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OPERATIONAL AMPLIFIERS
OPERATIONAL AMPLIFIERS (OP-AMP)
The standard operational amplifier (op-amp) symbol is shown in Figure 12-1(a). It has two terminals, the inverting input
(-) and the non inverting input (+), and one output terminal. The typical op-amp operates with two dc supply voltages, one positive and the other negative, as shown in Figure 12-1(b). Usually these dc voltage terminals are left off the schematic symbol for simplicity but are always understood to be there.
The Ideal Op-Amp
The ideal op-amp has infinite voltage gain and infinite bandwidth. Also, it has an infinite impedance (open), so that it does not load the driving source. Finally, it has a zero output impedance. These characteristics are illustrated in Figure 12-2. The input voltage Vin appears between the two input terminals, and the output voltage is AvVin, as indicated by the internal voltage source symbol. The concept of infinite input impedance is a particularly valuable analysis tool for the various op-amp configurations.
The Practical Op-Amp
Limitations of an IC Op-Amp:
1) Op-amps have both voltage and current limitations.
2) Peak-to-peak output voltage, is usually limited to slightly less than the two supply voltages.
3) Output current is also limited by internal restrictions such as power dissipation and component ratings.
Characteristics of a Practical Op-Amp:
1) Very high voltage gain.
2) Very high input impedance.
3) Very low impedance.
4) Wide bandwidth.
The Differential Amplifier
A basic differential amplifier circuit and its symbol are shown in Figure 12-4. The diff-amp stages that make up part of the op-amp provide high voltage gain and common-mode rejection. Notice that the differential amplifier has two outputs where an op-amp has only one output.
Modes of Signal Operation
Single-Ended Input - When a diff-amp is operated in this mode, one input is grounded and the signal voltage is applied only to the other input, as shown in Figure 12-6. In the case where the signal voltage is applied to input 1 as in part (a), an inverted, amplified signal voltage appears at output 1 as shown. Also, a signal voltage appears in phase at the emitter of Q1. Since the emitters of Q1 and Q2 are common, the emitter signal becomes an input to Q2, which functions as a common-base amplifier. The signal is amplified by Q2 and appears, non inverted, at output 2. This action is illustrated in part (a).
In the case where the signal is applied to input 2 with input 1 grounded, as in Figure 12-6(b), an inverted, amplified signal voltage appears at output 2. in this situation, Q1 acts as a common-base amplifier, and a non inverted, amplified signal appears at output 2. This action is illustrated in part (b) of the figure.
Modes of Signal Operation
Differential Input – In this mode, two opposite-polarity (out-of-phase) signals are applied to the inputs, as shown in Figure 12-7(a). This type of operation is also referred to as double-ended. Each input affects the output.
Figure 12-7(b) shows the output signals due to the signal on input 1 acting alone as a single-ended input. Figure 12-7© shows the output signals due to the signal on input 2 acting alone as a single-ended input. Notice in parts (b) and © that the signals on output 1 are of the same polarity. The same is also true for output 2. By superimposing both output 1 signals and both output 2 signals, we get the total differential operation, as pictured in Figure 12-7(d).
Common-Mode Input – In this mode, two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs, as shown in Figure 12-8(a).
Figure 12-8(b) shows the output signals due to the signal on only input 1, and Figure 12-8© shows the output signals due to the signal on only input 2. Notice that the corresponding signals on output 1 are of the opposite polarity, and so are the ones on output 2. When the input signals are applied to both inputs, the outputs are superimposed and they cancel, resulting in a zero output voltage, as shown in Figure 12-8(d).
Common – Mode Input
Common-Mode Rejection Ratio (CMRR)
A Simple Op-Amp Arrangement
A Simple Op-Amp Arrangement
Input Offset Voltage
Input Bias Current
Input Impedance
Input Offset Current
Output Impedance
pnpn
DEVICES
From Q1;
IB1 = IE1 – IC1
= IE2 – (IC1max + IC1min)

Where: a1 = IC1max / IE1

  Therefore;
IB1 = IE1 – (a1IE1 + ICBO1)
 
IB1 = IE1 – a1IE1 – ICBO1
 
IB1 = (1 – a1)IE1 – ICBO1 (1)
But:
 
