Home  Audio Magazine  Stereo Review magazine  Good Sound  Troubleshooting 
OPERATIONAL AMPLIFIERS (OPAMPS) CAN be simply described as highgain directcoupled voltage amplifier ' blocks' that have a single output terminal but have both inverting and non inverting input terminals. Opamps can readily be used as inverting, non inverting, and differential amplifiers in both a.c. and d.c. applications, and can easily be made to act as oscillators, tone filters, and level switches, etc. Opamps are readily available in integrated circuit form, and as such act as one of the most versatile building blocks available in electronics today. One of the most popular opamps presently available is the device that is universally known as the "741" opamp In this article we shall describe the basic features of this device, and show a wide variety of practical circuits in which it can be used. BASIC OPAMP CHARACTERISTICS AND CIRCUITS In its simplest form, an opamp consists of a differential amplifier followed by offset compensation and output stages, as shown in Fig. 1a. The differential amplifier has inverting and non inverting input terminals, a highimpedance (constant current) tail to give a high input impedance and a high degree of common mode signal rejection. It also has a high impedance (constant current) load to give a high degree of signal voltage stage gain.
The output of the differential amplifier is fed to a directcoupled offset compensation stage, which effectively reduces the output offset voltage of the differential amplifier to zero volts under quiescent conditions, and the output of the compensation stage is fed to a simple complementary emitter follower output stage which gives a low output impedance. INVERTING INPUT OUT NONINVERTING INPUT
LINES OF SUPPLY Opamps are normally powered from split power supplies, providing + ve,  ve, and common (zero volt) supply rails, so that the output of the opamp can swing either side of the zero volts value, and can be set at a true zero volts ( when zero differential voltage is applied to the circuits input terminals.) The input terminals can be used independently (with the unused terminal grounded) or simultaneously, enabling the device to function as an inverting, non inverting, or differential amplifier. Since the device is directcoupled throughout, it can be used to amplify both a.c. and d.c. input signals. Typically, they give basic lowfrequency voltage gains of about 100 000 between input and output, and have input impedances of 1M or greater at each input terminal. Fig. 1b shows the symbol that is commonly used to represent an opamp, and 1c shows the basic supply connections that are used with the device. Note that both input and output signals of the opamp are referenced to the ground or zero volt line. SIGNAL BOX
The output signal voltage of the opamp is proportional to the DIFFERENTIAL signal between its two input terminals, and is given by Ao(e,e 2) where A. the open loop voltage gain of the opamp (typically 100 000). e1 = signal voltage at the non inverting input terminal. e2= signal voltage at the inverting input terminal. Thus, if identical signals are simultaneously applied to both input terminals, the circuit will ( ideally) give zero signal output If a signal is applied to the inverting terminal only, the circuit gives an amplified and inverted output. If a signal is applied to the noninverting terminal only, the circuit gives an amplified but noninverted output. By using external negative feedback components, the stage gain of the opamp circuit can be very precisely controlled. REFERENCE VOLTAGE (e2) SAMPLE VOLTAGE (et ) SUPPLY Ve SUPPLY Ve OUTPUT
Simple differential voltage comparator circuit. TRANSFER REQUEST Fig. 2a shows a very simple application of the opamp. This particular circuit is known as a differential voltage comparator, and has a fixed reference voltage applied to the inverting input terminal, and a variable test or sample voltage applied to the noninverting terminal. When the sample voltage is more than a few hundred microvolts above the reference voltage the opamp output is driven to saturation in a positive direction, and when the sample is more than a few hundred microvolts below the reference voltage the output is driven to saturation in the negative direction. Fig. 2b shows the voltage transfer characteristics of the above circuit. Note that it is the magnitude of the differential input voltage that dictates the magnitude of the output voltage, and that the absolute values of input voltage are of little importance. Thus, if a 1V reference is used and a differential voltage of only 200uV is needed to switch the output from a negative to a positive saturation level, this change can be caused by a shift of only 0.02% on a 1V signal applied to the sample input. The circuit thus functions as a precision voltage comparator or balance detector.
