1 SCHMITT TRIGGER (regenerative comparator) Schmitt trigger is an inverting comparator with positive feedback. It converts an irregular-shaped waveform to a square wave or pulse, also called as squaring circuit. The input voltage Vin triggers the output V0 every time it exceeds certain voltage levels called the upper threshold voltage Vut and lower threshold voltage Vlt, These threshold voltages are obtained by using the voltage divider where the voltage across R1 is fed back to the input. The voltage across R1 is a variable reference threshold voltage that depends on the value and polarity of the output voltage V0 When Vo = +VSat, the voltage across R1 is called the upper threshold voltage, Vut The input voltage Vin must be slightly more positive than Vut in order to cause the output switch from +VSat to -VSat. As long as Vin <Vut, Vo is at +Vsat. Vut =R1/R1+R2 (+VSat) On the other hand, when V0 = -Vsat, the voltage across R1 is referred to as lower threshold voltage, vin be slightly more negative than vlt in order to switch V0 from +Vsat to -Vsat. In other words, for vin values greater than vlt, vo is at -Vsat. Vlt is given by the following equation Vlt =R1/R1+R2 (-VSat) v p V ut V ut -v p +v sat -v sat Fig : I/O waveform (c) V o vs V in plot of. hysteresis voltage Thus, if the threshold voltages are made larger than the input noise voltages, the positive feedback will eliminate the false output transitions. Resistance ROM used to minimize the offset problems. In the triangular wave and sawtooth wave generators a noninverting comparator is used as a Schmitt trigger. When the input is a triangular wave, the output of the Schmitt trigger is a square wave, whereas if the input is a sawtooth wave, the output is a pulse waveform. The comparator with positive feedback is said to exhibit hysteresis, a dead-band condition That is, when the input of the comparator exceeds Vut, its output switches from +Vsat to -Vsat and reverts back to its original state, +Vsat, when the input goes below Vlt. == R 1 R 1+ R 2 (V SAT ) COMPARATOR CHARACTERISTICS The important characteristics of a comparator are these: 1. Speed of operation 2. Accuracy 3. Compatibility of output
2 The output of the comparator must switch rapidly between saturation levels and also respond instantly to any change of conditions at its inputs. This implies that the bandwidth of the op-amp comparator must be rather wide; in fact, the wider the bandwidth, the higher is the speed of operation. The speed of operation of the comparator is improved with positive feedback (hysteresis) The accuracy of the comparator depends on its voltage gain, common-mode rejection input offsets, and thermal drifts. High voltage gain requires a smaller difference voltage (hysteresis voltage to cause the comparator's output voltage to switch between saturation levels, On the other hand, a high CMRR helps to reject the commonmode input voltages, such as noise, at the input terminals. Finally, to minimize the offset problems, the input offset current and input offset voltage must be negligible; also, the changes in these offsets due to temperature variations should be very slight. Since the comparator is a form of analog-to-digital converter, its output must swing between two logic levels suitable for a certain logic family such as transistor-transistor logic (TTL). LIMITATIONS OF OP-AMPS AS COMPARATORS A general-purpose op-amp such as the 741 can be used in relatively less critical comparator applications in which speed and accuracy are not major factors. With positive feedback (hysteresis), the switching speed of the op-amp comparator can be improved and false transition due to noise can be eliminated. In addition, an offset voltagecompensating network and offset minimizing resistor can be used to minimize offset problems. However, the output voltage swing of an op-amp is relatively large because it is designed primarily as an amplifier. In other words, the output of an op-amp comparator is generally not compatible with a particular logic family such as the TTL, which requires input voltages of either approximately +5 V or 0 V. Therefore, to keep the output voltage swing within specific limits, op-amps are used with externally wired components such as zeners or diodes. The resulting circuits, in which the outputs are limited to predetermined values, are called limiters. 555 TIMER Applications One of the most versatile IC is 555 timer. It can be used in a number novel and useful application. Monostable and Astable multivibrators, dc to dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control, infrared transmitters, burglar and toxic gas alarms, voltage regulators, electric eyes, and many other. 555 timer can produce accurate and highly stable time delays or oscillation. The timer basically operates in one of two as a monostable (one-shot) multivibrator or as an astable (free running multivibrator). The device is available as an 8-pin metal can, an 8-pin mini DIP, or a 14-pin DIP. Operating temperature range of SE555 is -55 to +125 and NEE is 0 to +70C. It operates on +5 to +18 V supply voltag; It has an adjustable duty cycle; timing is from microseconds through hours; it has a high current output it can source or sink 200 ma PIN CONFIGURATION Pin 1: Ground. All voltages are measured with respect to this terminal. Pin 2: Trigger. The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. The output is low if the voltage at this pin is greater than 2/3 Vcc. However, when a negative going pulse of
