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Circuit Surgery Regular clinic by Ian Bell Op amps as comparators A n EEWeb forum post by user Maddan417 describes problems with a voltage comparator circuit using the LM741 op amp (see Fig.1), which does not operate as hoped. The circuit needs to switch at 1.2V for inputs ranging from 1V to 3.5V, but switches at 2.5V. Maddan417 is attempting to do this using an LM741 op amp, L7805 voltage regulators, a voltage divider, and a transistor. The 741 is supplied from a single 5V rail from the regulator, with the power source being an automotive alternator. Maddan417’s problems also included the op amp output not being 0V when the circuit has switched to a ‘low’ output state. Another contributor pointed out that the incorrect switching voltage is at least partly due to swapped or incorrectly calculated values for the resistors used to set the switching point. The values shown set the comparator threshold to 5 × 1kΩ/(1kΩ + 330Ω) = 3.76V, swapped they give 5 × 330Ω/(1kΩ + 330Ω) = 1.24V. The design used two regulators for the supply and potential divider, which is not necessary. The more fundamental problems are related to the use of the 741 – it is an op amp, not a comparator, its performance is poor, and it is used outside its recommended operating conditions in Maddan417’s circuit. In this article we will discuss use of the 741, and op amps in general, as comparators. The 741 Op amp evolution: (top) GAP/R K2-W: a vacuum-tube op amp from 1953; (below) uA741 IC op amp, first produced in 1967. (Images: Wikipedia) Practical Electronics | May | 2023 The 741 is a very old op amp – the uA741 was designed by Dave Fullagar of Fairchild in 1968. At the time, there were very few other op amps on the market. The 741 was preceded by the uA702 in 1963 and uA709 in 1965, also both from Fairchild. The uA702 was the first monolithic (single chip) IC op amp, previously op amps were modular solid-state devices using discrete transistors, or hybrid circuits using chip transistors, or combination of ICs and discrete components. Before that there were valve / vacuum tube op amps. If you would like a long, detailed read on op amp history try Chapter H: Op Amp History by Walt Jung from the +5V +5V R1 330Ω VIn – U1 LM741 + VOut R2 1kΩ Fig.1. Maddan417’s circuit. Op Amp Applications Handbook, 2005 – see: https://bit.ly/pe-may23-op1 Robert Widlar, the designer of the uA702 and uA709, moved to National Semiconductor and produced the LM101, which, like the LM741 is still in production. Dave Fullagar improved on the LM101 to produce the uA741, which quickly became very popular. As part of IEEE Spectrum’s ‘Chip Hall of Fame’ series of articles – https://bit.ly/pe-may23-op2 – you can see a letter from Fairchild’s marketing department in 1968 which concludes, ‘We’ve got a winner’. The various versions of the 741 have sold in hundreds of millions. The 741 deserves its place as an iconic electronic component, but its popularity and ubiquitousness in textbooks and online tutorials can lead to problems when inexperienced designers assume that it will be suitable in circuits where it isn’t. A common example of this is use at relatively low supply voltage circuits – the 741 datasheet (Texas Instruments, Rev D, 2015) specifies a recommended minimum supply voltage of ±10V. Maddan417’s circuit runs on 5V, so the 741 is well outside its recommended operating conditions in this application. Of course, the 741 will be OK in plenty of circuits where high performance and relatively low supply voltages are not required, and it is still likely to be found in commercial designs, many of which may have been around for a long time. On the other hand, the performance of the 741 is poor compared to more modern devices, 55 The maximum output voltage range (or output voltage swing) of an op amp is often specified relative to the supplies (under Differential – input voltage specific conditions). For VOut example, ‘within 1V of the VIn + Output voltage supplies’ would mean, on a Common-mode range: –9 to +9V input voltage ±10V supply, the output range range: –7 to +7V –10V would be ±9V (see Fig.2), on –VSupply a single 20V supply it would be +1 to +19V (see Fig.3). The Fig.2. Example op amp voltage ranges for split LM741’s specified output range supply. For this circuit, for normal linear operation, the is ±12V to ±14V on ±15V supply common-mode input voltage is equal to Vin. (ie, within 1 or 2V of the supply) but is worse for loads below 10kΩ. so there is relatively little to justify using An op amp does not experience any it in a new design, and many designers fundamental difference operating on a today would never consider it. However, split supply or a single supply of equal its relatively high maximum supply voltage magnitude (eg, ±10V split or single 20V). (44V/±22V), combined with low cost and An AC signal can have an offset applied well-understood behaviour are sometimes to it so that it varies around the mid-point cited as a reasons for using it. There are, of a single supply. Thus, to the op amp, however, plenty of other op amps which the conditions are the same for a ±5V operate at this, and higher voltages, so this output on a ±10V split supply, or a 5V to is not in itself a unique property. 