IB1 = IC2 and IC2 = IC2max + IC2min
 
IC2 = a2IE2 + ICBO2 (2)
Therefore:
 
a2IE2 + ICBO2 = (1 – a1) IE1 – ICBO1
But:
  IE2 = IE1 = IA (with respect to anode current)
Therefore; 
a2IA + ICBO2 = (1 – a1) IA – ICBO1
ICBO2 + ICBO1 = (1 – a1) IA – a2IA
  ICBO2 + ICBO1 = [ (1 – a1) – a2 ] IA
IA = (ICBO2 + ICBO1) / [ (1 – a1) – a2 ]
Forward Break over Voltage V(BR)F* - is that voltage above which the SCR enters the conduction region. The asterisk (*) is a letter to be added that is dependent on the condition of the gate terminal as follows:
O – open circuit from G to K
S – short circuit from G to K
R – resistor from G to K
V – fixed bias (voltage) from G to K
Holding Current (IH) – is that value of current below which the SCR switches from the conduction state to the forward blocking region under stated conditions.
Forward and Reverse Blocking Regions – are the regions corresponding to the open circuit for the controlled rectifier which block the flow of charge (current) from anode to cathode.
Reverse Breakdown Voltage – is equivalent to the zener or avalanche region of the fundamental two – layer semiconductor diode.
SCR APPLICATIONS
Series Static Switch
A half-wave series static switch is shown in
Fig. 17.11a. If the switch is closed as shown in
Fig. 17.11b, a gate current will flow during the positive portion of the input signal, turning the SCR on. Resistor R1 limits the magnitude of the gate current. When the SCR turns on, the anode-to cathode voltage (VF) will drop to the conduction value, resulting in a great reduced gate current and very little loss in the gate circuitry. For the negative region of the input signal, the SCR will turn off since the anode is negative with respect to the cathode. The diode D1 is included to prevent a reversal in gate current.
The waveform for the resulting load current and voltage are shown in Fig. 17.11b. The result is a half-wave-rectified signal through the load. If less than 180º conduction is desired, the switch can be closed at any phase displacement during the positive portion of the input signal.
2) Variable – Resistance Phase Control
A circuit capable of establishing a conduction angle between 90º and 180º is shown in Fig.17.12a. The combination of the resistors R and R1 will limit the gate current during the positive portion of the input signal. If R1 is set to its maximum value, the gate current may never reach turn-on magnitude. As R1 is decreased from the maximum, the gate current will increase from the same input voltage. In this way, the required turn-on gate current can be established in any point between 0º and 90º as shown in Fig. 17.12b.
As shown in Fig.17.12b, the control cannot be extended past a 90º phase displacement since the input is at its maximum at this point. If it fails to fire at this and lesser values of input voltage on the positive slope of the input, the same response must be expected from the negatively sloped portion of the signal waveform.
3) Battery – Charging Regulator
The fundamental components of the circuit are shown in Fig. 17.13.
As indicated in the figure, D1 and D2 establish a full-wave-rectified signal across SCR1 and the 12-V battery to be charged. When the full-wave-rectified input is sufficiently large to produce the required turn-on gate current (controlled by R1), SCR1 will turn on and charging of the battery will commence.
4) Temperature Controller
The schematic diagram of a 100-W heater control using an SCR appears in Fig. 17.14. It is designed such that the 100-W heater will turn on and off as determined by thermostat. In this application, the SCR serves as a current amplifier in a load-switching element. It is not an amplifier in the sense that it magnifies the current level of the thermostat. Rather, it is a device whose higher current level is controlled by the behavior of the thermostat.
5) Emergency-Lighting System
The last application for the SCR to be described is shown in Fig. 17.15. It is a single-source emergency-lighting system that will maintain the charge on a 6-V battery to ensure its availability and also provide dc energy to a bulb if there is a power shortage. A full-wave-rectified signal will appear across the 6-V lamp due to diode D2 and D1. The capacitor C1 will charge to a voltage slightly less than a difference between the peak value of the full-wave-rectified signal and the dc voltage across R2 established by the 6-V battery. In any event, the cathode of SCR1 is higher than the anode, and the gate-to-cathode voltage is negative, ensuring that the SCR is non conducting. The battery is charged through R1 and D1 at a rate determined by R1. Charging will only take place when the anode of D1 is more positive than its cathode. The dc level of the full-wave-rectified signal will ensure that the bulb is lit when the power is on.