Ve SUPPLY 0V OUTPUT The opamp can be made to function as a lowlevel inverting d.c. amplifier by simply grounding the non inverting terminal and feeding the input signal to the inverting terminal, as shown in Fig. 3a. The opamp is used ' openloop (without feedback) in this configuration, and thus gives a voltage gain of about 100 000 and has an input impedance of about 1M. The disadvantage of this circuit is that its parameters are dictated by the actual opamp, and are subject to considerable variation between individual devices. CLOSING LOOPS A far more useful way of employing the opamp is to use it in the closedloop mode, i.e., with negative feedback. Fig. 3b shows the method of applying negative feedback to make a fixed gain inverting d.c. amplifier. Here, the parameters of the circuit are controlled by feedback resistors R, and R,. The gain, A of the circuit is dictated by the ratios of A, and R,. and equals 1:1 2/ R The gain is virtually independent of the opamp characteristics, provided that the openloop gain (A„) is large relative to the closedloop gain (A). The input impedance of the circuit is equal to R,, and again is virtually independent of the opamp characteristics. 741 COOKBOOK It should be noted at this point that although R1 and R2 control the gain of the complete circuit, they have no effect on the parameters of the actual opamp, and the full open loop gain of the opamp is still available between its inverting input terminal and the output. Similarly, the inverting terminal continues to have a very high input impedance, and negligible signal current flows into the inverting terminal. Consequently, virtually all of the R, signal current also flows in R,, and Fig. 4a Basic non inverting d.c. amplifier signal currents i, and i, can be regarded as being equal, as indicated in the diagram. Since the signal voltage appearing at the output terminal end of R, is A times greater than that appearing at the inverting terminal end, the current flowing in R2 is A times greater than that caused by the inverting terminal signal only. Consequently, R, has an apparent value of R2/A when looked at from its inverting terminal end, and the R, R, junction thus appears as a low impedance VIRTUAL GROUND point. INPUT VIRTUAL GROUND POINT
INVERT OR NOT TO INVERT ... It can be seen from the above description that the Fig. 3b circuit is very versatile. Its gain and input impedance can be very precisely controlled by suitable choice of R, and R,, and are unaffected by variations in the opamp characteristics. A similar thing is true of the non inverting d.c. amplifier circuit shown in Fig. 4a. In this case the voltage gain is equal to ( R,f R2)/ R, and the input impedance is approximately equal to (Ao/A)Zin where Zin is the open loon input impedance of the opamp. A great advantage of this circuit is that it has a very high input impedance. FOLLOW THAT VOLTAGE The opamp can be made to function as a precision voltage follower by connecting it as a unitygain non inverting d.c. amplifier, as shown in Fig. 4b. In this case the input and output voltages of the circuit are identical, but the input impedance is very  high and is roughly equal to A,, X Z,„. The basic opamp circuits of Figs. 2a to 4b are shown as d.c. amplifiers, but can readily be adapted for a.c. use. Opamps also have many applications other than as simple amplifiers. They can easily be made to function as precision phase splitters, as adders or subtractors, as active filters or selective amplifiers, as precision half wave or full wave rectifiers, and as oscillators or multivibrators, etc.