15V output centred on 10V with a single 20V supply. However, there is often a Voltage ranges requirement to handle signals specifically There are a few voltage ranges to consider at, or close to 0V, which means that a when designing op amp circuits, and these single-supply circuit requires the op amp op amp specifications must be compatible to output signals at or close to the supply, with the operating requirements of the whereas a split supply does not. This is a circuit. We have already mentioned that problem for Maddan417’s single-supply the supply voltage range is a problem in circuit where there is a requirement to Maddan417’s circuit. The other voltage output 0V when the comparator output is range parameters are the output voltage low – the 741 is not able to achieve this. and input voltage ranges. Over the last two or three decades supply Op amps are often operated on split voltages have tended to reduce due to supplies – usually equal positive and the effects of advances in semiconductor negative supplies (see Fig.2). This enables technology. For example, op amps are the op amp circuit to easily handle AC often required in circuits together with signals (that have both positive and digital devices, such as microcontrollers, negative voltage excursions). A signal which require low supply voltages. It is varying around 0V is conveniently in the convenient to operate op amps on the middle of the supply range, which tends same voltage, where possible, so there is be where it is easiest to design op amp demand for lower voltage op amps. If an circuits to work effectively. As signals op amp can only handle signals to within get closer to (or even beyond) the supply 1V of the supply this is much more likely voltage range, op amp circuit design to be a problem with a 3.3V supply than becomes more challenging. For example, a 20V one. This, and the convenience of transistors may turn off, or potentially single-supply operation with outputs able damaging polarity reversals may occur, to go 0V, have led to the development of preventing correct operation. Many op op amps with ‘rail-to-rail’ outputs. amps, particularly earlier devices such Typically, for BJT op amps, rail-to-rail as the 741, are not designed to handle outputs can go to within a collectorsignals close to the supply. emitter saturation voltage (VCEsat) of the supply. VCEsat is +VSupply +20V dependent on the transistor’s collector current and hence the op amp’s output current. Differential – input voltage For moderate currents (in the VOut mA range) VCEsat is typically VIn + Output voltage Common-mode 100 to 300mV. Some op amp range: +1V to +19V input voltage designs take things further range: +3V to +17V and have internal circuits to generate voltages above and below the applied supply Fig.3. Example op amp voltage ranges for single supply. For this circuit, for normal linear operation, the voltage to facilitate full railto-rail signal ranges. common-mode input voltage is equal to Vin. +VSupply 56 +10V Input voltages Op amps have two inputs, so there are input voltage specifications relating to individual, differential and common-mode signals. Individual inputs have an absolute maximum voltage specification. For the 741 this is either the supply voltage or ±15V, whichever is lower. The differential voltage is the voltage between the inputs, again there is an absolute maximum rating for this, which is ±30V for the 741. In most op amp applications, the op amp is used as an amplifier with feedback. This keeps the inputs at more or less the same voltage, so differential input limitations are not an issue for normal operation. However, if used as a comparator, or if a linear amplifier is pushed into saturation by a large input signal, then larger differential inputs can occur. The common-mode input voltage is the average of the voltage on the two inputs. For non-inverting amplifiers and similar configurations (see Fig.3) the commonmode input is equal to the input voltage (remember the two inputs are at about the same voltage). Common-mode input range may also be specified relative to the supplies – for example, within 3V of the supplies, as shown in Fig.3. As with output ranges, some op amps have rail-torail common-mode input capability. The common-mode input range figure is not always quoted for the 741, but the Harris CA741 datasheet (1993) states ±12V for ±15V supplies, indicating within 3V of the supplies. Op amps as comparators Op amps can be used as comparators, but there are dedicated comparator chips which perform this function much better. In general, a dedicated comparator should always be used, but in some cases, for example where the comparator requirements are not demanding and/or a spare op amp is available on a multiop-amp package then they can be used. The following discussion will highlight the differences between op amps and comparators. A comparator’s output will typically switch between the positive and negative supply voltages (or ground and supply in single-supply circuits), going rail-to-rail, or sufficiently close to be easily interpreted as a logic 0 or 1, or to switch a transistor or other load effectively. For similar behaviour from an op amp, a rail-to-rail output device must be used, otherwise the problems observed with Maddan417’s circuit will occur. Comparators have no need to output intermediate voltages; however, the output may switch (or it may be possible to arrange for it to switch) to a different voltage from the main comparator supply to facilitate interfacing to logic circuits, or for switching loads on different Practical Electronics | May | 2023 Input Overdrive VRef tpd Output VOH 90% 50% VOL 10% tr Time Fig.4. Comparator propagation delay. supplies. This is more difficult