3 amplitude larger than 1/3 Vcc is applied to this pin, the comparator 2 output goes low, which in turn switches the output of the timer high. The output remains high as long as the trigger terminal is held at a low voltage. Pin 3: Output, There are two ways a load can be connected to the output terminal: either between pin 3 and ground (pin 1) or between pin 3 and supply voltage Vcc (pin 8). when the output is low the load current flows through the load connected between pin 3 and Vcc into the output terminal and is called the sink current. However, the current through the grounded load is zero when the output is low. For this reason, the load connected between pin 3 and Vcc is called the normally on load and that connected between pin 3 and ground is called the normally off load. On the other hand, when the output is high, the current through the load connected between pin 3 and Vcc (normally on load) is zero. However the output terminal supplies current to the normally off load. This current is called the source current. The maximum value of sink or source current is 200 ma Pin 4: Reset. The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to Vcc to avoid any possibility of false triggering. Pin 5: Control voltage. An external voltage applied to this terminal changes the threshold as well as the trigger voltage. In other words, imposing a voltage on this by connecting a pot between this pin and ground, the pulse width of the output waveform can be varied. When not used, the control pin should be bypassed to ground with a 0.01-µF capacitor to prevent any noise problems. Pin 6: Threshold. This is the noninverting input terminal of comparator 1 which monitors the voltage across the external capacitor. When the voltage at this pin is 2 threshold voltage 2/3 Vcc, the output of comparator 1 goes high, which in turn switches the output of the low. Pin 7: Discharge. This pin is connected internally to the collector of transistor Q1. When the output is high, Q1 is off and acts as an open circuit to the external capacitor C connected across it. On the other hand, when the output is low, Q1 is saturated and acts as a short circuit, shorting out the external capacitor C to ground. Pin 8: +Voc. The supply voltage of +5 v to +18 is applied to this pin with respect to ground (pin 1) 555 TIMER AS MONOSTABLE MULTIVIBRATOR A monostable multivibrator, often called a one-shot multivibrator, is a pulse-generating circuit in which the duration of the pulse is determined by the RC Network connected externally to the 555 timer. In a stable or standby state the output of circuit is zero os at logic low level. When an Trigger pulse is applied, the output is forced to go high ( Vcc). The time the Output remains high is determined by the external RC network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low state. The output stays low until the trigger pulse is applied. Then the cycle repeats.
4 Monostable operation. Initially when the output is low, that is, the circuit is in a stable state, transistor Q1 is on and capacitor C is shorted out to ground. However, upon application of a negative trigger pulse to pin 2, transistor Q1 is turned off, which releases the short circuit across the external capacitor C and drives the output high. The capacitor C now starts charging up toward Vcc through RA. However, when the voltage across the capacitor equals 2/3 Vcc, comp1 s o/p switches from low to high, which in turn drives the output to its low state via the output of the flip-flop. At the same time, the output of the flip-flop turns transistor Q1 on, and hence capacitor C rapidly discharges through the transistor. The output of the monostable remains low until a trigger pulse is again applied. Then the cycle repeats. As shown here, the pulse width of the trigger input must be smaller than the expected pulse width of the output waveform. Also, the trigger pulse must be a negative-going input signal with an amplitude larger than 1/3 Vcc by The time during which the output remains high is given Once triggered, the circuit's will remain in the high state until the set time T elapses. The output will not change state even if an input trigger is applied again during this time interval to. However, the circuit can be reset during the timing cycle by applying a negative pulse to the reset terminal. The output will then remain in the low state until a trigger is again applied.