OPAMP PARAMETERS An ideal opamp would have an infinite input impedance, zero output impedance, infinite gain and infinite bandwidth, and would give perfect tracking between input and output. Practical opamps fall far short of this ideal, and have finite gain, bandwidth, etc., and give tracking errors between the input and output signals. Consequently, various performance parameters are detailed on opamp data sheets, and indicate the measure of "goodness" of the particular device. The most important of these parameters are detailed below. OPENLOOP VOLTAGE GAIN, Ao . This is the lowfrequency voltage gain occurring directly between the input and output terminals of the opamp, and may be expressed in direct terms or in terms of dB. Typically, d.c. gain figures of modern opamps are 100 000, or 100dB. INPUT IMPEDANCE, Z or This is the impedance looking directly into the input terminals of the opamp when it is used open loop, and is usually expressed in terms of resistance only. Values of 1M are typical of modern op amps with bipolar input stages, while F.E.T. input types have impedances of a million meg or greater. OUTPUT IMPEDANCE, Z o. This is the output impedance of the basic opamp when it is used open loop, and is usually expressed in terms of resistance only. Values of a few hundred ohms are typical of modern opamps. INPUT BIAS CURRENT, I,. Many opamps use bipolar transistor input stages, and draw a small bias current from the input terminals. The magnitude of this current is denoted by 1,, and is typically only a fraction of a microamp. 741 COOKBOOK SUPPLY VOLTAGE RANGE, V, Opamps are usually operated from two sets of supply rails, and these supplies must be within maximum and minimum limits. If the supply voltages are too high the opamp may be damaged, and if the supply voltages are too low the opamp will not function correctly. Typical supply limits are: _ t 3V to ± 15V. INPUT VOLTAGE RANGE, V imaxr The input voltage to the opamp must never be allowed to exceed the supply line voltages, or the opamp may be damaged. V,i ,„, is usually specified as being one or two volts less than v,. OUTPUT VOLTAGE RANGE, Voi..4 If the op amp is over driven its output will saturate and be limited by the available supply voltages, so Vot, a ., is usually specified as being one or two volts less than V,. DIFFERENTIAL INPUT OFFSET VOLTAGE, V, In the ideal opamp perfect tracking would exist between the input and output terminals of the device, and the output would register zero when both inputs were grounded. Actual opamps are not perfect devices, however, and in practice slight imbalances exist within their input circuitry and effectively cause a small offset or bias potential to be applied to the input terminals of the opamp. Typically, this DIFFERENTIAL INPUT OFFSET VOLTAGE has a value of only a few millivolts, but when this voltage is amplified by the gain of the circuit in which the opamp is used it may be sufficient to drive the opamp output to saturation. Because of this, most opamps have some facility for externally nulling out the offset voltage. COMMON MODE REJECTION RATION, c.m.r.r. The ideal opamp produces an output that is proportional to the difference between the two signals applied to its input terminals, and produces zero output when identical signals are applied to both inputs simultaneously, i.e., in common mode. In practical opamps, common mode signals do not entirely cancel out, and produce a small signal at the opamps output terminal. The ability of the opamp to reject common mode signals is usually expressed in terms of common mode rejection ratio, which is the ratio of the opamps gain with differential signals to the opamps gain with common mode signals. C.m.r.r. values of 90dB are typical of modern opamps. TRANSITION FREQUENCY, f,. An opamp typically gives a lowfrequency voltage gain of about 100dB, and in the interest of stability its open loop frequency response is tailored so that the gain falls off as the frequency rises, and falls to unity at a transition frequency denoted f i. Usually, the response falls off at a rate of 6dB per octave or 20dB per decade. Fig. 5 shows the typical response curve of the type 741 opamp, which has an f i of 1MHz and a low frequency gain of 100dB. Note that, when the opamp is used in a closedloop amplifier circuit, the bandwidth of the circuit depends on the closed loop gain If the amplifier is used to give a gain of 60dB its bandwidth is only 1kHz, and if it is used to give a gain of 20dB its bandwidth is 100kHz. The f i figure can thus be used to represent a gain bandwidth product.
Table 1 Typical characteristics of the 741 opamp. SLEW RATE. As well as being subject to normal bandwidth limitations, opamps are also subject to a phenomenon known as slew rate limiting, which has the effect of limiting the maximum rate of change of voltage at the output of the device. Slew rate is normally specified in terms of volts per microsecond, and values in the range 1V/us to 10V/us are common with most popular types of opamp. One effect of slew rate limiting is to make a greater bandwidth available to small output signals than is available to large output signals. THE 741 OPAMP. Early types of i.c. opamp, such as the well known 709 type, suffered from a number of design weaknesses. In particular, they were prone to a phenomenon known as INPUT LATCHUP, in which the input circuitry tended to switch into a locked state if special precautions were not taken when connecting the input signals to the input terminals, and tended to selfdestruct if a short circuit were inadvertently placed across the opamp output terminals. In addition, the opamps were prone to bursting into unwanted oscillations when used in the linear amplifier mode, and required the use of external frequency compensation components for stability control. These weaknesses have been eliminated in the type 741 opamp. This device is immune to input latch up problems, has builtin output short circuit protection, and does not require the use of external frequency compensation components. The typical performance characteristics of the device are listed in Table 1. The type 741 op amp is marketed by most i.c manufacturers, and is very readily available Fig. 6 shows the two most commonly used forms of packaging of the device Throughout this chapter, all practical circuits are based on the standard 8 pin dual in line ( D.I.L. or DIP) version of the 741 opamp.
