breakdown voltage can reach 50 to 60 volts, and the pinch-off voltage can be controlled at about 1 volt.

Ion Implantation Process

● Supergain Transistor

It is the NPN transistor with the current gain of the common emitter up to 1000-5000. The base region is made through ion implantation. The doping density of the base region is an order of magnitude lower than that of the usual NPN transistor, and the thickness of the base region is also thinner than that of the usual NPN transistor.

● Subsurface Breakdown Diode

The usual breakdown diode uses the eb junction of NPN transistor, and its breakdown phenomenon occurs on the junction surface. While for the subsurface breakdown diode, a high-concentration P + type layer is formed with ion implantation under the N + type emitter region, and an N + -P + junction is formed below the surface.

The breakdown voltage of this transistor is lower than that of the surface junction, and the breakdown process is not affected by the surface condition. Besides, it has low noise and long-term stability.

2. High-Frequency Technology

The characteristic frequency of NPN transistors manufactured with the standard bipolar process is generally lower than 1000 MHz. When high-frequency and high-speed performance are required, we often use micro-machining, thin-layer epitaxy, and shallow junction techniques, which could make characteristic frequency reach 3000-5000 MHz.

● High Voltage Withstand Technology

The voltage resistance value of the linear bipolar process can usually achieve 50 to 60 volts. If we want to obtain a voltage resistance value of nearly 100 volts or higher, we can take the following measures:

① Increase the thickness of the N-type epitaxial layer to increase the breakdown voltage of the NPN transistor;

② When the metal interconnects with negative potential cross the lateral PNP transistor, the thickness of the oxide layer is increased to prevent parasitic MOS transistor effect.

③ Use the field electrode to protect the surface of the isolation junction, preventing the electric field from being too concentrated, or the breakdown voltage will be reduced.

● Linear CMOS Technology

This is a complex and versatile compatible linear technology that can simultaneously produce bipolar devices and CMOS devices. With this linear technologies, high-performance linear systems and high-density, high-speed logic circuits can be combined on one chip.

It uses refractory metal一molybdenum一 as the gate material, and the P-channel and N-channel MOSFET can be made on the N-type epitaxial layer of the linear bipolar chip with photoetching for only 10 times. Besides, it has two layers of interconnects of aluminum and molybdenum, and the P-channel and N-channel devices can be used in one N-region.

CMOS Transistor 

● Precision Components

Passive resistors are formed by diffusion layers or ion-implanted layers. Usually, making alloy film resistors on silicon chips can achieve better temperature stability, but the resistance tolerance of them is no more than 1%. A pulse laser beam with a diameter of about 10 microns is often applied for high-precision resistors.

III Types of Linear Integrated Circuit 

According to the function and use, linear integrated circuits can be divided into:

① General-purpose Circuits

Operational amplifiers, voltage comparators, voltage reference circuits, stabilized voltage supply circuits;

② Industrial Measurement and Control Circuits

Timers, waveform generators, detectors, sensor circuits, phase-locked loop circuits, analog multiplier circuits, motor driver circuits, power control circuits, and analog switch circuit;  

Simplest Phase-Locked Loop

③ Data Conversion Circuits

Digital-to-analog converters, analog-to-digital converters, and voltage-to-frequency converters ;

④ Communication Circuits

Telephone communication circuits and mobile communication circuits;

⑤ Consumer Electronics Circuits

Television circuits, video recorder circuits and audio circuits.

Due to the increasing development of large-scale integration technology and computer-aided design and measurement technology, linear circuits are developing from traditional standard units to custom integrated circuits with complex functions.

IV Basic Forms of Linear Integrated Circuits

Linear integrated circuits have many varieties and different designs. However, some functional unit circuits are used as basic components and widely used in many circuits.

The following are the linearity circuits analysis:

1. Differential Amplifier Circuit

It has a symmetrical structure. Transistor pair Q1 and Q2 with the same characteristics are called the differential pair. Due to the constant current-source biasing, if the base current is negligible, the sum of the collector current C1 and C2 is equal to I0, which is irrelevant to the input voltage U1 and U2. The input voltage only changes the distribution of bias current I0 in Q1 and Q2. 