If the power should fail, the capacitor C1 will discharge D1,R1, and R3 until the cathode of SCR1 is less positive than the anode. At the same time, the junction of R2 and R3 will become positive and establish sufficient gate-to-cathode voltage to trigger the SCR. Once fired, the 6-V battery discharges through the SCR1 and energizes the lamp and maintains its illumination. Once power is restored, the capacitor C1 recharges and reestablishes the non conducting state of SCR1 as described above.
APPLICATION OF SCS
Voltage Sensor
Temperature-, light-, or radiation-sensitive resistors whose resistance increases due to the application of any of the three energy sources described above can be accommodated by simply interchanging the location of RS and the variable resistor. The terminal identification of an SCS is shown in Fig. 17.18 with a packaged SCS.
One simple application for an SCS as a voltage-sensing device is shown in Fig. 17.19. It is an alarm system with n inputs from various stations. Any single input will turn that particular SCS on, resulting in an energized alarm relay and light in the anode gate circuit to indicate the location of the input (disturbance).
2) Alarm Circuit
Fig. 17.20 shows application of the SCS in the alarm circuit. The cathode gate potential is determined by the divider relationship established by RS and the variable resistor. However, if RS decreases, the potential of the junction will increase until the SCS is forward-biased, causing the SCS to turn on and energize the alarm relay.
The 100kΩ resistor is included to reduce the possibility of an accidental triggering of the device through a phenomenon known as the rate effect
GATE TURN-OFF SWITCH
The gate turn-off switch is a pnpn device. It has three external terminals, as indicated in Fig. 17.21a. Its graphical symbol is shown in Fig. 17.21b.
CHARACTERISTICS:
1) It can be turned on or off by applying the proper pulse to the cathode gate.
2) Improved switching characteristics. The fact that the turn-on time rather than considerably larger permits the use of this device in high-speed applications.
LIGHT-ACTIVATED SCR
SCR (LASCR) is a pnpn device whose state is controlled by the light falling on a silicon semiconductor layer of the device. As indicated in
Fig. 17.24a, a gate lead is also provided to permit triggering the device using typical SCR methods. The graphical symbols most commonly employed for LASCR are provided in Fig. 17.24b.
The terminal identification and a typical LASCR appear n Fig. 17.25a.
Some of the areas of application for the LASCR include optical light controls, relays, phase control, motor control, and a variety of computer applications.
The characteristics (light triggering) of a representative LASCR are provided in Fig. 17.25b. Note in this figure that an increase in junction temperature results in a reduction in light energy required to activate the device.
APPLICATIONS OF LASCR
AND / OR Circuits
One interesting application of an LASCR is in the AND and OR circuits of Fig. 17.26. Only when light falls on LASCR1 and LASCR2 will the short-circuit representation for each be applicable and the supply voltage appear across the load. For the OR circuit, light energy applied to LASCR1 or LASCR2 will result in the supply voltage appearing across the load. The LASCR is most sensitive to light when the gate terminal is open.
2) Latching Relay
Another application of LASCR appears in Fig. 17.27. It is the semiconductor analog of an electromechanical relay. Note that it offers complete isolation between the input and the switching element. The energizing current can be passed through a light emitting diode or a lamp, as shown in the figure. The incident light will cause the LASCR to turn on and permit a flow of charge (current) through the load as established by the dc supply. The LASCR can be turned off using the reset switch S1. This system offers the additional advantages over an electromechanical switch of long life, microsecond response, small size, and the elimination of contact bounce.
SHOCKLEY DIODE
DIAC
Proximity Detector
The use of the diac in a proximity detector is shown in Fig. 17.31. Note the use of an SCR in series with the load and the programmable unijunction transistor connected to the sensing electrode.