The 741 opamp can be provided with external offset nulling by wiring a 10k pot between its two null terminals and taking the pot slider to the negative supply rail, as shown in Fig. 7 Having cleared up these basic points, let's now go on and look at a range of practical applications of the 741 opamp.
BASIC LINEAR AMPLIFIER PROJECTS. (Figs. 8 to 11). Figs. 8 to 11 show a variety of ways of using the 741 in basic linear amplifier applications. The 741 can be made to function as an inverting amplifier by grounding the non inverting input terminal and feeding the input signal to the inverting terminal. The voltage gain of the circuit can be precisely controlled by selecting suitable values of external feedback resistance. Fig 8a shows the practical connections of an inverting d.c. amplifier with a preset gain of x100 The voltage gain is determined by the ratios of R, and R7, as shown in the diagram. The gain can be readily altered by using alternative R1 and/ or R, values If required, the gain can be made variable by using a series combination of a fixed and a variable resistor in place of R7, as shown in the circuit of Fig. 8b, in which the gain can be varied over the range xl to x100 via R VARIATIONS A variation of the basic inverting d.c. amplifier is shown in Fig. 9a. Here, the feedback connection to R, is taken from the output of the R3R, output potential divider, rather than directly from the output of the op amp, and the voltage gain is determined by the ratios of this divider as well as by the values of R1 and ...
... R2. The important feature of this circuit is that it enables R,, which determines the input impedance of the circuit, to be given a high value if required, while at the same time enabling high voltage gain to be achieved. 741 COOKBOOK The basic inverting d.c. amplifier can be adapted for a.c. use by simply wiring blocking capacitors in series with its input and output terminals, as shown in the x100 inverting a.c. amplifier circuit of Fig. 9b.
NONINVERTING ... The amp can be made to function as a noninverting amplifier by feeding the input signal to its noninverting terminal and applying negative feedback to the inverting terminal via a resistive potential divider that is connected across the opamp output. Fig. 10a shows the connections for making a fixed gain (x100) d.c. amplifier. The voltage gain of the Fig. 10a circuit is determined by the ratios of R, and R2. If R2 is given a value of zero the gain falls to unity, and if R, is given a value of zero the gain rises towards infinity ( but in practice is limited to the openloop gain of the opamp). If required, the gain can be made variable by replacing R2 with a ... INPUT OUTPUT
... potentiometer and connecting the pot slider to the inverting terminal of the opamp, as shown in the circuit of Fig. 10b The gain of this circuit can be varied over the range x1 to x100 via R, ... AND RESISTANCE TO INPUTS A major advantage of the noninverting d.c. amplifier is that it has a very high input resistance. In theory, the input resistance is equal to the openloop input resistance (typically 1M) multiplied by the openloop voltage gain (typically 100 000) divided by the actual circuit voltage gain. In practice, input resistance values of hundreds of megohms can readily be obtained.
BLOCKING OUT The basic non inverting d.c. circuit of Fig. 10 can be modified to operate as a.c. amplifiers in a variety of ways. The most obvious approach here is to simply wire blocking capacitors in series with the inputs and outputs, but in such cases the input terminal must be d.c. grounded via a suitable resistor, as shown by 13 3 in the noninverting x100 a.c. amplifier of Fig. 11a. If this resistor is not used the opamp will have no d.c. stability, and its output will rapidly drift into saturation. Clearly, the input resistance of the Fig. 11a circuit is equal to R,, and R. , must have a relatively low value in the interest of d.c. stability This circuit thus loses the noninverting amplifier's basic advantage of high input resistance.