5 Often in practice a decoupling capacitor (10µF) is used between Vcc (pin 8) and ground (pin 1) to eliminate unwanted voltage spikes in the output waveform. Sometimes, to prevent any possibility of mistriggering the monostable multivibrator on positive pulse edges, a waveshaping circuit consisting of R, C2, and diode D is connected between the trigger input pin 2 and Vcc pin 8. The values of R and C2 should be selected so that the time constant RC2 is smaller than the output pulse width T APPLICATION OF MONOSTABLE MULTIVIBRATOR Missing Pulse Detector Whenever, input trigger is low, the emitter diode of the transistor Q is forward biased. The capacitor C gets clamped to few tenths of a volte (0.7V). The output of the timer goes HIGH. The circuit is designed so that the time period of the monostable circuit is slightly greater (1/3 longer) than that of the triggering pulses, So long the trigger pulse train keeps coming at pin 2, the output remains HIGH. However,if a pulse misses, the trigger input is high and transistor Q is cut off. The 555 timer enters into normal state of monostable operation. The output goes Low after time T of the mono-shot. Thus this type circuit can be used to detect missing heartbeat. It can also be used for speed control and measurement. Frequency Divider A continuously triggered monostable circuit when triggered by a square wave generator can be used as a frequency divider, if the timing interval is adjusted to be longer than the period of the triggering square wave input signal. The monostable multivibrator will be triggered by the first negative going edge of square wave input but the output will remain HIGH (because of greater timing interval) for next negative going edge of the input square wave. The mono-shot will however be triggered on the third negative going input, depending on the choice of the time delay. In this way, the output can be made integral fractions of the frequency of the input triggering square wave. the Pulse width Modulation This is basically a monostable multivibrator with a modulating input signal applied at pin-5. By the application of continuous trigger at pin-2, a series of output pulses are obtained, the duration of which
6 depends on the modulating input at pin-5. The modulating signal applied at pin-5 gets superimposed upon the already existing voltage (2/3) Vcc at the inverting input terminal of comparator1. This in turn changes the threshold level of Comparator1 and the output pulse width modulation takes place. It may be noted from the output waveform that the pulse duration, that is, the duty cycle only varies, keeping the frequency same as that of the continuous input pulse train trigger. THE 555 AS AN ASTABLE MULTIVIBRATOR In astable multivibrator, often called a free-running multivibrator, is a rectangularwave-generating circuit. Unlike the monostable multivibrator, this circuit does not require an external trigger to change the state of the output, hence the name free-running. However, the time during which the output is either high or low is determined by the two resistors and a capacitor, which are externally connected to the 555 timer. Initially, when the output is high, capacitor C starts charging toward Vcc through RA and RB. However as soon as voltage across the capacitor equals 2/3 Vcc, comparator 1 triggers the flip-flop, and the output switches low. Now capacitor C starts discharging through transistor Q1. When the voltage across C equals l /3 Vcc, comparator 2's output triggers the flip-flop, and the output goes high. Then the cycle repeats. The capacitor is periodically charged and discharged between 2/3 Vcc and 1/3 Vcc, respectively. Fig. free running freq. vs RA,RB and C
7 The duty cycle of the square wave is 50%. The astable multivibrator will not produce square-wave output unless the resistance RA =0Ω. However, there is a danger in shorting resistance RA to zero. With RA =0Ω, terminal 7 is connected directly to Vcc. When the capacitor discharges through RB and Q1 (pin 7), an extra current is supplied to Q1 by Vcc through a short between terminal 7 and cc, which may damage Q1 and hence the timer. Fortunately, an alternative is available, Astable multivibrator can be used to produce a square wave output simply by connecting diode D across resistor RB. The capacitor C charges through RA and diode D to approximately 2/3 Vcc and discharges through Ra and terminal 7 (transistor Q1) until the capacitor voltage equals approximately 1/3 Vcc; then the cycle repeats. To obtain a square wave output (50% duty cycle), R must be a combination of a fixed resistor and potentiometer so that the potentiometer can be adjusted for the exact square wave. Astable Multivibrator Applications FSK Generator: In digital data communication, binary code is transmitted by shifting a carrier frequency between two preset frequencies. This type of transmission is called frequency shift keying (FSK) technique. A 555 timer in astable mode can be used to generate FSK signal. The standard digital data input frequency is 150 Hz. When input is HIGH, transistor Q1 is off and 555 timer works in the normal astable mode of operation. The frequency of the output waveform given as f o = 1.45 (R A + 2R B )C