to achieve with op amps. Often, comparator output circuits are designed to be easy to interface with specific types of logic. Comparators are therefore available with a variety of output configurations including push-pull, open drain, open collector and LVDS (lowvoltage differential signalling). Open drain and open collector require an external (pull-up) resistor connected from the output to the positive (digital) supply. Op amps are designed to be used with negative feedback – a ‘closed-loop’ system. All amplifiers have some delay from input to output, which results in increasing phase shift as signal frequency increases. At some point the phase shift reaches 180°, at which point the negative feedback network is actually delivering positive feedback. If the gain of the amplifier and feedback network together is greater than one at this frequency then oscillation will occur. The gain of most op amps is deliberately rolled off as frequency increases to prevent this instability – this is called compensation. Comparators are either used in ‘open-loop’ mode, or with positive feedback, so compensation is not required, leading to significant differences between the two types of devices. Op amps are high-gain, linear, differential amplifiers; so, as already noted, in normal operation the voltage difference between an op amp’s inputs is very small (typically microvolts to millivolts). Comparators often have much larger input differences. Not all op amps can tolerate large input voltage differences and they perform very poorly under such conditions. Op amp input impedance may drop significantly for large input differences due to conduction of protection diodes – this could upset circuits driving an op amp used as a comparator. Comparators are commonly used to compare voltages which are not close to half the supply range. For an op amp, this is a large common-mode input voltage. Again, not all op amps perform well under such conditions. If the common-mode input range is exceeded correct operation may not occur. Gain and offset are characteristics shared by op amps and comparators, however, the Practical Electronics | May | 2023 Fig.5. Configuring opamp2 to use the lm741 subcircuit. switching behaviour of comparators means that they have characteristics related to switching which are not relevant to the standard analogue amplifier usage of op amps. The switching characteristics are illustrated in Fig.4. Speed of switching When the comparator input voltage crosses the reference voltage the comparator output will switch. This will not happen instantaneously – the time taken for the comparator output to reach 50% of the resulting voltage change is the propagation delay (tpd). The time taken for the comparator output voltage to rise from 10% to 90% of its range is the rise time (tr). The amount of voltage applied to the comparator’s input beyond the switching threshold (reference voltage) is known as the overdrive. Propagation delay and rise time are usually sensitive to overdrive, with increasing overdrive resulting in faster switching times. Comparator speed is also usually dependent on supply voltage. The maximum rate of change of output voltage an op amp or comparator can deliver is the slew rate. Slew rate is important for op amps because it indicates how well the output voltage will track fastchanging analogue waveforms; failure to do so causes distortion. Slew rate also directly determines the maximum frequency at which an op amp can produce a pure sinewave at full output swing (the full power bandwidth), however, sinewave output is of no relevance to comparators. For any circuit used as a comparator, either the slew rate or the bandwidth may be the dominant factor in determining the propagation delay. Because comparators are just required to switch their outputs quickly the slew rate itself is not usually very important as a specification, it is the propagation delay and rise time which are quoted. The compensation applied to op amps tends to reduce their slew rate, making them relatively slow when used as comparators. Op amps are designed for applications where the output voltage does not go hard to the supply rails – this would normally imply clipping of the waveform and hence distortion. When op amp outputs are driven hard into saturation they tend to be slow to recover. Like compensation, this makes op amps poor comparators where fast switching is required. The internal circuitry of comparators is different, allowing them to recover very quickly. A further subtlety to this is that op amp saturation recovery time is likely to vary between individual devices, making the propagation delay somewhat unpredictable. Simulating the 741 We can simulate a 741 comparator circuit and compare it with a dedicated comparator using LTspice. However, this takes some effort as there is no 741 model provided as part of the LTspice download. If you search online for ‘741 SPICE model’ you will find a link on the Texas Instruments site for a ‘LM741 PSPICE Model’ (note Pspice, not LTspice): https://bit.ly/pe-may23-741 The download provides an lm741.lib file, which is a SPICE subcircuit definition. This code can be used, together with an appropriate associated symbol file to allow the model to be included in an LTspice schematic. Do be aware that although Fig.6. Entering the .lib directive. 