Thursday 20 April 2017

Analog Electronics

Theory
Operational Amplifiers
Operational Amplifiers (OAs) are highly stable, high gain dc difference amplifiers. Since there is no capacitive coupling between their various amplifying stages, they can handle signals from zero frequency (dc signals) up to a few hundred kHz. Their name is derived by the fact that they are used for performing mathematical operations on their input signal(s).
 Figure 1 shows the symbol for an OA. There are two inputs, the inverting input (-) and the non-inverting input (+). These symbols have nothing to do with the polarity of the applied input signals.
 Image result for operational amplifier
 Figure 1. Symbol of the operational amplifier. Connections to power supplies are also shown.

The output signal (voltage), vo, is given by
vo = A(v+ - v-)
v+ and v- are the signals applied to the non-inverting and to the inverting input, respectively. Α represents the open loop gain of the OAA is infinite for the ideal amplifier, whereas for the various types of real OAs, it is usually within the range of 104 to 106.
OAs require two power supplies to operate, supplying a positive voltage (+V) and a negative voltage (-V) with respect to circuit common. This bipolar power supply allows OAs to generate output signals (results) of either polarity. The output signal (vo) range is not unlimited. The voltages of the power supplies determine its actual range. Thus, a typical OA fed with -15 and +15 V, may yield a vo within the (approximately) -13 to +13 V range, called operational range. Any result expected to be outside this range is clipped to the respective limit, and OA is in a saturation stage.
The connections to the power supplies and to the circuit common symbols, shown in Figure 1, hereafter will be implied, and they will be not shown in the rest of the circuits for simplicity.
Because of their very high open loop gain, OAs are almost exclusively used with some additional circuitry (mostly with resistors and capacitors), required to ensure a negative feedback loop. Through this loop a tiny fraction of the output signal is fed back to the inverting input. The negative feedback stabilizes the output within the operational range and provides a much smaller but precisely controlled gain, the so-called closed loop gain.
Circuits of OAs have been used in the past as analog computers, and they are still in use for mathematical operations and modification of the input signals in real time. A large variety of OAs is commercially available in the form of low cost integrated circuits.
There is a plethora of circuits with OAs performing various mathematical operations. Each circuit is characterized by its own transfer function, i.e. the mathematical equation describing the output signal (vo) as a function of the input signal (vi) or signals (v1, v2, …, vn). Generally, transfer functions can be derived by applying Kirchhoff’s rules and the following two simplifying assumptions:
#1. The output signal (vo) acquires a value that (through the feedback circuits) practically equates the voltages applied to both inputs, i.e. v+  v-.
#2. The input resistance of both OA inputs is extremely high(usually within the range 106-1012 MΩfor the ideal OA this is infinite), thus no current flows into them.

Inverting Amplifier
The basic circuit of the inverting amplifier is shown in Figure 2.
 Image result for inverting amplifier

Figure 2. Inverting amplifier.

The transfer function is derived as follows: Considering the arbitrary current directions we have:
 i1 = (vi - vs)/Ri   and   i2 = (vs - vo)/Rf
 The non-inverting input is connected directly to the circuit common (i.e. v+ = 0 V), therefore (considering simplifying assumption #1) vs = v- = 0 V, therefore:
i1 = vi/Ri   and   i2 = - vo/Rf
 Since there is no current flow to any input (simplifying assumption #2), it is
 i1 = i2
Therefore, the transfer function of the inverting amplifier is
vo = -(Rf/Ri)vi
 Thus, the closed loop gain of the inverting amplifier is equal to the ratio of Rf (feedback resistor) over Ri (input resistor). This transfer function describes accurately the output signal as long as the closed loop gain is much smaller than the open loop gain A of the OA used (e.g. it must not exceed 1000), and the expected values of voare within the operational range of the OA.

 

Summing Amplifier

The summing amplifier  is a logical extension of the previously described circuit, with two or more inputs. Its circuit is shown in Figure 3.
 Image result for summing amplifier

Figure 3.  Summing amplifier.

The transfer function of the summing amplifier (similarly derived) is:
 vo = -(v1/R1  +  v2/R2  +  …  +  vn/Rn)Rf
Thus if all input resistors are equal, the output is a scaled sum of all inputs, whereas, if they are different, the output is a weighted linear sum of all inputs.
The summing amplifier is used for combining several signals. The most common use of a summing amplifier with two inputs is the amplification of a signal combined with a subtraction of a constant amount from it (dc offset).  

 

Difference amplifier

Difference amplifier precisely amplifies the difference of two input signals. Its typical circuit is shown in Figure 4.

 Image result for difference amplifier

 

Figure 4. Difference amplifier.