8 When the input is Low, Q1 goes ON and connects the resistance Rc across RA. The output frequency is now given by f o = Pulse-Position Modulator 1.45 (R A R C + 2R B )C The pulse-position modulator can be constructed by applying a modulating signal to pin 5 of a 555 timer connected for astable operation. The output pulse position varies with the modulating signal, since the threshold voltage and hence the time delay is PHASE LOCKED LOOPS The phase-locked loop principle has been used in applications such as FM (frequency modulation) stereo decoders, motor speed controls, tracking filters, frequency synthesized transmitters and receivers, FM demodulators, frequency shift keying (FSK) decoders, and a generation of local oscillator frequencies in TV and in FM tuners. The phase-locked loop consists of (1) a phase detector, (2) a low-pass filter, and, (3) Error amp. 4) voltage controlled oscillator. The phase detector or comparator compares the input frequency fin with feedback frequency fout. The output of the phase detector is proportional to the phase difference between fin& fout. The output of the phase detector is a dc voltage & therefore is often referred to as the error voltage. The output of the phase detector is then applied to the LPF, which removes the high frequency noise and produces a dc level. This dc level in turn, is input to the VCO.
9 The output frequency of VCO is directly proportional to the dc level. The VCO frequency is compared with input frequency and adjusted until it is equal to the input frequencies. PLL goes through 3 states, i) free running ii) Capture iii) Phase lock. Before the input is applied, the PLL is in free running state. Once the input frequency is applied the VCO frequency starts to change and PLL is said to be in the capture mode. The VCO frequency continuous to change until it equals the input frequency and the PLL is in phase lock mode. When Phase locked, the loop tracks any change in the input frequency through its repetitive action. If an input signal vs of frequency fs is applied to the PLL, the phase detector compares the phase and frequency of the incoming signal to that of the output Vo of the VCO. If the two signals differ in frequency of the incoming signal to that of the output vo of the VCO. If the two signals differ in frequency and/or phase, an error voltage Ve is generated. Lock in range(tracking range): Once PLL is locked it can track frequency changes in incoming signal. The lock range is defined as the range of frequencies over which the PLL system maintain lock with incoming signal Capture range: Capture range is the frequency range in which the PLL acquires phase lock. Capture range is always smaller than the lock range Pull in time : Total time taken by PLL to establish lock is called pill in time The phase detector is basically a multiplier and produces the sum (fs + fo) and difference (fs - fo) components at its output. The high frequency component (fs + fo) is removed by the low pass filter and the difference frequency component is amplified then applied as control voltage Vc to VCO. The signal Vc shifts the VCO frequency in a direction to reduce the frequency difference between fs and fo. Once this action starts, we say that the signal is in the capture range. The VCO continues to change frequency till its output frequency is exactly the same as the input signal frequency. The circuit is then said to be locked. PHASE DETECTOR/COMPARATOR Phase detection is the most important part of the PLL system. There are two types of detectors used, analog and digital. Analog Phase Detector The principle of analog phase detection using switch type phase detector is shown in Fig.. An electronic switch S is opened and closed by signal coming from VCO (normally a square wave). The input signal is, therefore, chopped at a repetition rate determined by VCO frequency. positive, the output waveform ve will be half sinusoids (shown hatched). Similarly, the output waveform for 90 and 180" is shown in Fig, 9.3 (d, e). This type of phase detector is called a half wave detector, since the phase information for only one-half of the input waveform is detected and averaged. The output of the phase comparator when filtered through a low pass filter gives an error signal which is the average value