DRIFTING INTO STABILITY A useful development of the Fig. 11a circuit is shown in Fig. 11b. Here, the values of R1 and R2 are increased and a blocking capacitor is interposed between them At practical operating frequencies this capacitor has a negligible impedance, so the voltage gain is still determined by the ratios of the two resistors. Because of the inclusion of the blocking capacitor, however, the inverting terminal of the opamp is subjected to virtually 100% d.c. negative feedback from the output terminal of the opamp, and the circuit thus has excellent d.c. stability. The low end of R, is connected to the C3R, junction, rather than directly to the ground line, and the signal voltage appearing at this point is virtually identical with that appearing at the noninverting terminal of the opamp'
Consequently, identical signal voltages appear at both ends of 13,, and the apparent impedance of this resistor is increased close to infinity by bootstrap action. This circuit thus has good d.c. stability and a very high input impedance In practice, this circuit gives a typical input impedance of about 50M. VOLTAGE FOLLOWER PROJECTS (Figs. 12 to 13). A 741 can be made to function as a precision voltage follower by connecting it as a unitygain non inverting amplifier. Fig. 12a shows the practical connections for making a d.c. voltage follower. Here, the input signal is applied directly to the noninverting terminal of the opamp, and the inverting terminal is connected directly to the output, so the circuit has 100% d.c. negative feedback and acts as a unitygain noninverting d.c. amplifier. The output signal voltage of the circuit is virtually identical to that of the input, so the output is said to 'follow' the input voltage. The great advantage of this circuit is that it has a very high input impedance (as high as hundreds of megohms) and a very low output impedance (as low as a few ohms). The circuit acts effectively as an impedance transformer.
PRACTICE, AND ITS LIMITS In practice the output of the basic Fig. 12a circuit will follow the input to within a couple of millivolts up to magnitudes within a volt or so of the supply line potentials. If required, the circuit can be made to follow to within a few microvolts by adding the offset null facility to the opamp. The d.c. voltage follower can be adapted for a.c. use by wiring blocking capacitors in series with its input and output terminals and by d.c.coupling the noninverting terminal of the opamp to the zero volts line via a suitable resistor, as shown by R, in Fig. 12b. R, should have a value less than a couple of megohms, and restricts the available input impedance of the voltage follower. LACED UP OHMS If a very high input impedance a.c. voltage follower is needed, the circuit of Fig. 12c can be used. Here, R1 is bootstrapped from the output of the opamp, and its apparent impedance is greatly increased. This circuit has a typical impedance of hundreds of megohms.
Fig. 12c Very high inputimpedance a.c. voltage follower. DRIVING CIRCUITS AMPLY The 741 opamp is capable of providing output currents up to about 5mA, and this is consequently the currentdriving limit of the three voltage follower circuits that we have looked at so far. The currentdriving capabilities of the circuits can readily be increased by wiring simple or complementary emitter follower booster stages between the opamp output terminals and the outputs of the actual circuits, as shown in Figs. 13a and 13b respectively. Note in each case that the baseemitter junction(s) of the output transistor(s) are included in the negative feedback loop of the circuit. Consequently, the 600mV knee voltage of each junction is effectively reduced by a factor equal to the openloop gain of the opamp, so the junctions do not adversely effect the voltagefollowing, characteristics of either circuit. The Fig. 13a circuit is able to source current only, and can be regarded as a unidirectional, positivegoing, d.c. voltage follower. The Fig. 13b circuit can both source and sink output currents, and thus gives bidirectional follower action. Each circuit has a currentdriving capacity of about 50mA• This figure is dictated by the limited power rating of the specified output transistors . The drive capability can be increased by using alternative transistors. Fig. 13a Unidirectional d.c. voltage follower with boosted output (variable from 0V to +8V at 50mA.) Fig. 13b Bidirectional d.c. voltage follower with boosted output (variable from 0V to± ±8V at 50mA). MISC AMP PROJECTS (Figs. 14 to 22) Figs. 14 to 22 show a miscellaneous assortment of 741 amplifier projects, ranging from d.c. adding circuits to frequencyselective amplifiers. Fig. 14 shows the circuit of a unitygain inverting d.c. adder, which gives an output voltage that is equal to the sum of the three input voltages. Here, input resistors R, to 13, and feedback resistor R4 each have the same value, and the circuit thus acts as a unitygain inverting d.c. amplifier between each input terminal and the output. Since the current flowing in each input resistor also flows in feedback resistor R4 , the total current flowing in R, is equal to the sum of the input currents, and the output voltage is equal to the negative sum of the input voltages. The circuit is shown with only three input connections, but in fact can be provided with any number of input terminals. The circuit can be made to function as a socalled ' audio mixer' by wiring blocking capacitors in series with each input terminal and with the output terminal.