 

If Ri = Ri΄ and Rf = Rf΄, then the transfer function of the difference amplifier is:

vo = (v2 - v1) Rf/Ri

The difference amplifier is useful for handling signals referring not to the circuit common, but to other signals, known as floating signal sources. Its capability to reject a common signal makes it particularly valuable for amplifying small voltage differences contaminated with the same amount of noise (common signal).
In order for the difference amplifier to be able to reject a large common signal and to generate at the same time an output precisely proportional to the two signals difference, the two ratios p = Rf/Ri and q = Rf΄/Ri΄ must be precisely equal, otherwise the signal output will be:
vo = [q(p+1)/(q+1)]v2 - pv1

Differentiator

The differentiator generates an output signal proportional to the first derivative of the input with respect to time. Its typical circuit is shown in Figure 5.
 Image result for Differentiator circuit

Figure 5. Differentiator.

 

The transfer function of this circuit is
vo = -RC(dvi/dt)
 Obviously, a constant input (regardless of its magnitude) generates a zero output signal. A typical usage of the differentiator in the field of chemical instrumentation is obtaining the first derivative of a potentiometric titration curve for the easier location of the titration final points (points of maximum slope).

Integrator

The integrator generates an output signal proportional to the time integral of the input signal. Its typical circuit is shown in Figure 6.
Image result for integrator circuit

Figure 6. Integrator

 vo = -(1/RC)∫vi(t)dt
The output remains zero as far as switch S remains closed. The integration starts (t = 0) when S opens. The output is proportional to the charge accumulated in capacitor C, which serves as the integrating device. A typical application of the (analog) integrator in chemical instrumentation is the integration of chromatographic peaks, since its output will be proportional to the peak area.  
If the input signal is stable then the output from the integrator will be given by the equation
vo = -(vi/RC) t
i.e. the output signal will be a voltage ramp. Voltage ramps are commonly used for generating the linear potential sweep required in polarography and many other voltammetric techniques.

Memory Devices

A memory is just like a human brain. It is used to store data and instruction. Computer memory is the storage space in computer where data is to be processed and instructions required for processing are stored.
The memory is divided into large number of small parts. Each part is called a cell. Each location or cell has a unique address which varies from zero to memory size minus one.
For example if computer has 64k words, then this memory unit has 64 * 1024 = 65536 memory location. The address of these locations varies from 0 to 65535.
Memory is primarily of two types
  • Internal Memory − cache memory and primary/main memory
  • External Memory − magnetic disk / optical disk etc.
Memory Hiearchy
Characteristics of Memory Hierarchy are following when we go from top to bottom.
  • Capacity in terms of storage increases.
  • Cost per bit of storage decreases.
  • Frequency of access of the memory by the CPU decreases.
  • Access time by the CPU increases.

RAM

A RAM constitutes the internal memory of the CPU for storing data, program and program result. It is read/write memory. It is called random access memory (RAM).
Since access time in RAM is independent of the address to the word that is, each storage location inside the memory is as easy to reach as other location & takes the same amount of time. We can reach into the memory at random & extremely fast but can also be quite expensive.
RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence, a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.
RAM is of two types
  • Static RAM (SRAM)
  • Dynamic RAM (DRAM)

Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power remains applied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6-transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis.
Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher.
Static RAM is used as cache memory needs to be very fast and small.

Dynamic RAM (DRAM)

DRAM, unlike SRAM, must be continually refreshed in order for it to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small. All DRAMs are made up of memory cells. These cells are composed of one capacitor and one transistor.

ROM

ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture.
A ROM, stores such instruction as are required to start computer when electricity is first turned on, this operation is referred to as bootstrap. ROM chip are not only used in the computer but also in other electronic items like washing machine and microwave oven.
Following are the various types of ROM −

MROM (Masked ROM)

The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kind of ROMs are known as masked ROMs. It is inexpensive ROM.

PROM (Programmable Read Only Memory)

PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM programmer. Inside the PROM chip there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

EPROM (Erasable and Programmable Read Only Memory)

The EPROM can be erased by exposing it to ultra-violet light for a duration of upto 40 minutes. Usually, an EPROM eraser achieves this function. During programming an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use the quartz lid is sealed with a sticker.

EEPROM (Electrically Erasable and Programmable Read Only Memory)

The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.

Serial Access Memory

Sequential access means the system must search the storage device from the beginning of the memory address until it finds the required piece of data. Memory device which supports such access is called a Sequential Access Memory or Serial Access Memory. Magnetic tape is an example of serial access memory.