10 of the output waveform shown by dotted line. It may be seen that the error voltage is zero when the shift between the two inputs phase is 90%. So, for perfect lock, the VCO output Bhould be 90 out of phase with to the input signal. Analysis A phase comparator is basically a multiplier which multiplies the input signal by the VCO signal. Thus the phase comparator output is where K is the phase comparator gain (or attenuation constant) and is the phase shift between the input signal and the VCO output. It can be simplified as, Digital Phase Detector: This uses an exclusive OR gate. The output of the Ex-OR gate is high only when Fin or fout is high. The DC output voltage of the Ex-OR phase detector is a function of the phase difference between its two outputs. The maximum dc output voltage occurs when the phase difference is Π radians or 180 degrees. The slope of the curve between 0 or Π radians is the conversion gain kφ of the phase detector for eg; if the Ex-OR gate uses a supply voltage Vcc = 5V, the conversion gain K φ is` K φ = Π/ 5 = 1.59 V / RAD Low Pass filter: The low pass filter not only removes the high frequency components and noise, but also controls the dynamic characteristics of the PLL. These characteristics include capture range, lock range, band-width and transient response. If filter band-width
11 is reduced, the response time increases. However, reducing the band-width of the filter also reduces the capture range of the PLL. The filter serves one more important purpose. The charge on the filter capacitor gives a short time 'memory' to the PLL. Thus, even if the signal becomes less than the noise for a few cycles, the dc voltage on the capacitor continues to shift the frequency of the VCO till it picks up signal again. This produces a high noise immunity and lock in range stability. VCO Referring to the circuit in the above figure, the capacitor C1 is linearly charged or discharged by a Constant current source/sink. The amount of current can be controlled by changing the voltage Vc Applied at the modulating input (pin 5) or by changing the timing resistor R1 external to the IC Chip. The voltage at pin 6 is held at the same voltage as pin 5. Thus, if the modulating voltage at Pin 5 is increased, the voltage at pin 6 also increases, resulting in less voltage across R1 and Thereby decreasing the charging current. The voltage across the capacitor C1 is applied to the inverting input terminal of Schmitt Trigger via buffer amplifier. The output voltage swing of the schmitt trigger is designed to VCC and 0.5 VCC. If RA= Rb in the positive feedback loop, the voltage at the non-inverting input terminal of Schmitt trigger swings from 0.5 VCC to 0.25 VCC. When the voltage on the capacitor C1 exceeds 0.5 Vcc during charging, the output of the schmitt trigger goes low (0.5 vcc). The capacitor now Discharges and when it is at 0.25 vcc, the output of schmitt trigger goes high (v cc). Since the This gives a triangular voltage waveform across C1 which is also available at pin 4. The square Wave output of the schmitt trigger is inverted by buffer amplifier at pin 3. The output waveforms are shown near the pins 4 and 3. The output frequency of the vco can be given as follows:
12 The output frequency of the VCO can be changed either by (i) R1, (ii) c1 or (iii) the voltage Vc at the modulating input terminal pin 5. The voltage vc can be varied by connecting a R1-R2circuit. The components R1 and c1 are first selected so that VCO output frequency lies in the centre of the operating frequency range. Now the modulating input voltage is usually varied from 0.75 Vcc to Vcc which can produce a frequency variation of about 10 to 1. MONOLITHIC PHASE LOCKED LOOPS (PLL IC 565) It consists of phase detector, amplifier, low pass filter and VCO.As shown in the block diagram the phase locked feedback loop is not internally connected. Therefore, it is necessary to connect output of VCO to the phase comparator input, externally. The PLL can lock to and track an input signal over typically ±60% bandwidth
13 w.r.t fo as the center frequency. The lock range fl and the capture range fc of the PLL are given by the following equations are, The important electrical characteristics of the 565 PLL Operating frequency range: 0.001Hz to 500 Khz. Operating voltage range: ±6 to ±12v Input level required for tracking: 10mv rms min to 3 Vpp max Input impedance: 10 K ohms typically. Output sink current: 1mA Output source current: 10 ma The center frequency of the PLL is determined by the free running frequency of the VCO, which is given by where R1&C1 are an external resistor & a capacitor connected to pins 8 & 9. The VCO free-running frequency fout is adjusted externally with R1 & C1 to be at the center of the input frequency range. C1 can be any value, R1 must have a value between 2 k ohms and 20 K ohms. Capacitor C2 connected between 7 & +V. The filter capacitor C2 should be large enough to eliminate variations in the demodulated output voltage in order to stabilize the VCO frequency. APPLICATION Frequency Multiplier: In this application, the loop is broken and a frequency divider network is inserted between VCO and phase detector as shown in figure below. The multiplication factor can be obtained by selecting a proper Input scaling factor N of the counter. Frequency multiplication can also be obtained by using PLL in its harmonic locking mode. If the input signal is rich in harmonics e.g. square wave, pulse train etc., then VCO can be directly locked to the n-th harmonic of the input signal without