FIG. 16 shows how a 741 can be used as a unitygain differential d.c. amplifier. The output of this circuit is equal to the difference between the two input signals or voltages, or to ere 2. Thus, the circuit can also be used as a subtractor. In this type of circuit the component values are chosen such that R , / 13 2= R4/ R3, in which case the voltage gain Av R,/ R1. The circuit can thus be made to give voltage gain if required.
FIG. 17 shows the amp can be made to act as a nonlinear (semi log) a.c. voltage amplifier by using a couple of ordinary silicon diodes as feedback elements. The voltage gain of the circuit depends on the magnitude of applied input signal, and is high when input signals are low, and low when input signals are high. The measured performance of the circuit is shown in the table, and can be varied by using alternative R, values.
FIG. 18 shows how the 741 can be used together with a junctiontype fieldeffect transistor (JFET) to make a socalled constantvolume amplifier. The action of this type of circuit is such that its peak output voltage is held sensibly constant, without distortion, over a wide range of input signal levels, and this particular circuit gives a sensibly constant output over a 30dB range of input signal levels. The measured performance of the circuit is shown in the table. C, determines the response time of the amplifier, and may be altered to satisfy individual needs.
ACTION TAKEN The action of the Fig. 18 circuit relies on the fact that the JFET can act as a voltagecontrolled resistance which appears as a low value when zero bias is applied to its gate and as a high resistance when its gate is negatively biased. The JFET and 13, act as a gaindetermining a.c. voltage divider (via C2), and the bias to the JFET gate is derived from the circuits output via the D,C, network. When the circuit output is low the JFET appears as a low resistance, and the opamp gives high voltage gain. When the circuit output is high the JFET appears as a high resistance, and the opamp gives low voltage gain. The output level of the circuit is thus held sensibly constant by negative feedback.
741 COOKBOOK CHOOSE YOUR FREQUENCY The 741 op amp can be made to function as a frequency selective amplifier by connecting frequencysensitive networks into its feedback loops. Fig. 19 shows how a twinT network can be connected to the opamp so that it acts as a tuned (acceptor) amplifier, and Fig. 20 shows how the same twinT network can be connected so that the op amp acts as a notch (rejector) filter. The values of the twinT network are chosen such that R2= R3= 2 X R 4 , and C2= C,/ 2, in which case its centre (tuned) frequency = 1 / 6.28 R2.C 2. With the component values shown, both circuits are tuned to approximately 1kHz.
Fig. 20 1 kHz notch (reject) filter. Fig. 22 Variable highpass filter, covering 235Hz to 2.8kHz. Finally, to complete this section, Figs. 21 and 22 show the circuits of a couple of variablefrequency audio filters. The Fig. 21 circuit is that of a lowpass filter which covers the range 2.2kHz to 24kHz, and the Fig. 22 circuit is that of a highpass filter which covers the range 235Hz to 2.8kHz. In each case, the circuit gives unity gain to signals beyond its cutoff frequency, and gives a 2nd order response (a change of 12dB per octave) to signals within its range. INSTRUMENTATION PROJECTS (Figs. 23 to 31) Figs. 23 to 31 show a variety of instrumentation projects in which the 741 can be used. The circuits range from a simple voltage regulator to a linear scale ohmmeter.