57 Fig.9. New folder in the component selector. Fig.7. Symbol Attribute Editor for a component in the library. MyComponents). Copy (not move!) the opamp2.asy symbol file from the Opamps subfolder of the existing sym folder to your new subfolder and rename the new file lm741.asy. Also create another new folder for the model file – a subfolder of …\Documents\LTspiceXVII is appropriate, with a name like MySub (the folder with the original LTspice model files is called Sub). Put the downloaded lm741.lib file in your new folder. Go to you your symbol folder and double-click the lm741.asy file to open it in the LTspice symbol editor. Alternatively, open LTspice and do File > Open. In the file open dialog change the file type to Symbols (*.asy) and open the lm741.asy file. Then, in LTspice open the symbol attribute editor from the menu using: Edit > Attributes > Edit Attributes Fig.8. Configuring the LM741 symbol attributes. SPICE is a de facto standard, and hence models are often compatible, sometimes it is necessary to edit models created for a different SPICE tool. Fortunately, in this case, the model works with LTspice. The 741 model can be used as a one-off, or added to the LTspice library so that it can be used like other components. For one-off use, place the downloaded lm741.lib file in the folder to be used for the circuit simulation files. Create a new schematic in the same folder, using the opamp2 component (from the Opamps part of the component selector) where you need a 741. Right click on the symbol and change the Value attribute to lm741 (see Fig.5). Then click on the SPICE Directive (.op) button and enter the .lib command with the full path to the lm741.lib file (see Fig.6), for example: Referring to Fig.7, in the Symbol Attribute Editor change the Value attribute to LM741 and insert the full path to the lm741.lib file as the Modelfile attribute value. You can also add a description if you want (see Fig.8). Save the symbol and close LTspice. Open LTspice and create a new schematic. When you add a component, the new folder in the symbol library should be listed (See Fig.9). In this folder you will find the LM741 (see Fig.10), which can be added to the schematic in the usual way. Note that the Description attribute is displayed in the component selector along with the symbol. LT1018 vs LM741 simulation Fig.11 shows an LTspice schematic for comparing the 741 with a comparator. The comparator is an LT1018, which is .lib D:\LTSpice\Comparator741\lm741.lib (Do remember to use the path to the file on your computer.) Click to place the directive text on the schematic. Adding components to the LTspice library It is possible to create a new folder in the LTSpice library that will appear on the list of components when editing schematics. Placing symbol files in this folder and associating these with .lib SPICE model files downloaded from manufacturers will make those components available for use in any simulation. The following instructions describe what to do for the 741, but other models can be added using this procedure. Find the location of the LTspice symbols library (sym folder) on your computer (eg, C:\Users\username\Documents\LTspiceXVII\lib\ sym) and create a new subfolder here with a name of your choice (eg, 58 Fig.10. LM741 in the component selector. Practical Electronics | May | 2023 Top to bottom: Fig.11. LTspice simulator schematic for comparison of an LT1018 comparator with an LM741 op amp when it is used as a comparator. Fig.12. Results from simulation circuit shown in Fig.11. Fig.13. Results from simulation circuit shown in Fig.11 (zoom in on output switching). Input threshold is crossed at 6ms. available in the LTspice library. The output stage of the LT1018 includes a pull-up current source, eliminating the need for an external resistor, which is commonly needed with similar comparators like the LM393. The simulation uses both devices in a basic comparator configuration on a single supply, which is 20V to correspond with the LM741 recommended minimum. The comparator reference is at the centre of the supply range. This provides conditions which should not take the 741 out of its recommended operating conditions. This doesn’t match Maddan417’s circuit, but we can’t be confident that the 741 model works well outside the recommended conditions. The results of the simulation are shown in Fig.12, and Fig.13 shows a zoom in on the output switching. These shows that the output of the LM741 only gets to within about ±1V of the supplies – as described by Maddan417 and discussed above. The LT1018’s output goes to values very close to the supplies. This again highlights the fact that interfacing an op amp output to logic or load switching is often more difficult than with a comparator device. The simulation (Fig.13) also shows the LM741 op amp responds much more slowly than the LT1018 comparator, as discussed above. Finally, Fig.12 show the current into the non-inverting inputs of the two devices. The current into the LT1018 is orders of magnitude lower than for the 741. In general, old op amp designs have much higher input currents than modern devices. This may be very significant in some applications. Op amps can be used as comparators, but not without difficulties, and only in relatively slow-speed applications – the op amp’s datasheet should be consulted to make sure that it is suitable for the application to avoid problems such as those experienced by Maddan417. Simulation files Most, but not every month, LTSpice is used to support descriptions and analysis in Circuit Surgery. The examples and files are available for download from the PE website. Practical Electronics | May | 2023 59