As a human body approaches the sensing electrode, the capacitance between the electrode and the ground increases. The programmable UJT (PUT) is a device that will fire (enter the short-circuit state) when the anode voltage (VA) is at least 0.7V (for silicon) greater than the gate voltage (VG). Before the programmable device turns on, the system is essentially as shown in Fig. 17.32.
TRIAC
APPLICATION OF TRIAC
Phase (Power) Control
One fundamental application of the triac is presented in Fig. 17.34. In this capacity, it is controlling the ac power to the load by switching on and off during the positive and negative regions of the input sinusoidal signal. The advantage of this configuration is that during the negative portion of the input signal, the same type of response will result since both the diac and the triac can fire in the reverse direction. The resulting waveform for the current through the load is provided in Fig. 17.34. By varying the resistor R, one can control the conduction angle.
OTHER
DEVICES
UNIJUNCTION TRANSISTOR
PHOTOTRANSISTORS
The current induced by photoelectric effects is the base current of the transistor. If we assign the notation Iλ for the photo induced base current, the resulting collector current, on an approximate basis, is
IC ≈ hfe Iλ
A representative set of characteristics for a phototransistor is provided in Fig. 17.50 along with the symbolic representation of the device. A curve of base current versus flux density is provided in Fig. 17.51a. Note the exponential increase in base current with increasing flux density. In the same figure, a sketch of the phototransistor is provided with the terminal identification and the angular alignment.
Some of the areas of application for the phototransistor include punch-card readers, computer logic circuitry, lighting control, level indication, relays, and counting systems.
High-Isolation AND Gate
A high-isolation AND gate is shown in Fig. 17.52 using three phototransistors and three LEDs (light emitting diode). The LEDs are semiconductor devices that emit light at an intensity determined by the forward current through the device. The terminology high isolation simply refers to the lack of an electrical connection between the input and output circuits.
OPTO-ISOLATORS
The opto-isolator is simply a package that contains both an infrared LED and a photo detector such as a silicon diode, transistor Darlington pair, or SCR. The wavelength response of each device is tailored to be as identical as possible to permit the highest measure of coupling possible. In Fig. 17.53, two possible chip configurations are provided.
The maximum ratings and electrical characteristics for the IL-1 model are provided in Fig. 17.54.
The typical opt electric characteristic curves for each channel are provided in Figs. 17.55 through 17.59. Note the very pronounced effect of temperature on the output current at low temperatures but the fairly level response at or above room temperature (25ºC). In Fig. 17.55, we do not reach 1μA until the temperature rises above 75ºC. The transfer characteristics of Fig. 17.56 compare the input LED current (which establishes the luminous flux) to the resulting collector current of the output transistor (whose base current is determined by the incident flux). In fact, Fig. 17.57 demonstrates that the VCE voltage affects the resulting collector current only very slightly. It is interesting to note in Fig. 17.58 that the switching time of an opto-isolator decreases with increased current, whereas for many devices it is exactly the reverse. The relative output versus temperature appears in Fig. 17.59.
The schematic representation for a transistor coupler appears in Fig. 17.53. The schematic representation for a photodiode, a photo-Darlington, and a photo-SCR opto-isolator appear in Fig. 17.60.
PROGRAMMABLE UNIJUNCTION TRANSISTOR (PUT)

VG = VBB = η VBB
Where:
η =
as defined for the UJT.
The firing potential (VP) or voltage necessary to “fire” the device is given by:
Vp = ηVBB + VD
as defined for the UJT. However, VP represents the voltage drop VAK in the figure (the forward voltage drop across the conducting diode). For silicon, VD is typically 0.7V.
Therefore:
VAK = VAG + VGK
Vp = VD + VG
And:
Vp = ηVBB + 0.7V
The subscript is included to indicate that any R greater than Rmax will result in a current less than IP. The level of R must also be such to ensure it is less than IV if oscillations are to occur. In other words, we want the device to enter the unstable region and then return to the “off” state. From reasoning similar to that above:
Rmin =
The discussion above requires that R be limited to the following for an oscillatory system:
Rmin < R < Rmax
The waveforms of VA, VG, and VK appear in the figure. Note that T determines the maximum voltage VA can charge to. Once the device fires, the capacitor will rapidly discharge through the PUT and RK, producing the drop shown. Of course, VK will peak at the same time due to the brief but heavy current. The voltage VG will rapidly drop down from VG to a level just greater than 0V. When the capacitor voltage drops to a low level, the PUT will once again turn off and the charging cycle will be repeated. The effect on VG and VK is shown in figure above.