FIG. 23 shows the circuit of a simple variablevoltage power supply, which gives a stable output that is fully adjustable from 0V to 12V at currents up to a maximum of about 50mA. The operation of the circuit is quite simple. ZD, is a zener diode, and is energized from the positive supply line via R,. A constant reference potential of 12V is developed across the zener diode, and is fed to variable potential divider RV, The output of this divider is fully variable from 0V to 12V, and is fed  to the non inverting input of the opamp. The opamp is wired as a unitygain voltage follower, with Q, connected as an emitter follower current booster stage in series with its output. Thus, the output voltage of the circuit follows the voltage set at the op amp input via RV,. and is fully variable from 0V to 12V. Note that the circuit uses an 18V positive supply and a 9V negative supply. Also note that the voltage range of the above circuit can be increased by using higher zener and unregulated supply voltages, and that its current capacity can be increased by using one or more power transistors in place of Q. FIG. 24 shows how a 741 opamp can be used as the basis of a stabilized power supply unit ( P SU.) that covers the range 3V to 30V at currents up to 1A. Here, the voltage supply to the opamp is stabilized at 33V via ZD,, and a highly temperaturestable reference of 3V is fed to the input of the opamp via ZD 2. The opamp and output transistors Q1 Q2 are wired as a variablegain non inverting d.c. amplifier, with gain variable from unity to x10 via RV,, and the output voltage is thus fully variable from 3V to 30V via RV,. The output voltage is fully stabilized by negative feedback.
FIG. 25 shows how overload protection can be applied to the above circuit. Here, currentsensing resistor R , is wired in series with the output of the regulator. and cutout transistor Q 3 is driven from this resistor and is wired so that its basecollector junction is able to short the baseemitter junction of the Q1  Q2, output transistor stage. Normally. Q., is inoperative, and has no effect on the circuit, but when P.S.U. output currents exceed 1A a potential in excess of 600mV is developed across R, and biases C1, on, thus causing C1, to shunt the baseemitter Junction of the 0,C1, output stage and hence reducing the output current Heavy negative feedback takes place in this action, and the output current is automatically limited to 1A, even under shortcircuit conditions.
FIG. 26a shows how a 741 can be used in conjunction with a couple of silicon diodes as a precision half wave rectifier. Conventional diodes act as imperfect rectifiers of lowlevel a.c signals, because they do not begin to conduct significantly until the applied signal voltage exceeds a ' knee' value of about 600m V. When diodes are wired into the negative feedback loop of the circuit as shown the ' knee' voltage is effectively reduced by a factor equal to the open loop gain of the opamp, and the circuit thus acts like a near perfect rectifier The overall voltage gain of the Fig. 26a circuit is dictated by the ratios of R1 and R, to R3, as in the case of a conventional inverting amplifier, and this circuit thus gives a gain of unity The circuit can be made to act as a precision halfwave a.c. / d.c. converter by designing it to give a voltage gain of 2.22 to give formfactor correction, and by integrating its rectifier output, as shown in Fig. 26b. Note that each of the Fig. 26 circuits has a high output impedance, and the outputs must both be fed into loads haying impedances less than about 1M
FIG. 27 shows how opamp can be used as a highperformance d.c. voltmeter converter, which can be used to convert any 1V f.s.d. meter with a sensitivity better than 1k / V into a voltmeter that can read any value in the range 1mV to 10V f.s.d. at a sensitivity of 1M / V• The voltage range is determined by the R, value, and the table shows some suitable values for common voltage ranges. FIG. 28 shows a simple circuit that can be used to convert a 1 mA f.s.d. meter into a d.c voltmeter with any f.s.d. value in the range 100mV to 1000V, or into a d.c. current meter with any f.s.d. value in the range 1 uA to 1A. Suitable component values for different ranges are shown in the tables. Fig. 27 Highperformance d.c. voltmeter converter. Fig. 28 Simple d.c. voltage or current meter. FIG. 29 shows the circuit of a precision d.c. millivoltmeter, which uses a 1 mA f.s.d meter to read f.s.d. voltages from 1mV to 1000mV in seven switchselected ranges. FIG. 30 shows the basic circuit of a precision a.c. volt or millivolt meter. This circuit can be used with any movingcoil meter with a full scale current value in the range 100uA to 5mA, and can be made to give any full scale a.c. voltage reading in the range 1 mV to 1000mV. The tables show the alternative values of R, and R, that must be used to satisfy different