The relationship between the difference between C1 and C2 and the input differential voltage U1-U2 is: 

which is a tangent function.

When the drive signal is very small 

,

it is a linear amplifier that can be used to identify the small difference between the two input signals and can also be used as an ordinary single-ended input amplifier.

When 

,

It becomes a limiting amplifier, which can be used for the phase comparison of two signals. The limiting effect is not caused by the saturation of the transistor, but because the constant current biasing limits the increase of the collector current, it has good frequency response characteristics.

There are many types of differential amplifiers. For single-ended output, a load resistor or a current mirror can be used to replace the resistor. The biasing constant-current source can also be replaced with a resistor sometimes. In addition, resistors can also be inserted between the emitters of two transistors to change the performance of the amplifier.

2. Analog Multiplier Circuit

It can accept the input of two analog signals and produce an output signal proportional to the product of the two. The figure above shows the Gilbert cell circuit. The core part is two pairs of cross-connected differential transistors composed of Q5-Q8. 

The signal currents Ix and Iy are generated by the voltage-current conversion circuit. If the resistance Rx and Ry are large enough, then

Therefore, U2 is proportional to the product of the input voltage Ux and Uy, which is 

The input voltage of the Gilbert multiplier is allowed to have positive or negative polarity, therefore it’s often called the four-quadrant multiplier. It can be combined with operational amplifiers to complete multiplication and division, square and root operations, and can also be used for phase detection, frequency doubling, and gain control.

3. Energy Gap Voltage Reference Circuit

It's a reference voltage source capable of producing a low voltage output. As shown in the picture, it uses a voltage U_PTAT proportional to the absolute temperature to compensate for the negative temperature coefficient of the forward emitter voltage. 

U_PTAT is produced on the resistor R2 by the transistor Q1 and Q2 with different current densities. The theoretical value of the reference voltage UR is about 1.2V. This value does not change with temperature and is approximately equal to the forbidden bandwidth of the semiconductor silicon, so it’s called the energy gap voltage reference. Due to machining errors in doping concentration and device geometry, the actual temperature coefficient of the reference voltage is usually on the order of 10 ppm/℃.

V Examples of Linear Integrated Circuit 

1. Lm386 Bipolar Linear Integrated Circuit

Lm386 audio power amplifier is mainly used in low-voltage consumer products. In order to minimize the peripheral components, the internal voltage gain is set to be 20. However, if an external resistor and capacitor is connected between pin 1 and pin 8, the voltage gain can be adjusted to any value up to 200.

When the supply voltage is 6V, its static power consumption is only 24mW, which is suitable for battery-powered occasions.

● Features

① Low static power consumption, which is about 4mA, and can be powered by batteries

② Wide working voltage range from 4V-12V

③ Few peripheral components

④ Voltage gain can be adjustable from 20-200

⑤ Low distortion

LM386 Audio Amplifier with Gain = 20/200

● Applications

① AM / FM amplifiers

② Line drivers

③ Portable recorders and audio power amplifiers

④ Ultrasonic actuators

⑤ Speaker system of the hands-free telephones

⑥ Small servo drivers

⑦ TV audio systems

⑧ Source transformations

● Pay attention to

① Output power range

② Matching of the front and backstages

③ Distortion range

2. 34063 Bipolar Linear Integrated Circuit

34063 is a single-chip bipolar linear integrated circuit dedicated to DC converters. The control part of the DC converter contains a bandgap reference source for temperature compensation.

34063 can output a switching current of 1.5A, and use the minimum external components to form a switching-type boost converter, buck converter or power inverter.

● Features

① Short circuit current limits low quiescent current.

② Output switching current can reach 1.5A with no external transistor, and the output voltage is adjustable.

③ Can be formed as a boost/bulk converter or power inverter when the working oscillation frequency changes from 100HZ to 100KHZ 

Conclusion

In this passage, we first learned about the development process of linear integrated circuit. And then we have a basic understanding of the production process and the high-frequency technology. The following two parts help us command types and forms of the linear integrated system. And in the final chapter, we analyze the two bipolar linear integrated circuit一 Lm386 and 34063. Hope this article be helpful to you!

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