Direct Access Memory

Direct access memory or Random Access Memory, refers to conditions in which a system can go directly to the information that the user wants. Memory device which supports such access is called a Direct Access Memory. Magnetic disks, optical disks are examples of direct access memory.

Cache Memory

Cache memory is a very high speed semiconductor memory which can speed up CPU. It acts as a buffer between the CPU and main memory. It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs, are transferred from disk to cache memory by operating system, from where CPU can access them.

Advantages

  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.
  • It stores the program that can be executed within a short period of time.
  • It stores data for temporary use.

Disadvantages

  • Cache memory has limited capacity.
  • It is very expensive.
Virtual memory is a technique that allows the execution of processes which are not completely available in memory. The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory is the separation of user logical memory from physical memory.
This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available. Following are the situations, when entire program is not required to be loaded fully in main memory.
  • User written error handling routines are used only when an error occurred in the data or computation.
  • Certain options and features of a program may be used rarely.
  • Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used.
  • The ability to execute a program that is only partially in memory would counter many benefits.
  • Less number of I/O would be needed to load or swap each user program into memory.
  • A program would no longer be constrained by the amount of physical memory that is available.
  • Each user program could take less physical memory, more programs could be run the same time, with a corresponding increase in CPU utilization and throughput.

Auxiliary Memory

Auxiliary memory is much larger in size than main memory but is slower. It normally stores system programs, instruction and data files. It is also known as secondary memory. It can also be used as an overflow/virtual memory in case the main memory capacity has been exceeded. Secondary memories cannot be accessed directly by a processor. First the data/information of auxiliary memory is transferred to the main memory and then that information can be accessed by the CPU. Characteristics of Auxiliary Memory are following −
  • Non-volatile memory − Data is not lost when power is cut off.
  • Reusable − The data stays in the secondary storage on permanent basis until it is not overwritten or deleted by the user.
  • Reliable − Data in secondary storage is safe because of high physical stability of secondary storage device.
  • Convenience − With the help of a computer software, authorised people can locate and access the data quickly.
  • Capacity − Secondary storage can store large volumes of data in sets of multiple disks.
  • Cost − It is much lesser expensive to store data on a tape or disk than primary memory.

CPU Architecture

Microprocessing unit is synonymous to central processing unit, CPU used in traditional computer. Microprocessor (MPU) acts as a device or a group of devices which do the following tasks.
  • communicate with peripherals devices
  • provide timing signal
  • direct data flow
  • perform computer tasks as specified by the instructions in memory

8085 Microprocessor

The 8085 microprocessor is an 8-bit general purpose microprocessor which is capable to address 64k of memory. This processor has forty pins, requires +5 V single power supply and a 3-MHz single-phase clock.

Block Diagram

8080 Mircroprocessor block diagram

ALU

The ALU perform the computing function of microprocessor. It includes the accumulator, temporary register, arithmetic & logic circuit & and five flags. Result is stored in accumulator & flags.

Block Diagram

ALU

Accumulator

It is an 8-bit register that is part of ALU. This register is used to store 8-bit data & in performing arithmetic & logic operation. The result of operation is stored in accumulator.

Diagram

Accumulator

Flags

Flags are programmable. They can be used to store and transfer the data from the registers by using instruction. The ALU includes five flip-flops that are set and reset according to data condition in accumulator and other registers.
  • S (Sign) flag − After the execution of an arithmetic operation, if bit D7of the result is 1, the sign flag is set. It is used to signed number. In a given byte, if D7 is 1 means negative number. If it is zero means it is a positive number.
  • Z (Zero) flag − The zero flag is set if ALU operation result is 0.
  • AC (Auxiliary Carry) flag − In arithmetic operation, when carry is generated by digit D3 and passed on to digit D4, the AC flag is set. This flag is used only internally BCD operation.
  • P (Parity) flag − After arithmetic or logic operation, if result has even number of 1s, the flag is set. If it has odd number of 1s, flag is reset.
  • C (Carry) flag − If arithmetic operation result is in a carry, the carry flag is set, otherwise it is reset.

Register section

It is basically a storage device and transfers data from registers by using instructions.
  • Stack Pointer (SP) − The stack pointer is also a 16-bit register which is used as a memory pointer. It points to a memory location in Read/Write memory known as stack. In between execution of program, sometime data to be stored in stack. The beginning of the stack is defined by loading a 16-bit address in the stack pointer.
  • Program Counter (PC) − This 16-bit register deals with fourth operation to sequence the execution of instruction. This register is also a memory pointer. Memory location have 16-bit address. It is used to store the execution address. The function of the program counter is to point to memory address from which next byte is to be fetched.
  • Storage registers − These registers store 8-bit data during a program execution. These registers are identified as B, C, D, E, H, L. They can be combined as register pair BC, DE and HL to perform some 16 bit operations.