basic meter sensitivities, and the values of R3 and R4 that must be used for different f.s.d. voltage sensitivities. HOME OHM Finally, to conclude, Fig. 31 shows how the 741 opamp can be used in conjunction with a 1mA f.s.d. meter to make a linear scale ohmmeter that has five decade ranges from 1k to 10M. The circuit is divided into two parts, and consists of a voltage generator that is used to generate a standard test ... Fig. 29 Precision d.c. millivoltmeter. Fig. 30 Precision a.c. volt/millivolt meter. Test Resistor ... voltage, and a readout unit which indicates the value of the resistor under test. The voltage generator section of .the circuit comprises zener diode ZD,, transistor Q,, and resistors R, to R,. The action of these components is such that a stable reference potential of 1V is developed across R,, but is adjustable over a limited range via RV,. This voltage is fed to the input of the opamp readout unit. The opamp is wired as an inverting d.c. amplifier, with the 1mA meter and RV, forming a 1V f.s.d. meter across its output, and with the opamp gain determined by the values of ranging resistors 13 5 to R, and by negative feedback resistor R. Since the input to the amplifier is fixed at 1V, the output voltage reading of the meter is directly proportional to the value of R., and equals full scale when R. and the ranging resistor values are equal. Consequently, the circuit functions as a linearscale ohmmeter. CALIBRATION The procedure for initially calibrating the Fig. 31 circuit is as follows: First, switch the unit to 10k range and fix an accurate 10kU resistor in the R. position. Now adjust RN/ , to give an accurate 1V across R., and then adjust RV, to give a precise full scale reading on the meter. All adjustments are then complete, and the circuit is ready for use. MISCELLANEOUS 741 PROJECTS The 741 opamp can be used as the basis of a vast range of miscellaneous projects, including oscillators and sensing circuits. Four such projects are described in this final section. FIG. 32 shows how the 741 opamp can be connected as a variablefrequency Wienbridge oscillator, which covers the basic range 150Hz to 1.5kHz, and uses a lowcurrent lamp for amplitude stabilization. The output amplitude of the oscillator is variable via RV, and has a typical maximum value of 2.5V r.m.s. and a t.h.d. value of 0.1%. The frequency range of the circuit is inversely proportional to the C1  C, values. The circuit can give a useful performance up to a maximum frequency of about 25kHz. Fig. 33 shows how either a 741 or a 709 opamp can be connected as a simple variablefrequency squarewave generator that covers the range 500Hz to 5kHz via a single variable resistor. (The circuit produces a good symmetrical waveform.) The frequency of oscillation is inversely proportional to the C, value, and can be reduced by increasing the C, value, or viceversa. The amplitude of the square wave output signal can be made variable, if required, by wiring a 10k variable potential divider across the output terminals of the circuit and taking the output from between the pot slider and the zero volts line.
FIGS. 34 and 35 show a couple of useful ways of using the 741 opamp in the openloop differential voltage comparator mode. In each case, the circuits are powered from singleended 12V supplies, and have a fixed half supply reference voltage applied to the noninverting opamp terminal via the R1  R, potential divider and have a variable voltage applied to the inverting opamp terminal via a variable potential divider. The circuit action is such that the opamp output is driven to negative saturation (and the relay is driven on) when the variable input voltage is greater than the reference voltage. Conversely, the opamp output is driven to positive saturation (and the relay is cut off) when the variable input voltage is less than the reference voltage.
FROSTY RECEPTION The Fig. 34 circuit is that of a precision frost or undertemperature switch, which drives the relay on when the temperature sensed by thermistor TH, falls below a value preset via RV,. The circuit action can be reversed, so that it operates as a fire or overtempera ture switch, by simply transposing the RV, and the TH, positions. In either case, TH, can be any negativetemperaturecoefficient thermistor that presents a resistance in the range 9009 to 9k9 at the required trip temperature.
LIGHT WORK The Fig. 35 circuit is that of a precision lightactivated switch, which turns the relay on when the illumination level sensed by light dependent resistor LDR exceeds a value preset by RV, The circuit action can be reversed so that the relay turns on when the illumination falls below a preset level by simply transposing the RV, and LDR positions. In either case, the LDR can be any cadmiumsulphide photocell that presents a resistance in the range 9009, to 9k9 at the desired switchon level.