14 connecting any frequency divider in between. However, as the amplitude of the higher order harmonics becomes less, effective locking may not take place for high values of n. Typically n is kept less than 10. The circuit can also be used for frequency division. Since the VCO output square wave is rich in harmonics, it is possible to lock the m-th harmonic of the VCO out with the input signal f. The output fo of VCO is now given by Frequency Translation A schematic for shifting the frequency of an oscillator by a small factor is shown. It can be seen that a mixer (or multiplier) and a low pass filter are connected external to the PLL. The signal fa which has to be shifted and the output frequency of the VCO are applied as inputs to the mixer. The output of the mixer contains the sum and difference of fs and fo. However, the output of LPF contains only the difference signal. The translation or offset frequency f1 is applied to the phase comparator AM DETECTOR PLL may be used to demodulate AM signals as shown in Fig The PLL is locked to the carrier frequency of the incoming AM signal. unmodulated is fed to the multiplier, Since VCO output is always 90" out of phase with the incoming AM signal under the locked condition, the AM input signal is also shifted in phase by 90 o before being fed to the multiplier, This makes both the signals applied to the multiplier in same phase, The output of the multiplier contains both the sum and the difference signals, the demodulate output is obtained after filtering high frequency components by the LPF, since the PLL responds only to the career frequency which are very close to the VCO output FM detector There is shift in carrier frequency about the mean value according to modulating signal at FM transmitter. The deviation or shift in carrier frequency from centre value is converted to low voltage or high voltage, is demodulation. Assume the loop is in locked condition, so VCO frequency and input frequency is same. FM signal is applied as input to phase detector. Phase detector produce error voltage proportional to frequency shift. This signal is passed through
15 LPF and amplifier to give controlled voltage. Thus controlled voltage is proportional to change in frequency. As input frequency is shifted up or down, VCO voltage also varies accordingly SERIES OP-AMP REGULATOR A voltage regulator is an electronic circuit that provides a stable de voltage independent of the load current, temperature and ac line voltage variations. The circuit consists of following four parts: 1. Reference voltage circuit 2. Error amplifier 3. Series pass transistor 4. Feedback network. It can be seen from that the power transistor Q1 is in series with the unregulated dc voltage Vin and the regulated output voltage Vo. So it must absorb the difference between these two voltages whenever any fluctuation in output voltage VO occurs. The transistor Q1 is also connected as an emitter follower and therefore provides sufficient current gain to drive the load. The output voltage is sampled by the R1 -R2 divider and fed back to the (-) input terminal of the op amp error amplifier. This sampled voltage is compared with the reference voltage Vref (usually obtained by a zener diode). The output VO. of the error amplifier drives the series transistor Q1. If the output voltage increases, say, due to variation in load current, the sampled voltage also increases where B This, in turn, reduces the output voltage VO of the diff-amp due to the phase difference provided by the op amp amplifier. VO is applied to the base of Q1, which is used as an emitter follower. So VO follows VO that is V0 reduces. Hence the increase in V0 is nullified. Similarly, reduction in output voltage also regulated. IC VOLTAGE REGULATORS With the advent of micro-electronics, it is possible to incorporate the complete circuit on a monolithic silicon chip. This gives low cost, high reliability, reduction in size and excellent performance. Examples of monolithic regulators are 78 xx/79 XX series and 723 general purpose regulators. Fixed Voltage Series Regulator 78 xx series are three terminal, positive fixed voltage regulators. There are seven output voltage options available such as 5, 6, 8, v. 78 xx, the last two numbers XX indicate the output voltage. Thus 7815 represents a 15 V regulator. There are also available 79 XX series of fixed output, negative voltage regulators which are complements to the 78 XX series devices. There are two extra voltage options of -2 V and -5.2 V available in 79 XX series.