Time and Control Section

This unit is responsible to synchronize Microprocessor operation as per the clock pulse and to generate the control signals which are necessary for smooth communication between Microprocessor and peripherals devices. The RD bar and WR bar signals are synchronous pulses which indicates whether data is available on the data bus or not. The control unit is responsible to control the flow of data between microprocessor, memory and peripheral devices.

PIN diagram

PIN diagram
All the signal can be classified into six groups
S.N.GroupDescription
1Address bus
The 8085 microprocessor has 8 signal line, A15 - A8 which are uni directional and used as a high order address bus.
2Data bus
The signal line AD7 - AD0 are bi-directional for dual purpose. They are used as low order address bus as well as data bus.
3Control signal and Status signal
Control Signal
RD bar − It is a read control signal (active low). If it is active then memory read the data.
WR bar − It is write control signal (active low). It is active when written into selected memory.
Status signal
ALU (Address Latch Enable) − When ALU is high. 8085 microprocessor use address bus. When ALU is low. 8085 microprocessor is use data bus.
IO/M bar − This is a status signal used to differentiate between i/o and memory operations. When it is high, it indicate an i/o operation and when it is low, it indicate memory operation.
S1 and S0 − These status signals, similar to i/o and memory bar, can identify various operations, but they are rarely used in small system.
4Power supply and frequency signal
Vcc − +5v power supply.
Vss − ground reference.
X, X − A crystal is connected at these two pins. The frequency is internally divided by two operate system at 3-MHz, the crystal should have a frequency of 6-MHz.
CLK out − This signal can be used as the system clock for other devices.
5Externally initiated signal
INTR (i/p) − Interrupt request.
INTA bar (o/p) − It is used as acknowledge interrupt.
TRAP (i/p) − This is non maskable interrupt and has highest priority.
HOLD (i/p) − It is used to hold the executing program.
HLDA (o/p) − Hold acknowledge.
READY (i/p) − This signal is used to delay the microprocessor read or write cycle until a slow responding peripheral is ready to accept or send data.
RESET IN bar − When the signal on this pin goes low, the program counter is set to zero, the bus are tri-stated, & MPU is reset.
RESET OUT − This signal indicate that MPU is being reset. The signal can be used to reset other devices.
RST 7.5, RST 6.5, RST 5.5 (Request interrupt) − It is used to transfer the program control to specific memory location. They have higher priority than INTR interrupt.
6Serial I/O ports
The 8085 microprocessor has two signals to implement the serial transmission serial input data and serial output data.

Instruction Format

Each instruction is represented by a sequence of bits within the computer. The instruction is divided into group of bits called field. The way instruction is expressed is known as instruction format. It is usually represented in the form of rectangular box. The instruction format may be of the following types.

Variable Instruction Formats

These are the instruction formats in which the instruction length varies on the basis of opcode & address specifiers. For Example, VAX instruction vary between 1 and 53 bytes while X86 instruction vary between 1 and 17 bytes.

Format

Variable Instruction Format

Advantage

These formats have good code density.

Drawback

These instruction formats are very difficult to decode and pipeline.

Fixed Instruction Formats

In this type of instruction format, all instructions are of same size. For Example, MIPS, Power PC, Alpha, ARM.

Format

Fixed Instruction Format

Advantage

They are easy to decode & pipeline.

Drawback

They don't have good code density.

Hybrid Instruction Formats

In this type of instruction formats, we have multiple format length specified by opcode. For example, IBM 360/70, MIPS 16, Thumb.

Format

Hybrid Instruction Format

Advantage

These compromise between code density & instruction of these type are very easy to decode.

Addressing Modes

Addressing mode provides different ways for accessing an address to given data to a processor. Operated data is stored in the memory location, each instruction required certain data on which it has to operate. There are various techniques to specify address of data. These techniques are called Addressing Modes.
  • Direct addressing mode − In the direct addressing mode, address of the operand is given in the instruction and data is available in the memory location which is provided in instruction. We will move this data in desired location.
  • Indirect addressing mode − In the indirect addressing mode, the instruction specifies a register which contain the address of the operand. Both internal RAM and external RAM can be accessed via indirect addressing mode.
  • Immediate addressing mode − In the immediate addressing mode, direct data is given in the operand which move the data in accumulator. It is very fast.
  • Relative addressing mode − In the relative address mode, the effective address is determined by the index mode by using the program counter in stead of general purpose processor register. This mode is called relative address mode.
  • Index addressing mode − In the index address mode, the effective address of the operand is generated by adding a content value to the contents of the register. This mode is called index address mode.