16 These regulators are available in two types of packages.metal package (TO 3 type) Plastic package (TO 220 type) A capacitor C (0.33 HF) is usually connected between input terminal and ground to cancel the inductive effects due to long distribution leads. The output capacitor Co (1µF) improves the transient response. Characteristics There are four characteristics of three terminal IC regulators which must be mentioned. 1. Vo The regulated output voltage is fixed at a value as specified by the manufacturer. There are a number of models available for different output voltages, for example, 78 XX series has output voltage at 5, 6, 8 etc. 2. vin > Vo + 2 volts: The unregulated input voltage must be at least 2 V more than the regulated output voltage. For example, if Vo 5 V, then Vin = 7v 3. Io( max) :The load current may vary from 0 to rated maximum output current. The IC usually provided with a heat sink, otherwise it may not provide the rated maximum output current. 4. Thermal shutdown: The IC has a temperature sensor (built-in) which turns off the when it becomes too hot (usually 125. C to 150 C). The output current will drop and remains there until the IC has cooled significantly. Line/Input Regulation It is defined as the percentage change in the output voltage for a change in the input voltage. It is usually expressed in millivolts or as a percentage of the output voltage. Typical value of line regulation from the data sheet of 7805 is 3 mv. Load Regulation It is defined as the percentage change in the output voltage for a change in the load current. It is usually expressed in millivolts or as a percentage of the output voltage. Typical value of load regulation from the data sheet of 7805 is 15 mv for 5mA < Io < 1.5A. Ripple Rejection IC regulator not only keeps the output voltage constant but also reduces the amount of ripple voltage. It is usually expressed in db. Typical value for 7805 is 78dB. Current Source Three terminal regulator can be used as a current source. The circuit where 7805 has been wired to supply a current of 1ampere to a 10 ohm, 10 watt load
17 IQ is the quiescent current and is about 4.2 ma for 7805 So the value of R required is R = 5V/1a = 5Ω Thus choose R = 5Ω to deliver 1A current to a load of 10Ω Boosting IC Regulator output Current It is possible to boost the output current of a three terminal regulator simply by connecting an external pass transistor in parallel with the regulator as shown in Fig. 6.4 (b). Let us now see how the circuit works. For low load currents, the voltage drop across R1 is insufficient to drive the transistor Q1 and the regulator itself is able to supply the load current. However, as IL increases, the voltage drop across R1 increases. When this voltage drop is approximately 0.7 V, the transistor Q1 turns on. It can be easily seen that if IL = 100 ma, the voltage drop across R1 is equal to 7 Q x 100 ma = 0.7v. if IL increases more than 100 ma, the transistor Q1 turns on and supplies the extra current required. Since VEB (ON) remains fairly constant, the excess current comes from Q1 s Base after amplification by B. The regulator adjusts IB so that
18 Adjustable Regulator In the laboratory, one may need variable regulated or a voltage that is not available as standard fixed voltage regulator. This can be achieved by using a fixed three terminal regulator. Note that the ground (GND) terminal of the fixed three terminal regulator is floating. The output voltage where VR is the regulated voltage difference Between the OUT and GND terminals. The effect of lq is minimized by choosing R2 small enough to minimize the term IQR2. The minimum output voltage is the value of the fixed voltage available from the regulator. The LM117, 217, 17 positive regulators and LM137, 237, 337 negative regulators have been specially designed to be used for obtaining adjustable output voltages. It is possible to adjust output voltage from 1.2 v to 40 V and current upto 1.5 A. 723 GENERAL PURPOSE REGULATOR The three terminal regulators discussed earlier have the following limitations: 1. No short circuit protection 2. output voltage (positive or negative) is fixed. These limitations have been overcome in the 723 general purpose regulator, which can adjusted over a wide range of both positive or negative regulated voltage. This IC is inherently low current device, but can be boosted to provide 5 amps or more current by connect external components. The limitation of 723 is that it has no in-built thermal protection also has no short circuit current limits. It has two separate sections. The zener diode, a constant current source and reference amplifier produce a fixed voltage of about 7 volts at the terminal Vref. The constant current source forces the zener operate at a fixed point so that the zener outputs a fixed voltage.
19 The other section of the IC consists of an error amplifier, a series pass transistor Q1 and a current limit transistor Q2. The error amplifier compares a sample of the output voltage applied at the INV input terminal to the reference voltage Vref applied at the NI input terminal. The error signal controls the conduction of Q1. These two sections are not internally connected but the various points are brought out on the IC package. 723 regulated IC is available in a 14-pin dual-in-line package or pin metal-can. The difference between VNI and the output voltage Vo which is directly fed back to the INV terminal is amplified by the error amplifier. The output of the error amplifier drives the pass transistor Q1 so as to minimize the difference between the NI and INV inputs of error amplifier. Since Q1 is operating as an emitter follower The output voltage becomes low, the voltage at the INV terminal of error amplifier also goes down. This makes the output error amp become more positive, driving transistor Q1 more into conduction. This reduces the across and drives current into the load causing voltage across load to increase. So the initial drop in the load voltage has compensated. Similarly, any increase in load voltage, or changes in the input voltage get regulated. The reference voltage is typically 7.15 v. So the output voltage Vo is which will always be less than 7.15 v, So in the circuit is used as low voltage regulator
20 If it is desired to produce regulated output voltage greater than 7 V, then the below circuit can be used. The NI terminal is connected directly to Vref through R3. So the voltage at the NI terminal is Vref.The error amplifier operates as a non-inverting amplifier with a voltage gain of So the output voltage for the circuit is Current Limit Protection The circuits of for 723 regulator have no protection. If the load demands more current e.g. under short circuit conditions, the IC tries to provide it at a constant output voltage getting hotter all the time. This may ultimately burn the IC. The IC is, therefore, provided with a current limit facility. Current limiting refers to ability of a regulator to prevent the load current from increasing above a present value. Characteristic curve of a current limited power supply is shown The voltage remains constant for load current below Ilimit. As current approaches to the limit, output voltage drops. The current limit Ilimit is set by connecting external resistor also between the terminals CL and CS terminals. The CL terminal is connected to the output terminal V0 and CS terminal to the load.
21 The load current produces a small voltage drop Vsense, across Rsc. This voltage Vsense applied directly across the base emitter junction of Q2. When this voltage is 0.5 volt, transistor Q2 begins to turn ON. Now a part of the current from error amplifier reduces emitter current of Q1. So any increase in the load current will get nullified. Similarly, if the load current decreases, VBE of Q2 drops, repeating the cycle in such a manner that the load current is held constant to produce a voltage across Rsc sufficient to turn ON Q2. This voltage is typically 0.5v This method of current limiting is also referred to as current sensing technique. current Foldback In current limiting technique, the load current is maintained at a present value and when overload condition occurs, the output voltage Vo drops to zero. However, if the load is short circuited, maximum current does flow through the regulator. To protect the regulator, one must devise a method which will limit the short circuit current and yet allow higher currents to the load. Current foldback is the method used for this. As current demand increases, the output voltage is held constant till a present current level (knee) is reached. If the current demand exceeds this level, both output voltage and output current decrease. In order to understand the operation of the circuit, consider the circuit, voltage at terminal CL is divided by R3-R4, network. The current limit transistor Q2 conducts only when the drop across the resistance Rsc is large enough to produce a base-emitter voltage of Q2 to be at least 0.5 V. As Q2 starts conducting, transistor Q1 begins to turn off and the current I, decreases. This reduces the voltage V1 at the emitter of Q1 and also the output voltage Vo. The voltage at the base of Q2 (CL) will be V1R4/(R3+R4). Current Boosting The maximum current that 723 IC regulator can provide is 140 ma. For many application this is not sufficient. It is possible to boost the current level simply by adding boosting transistor Q1 to the voltage regulator. The collector current of the pass transistor Q1 comes from the unregulated dc supply. The output current from drives the base of the pass transistor Q1. This base current gets multiplied by the beta of the pass transistor, so that 723 has to provide only the base current. So, Iload = βpass transistor x Io