Basic Op Amps
The operational amplifier (Op Amp) is a staple item in electronic circuits andis a building block that often is one of the main components in linear audio andvideo circuitry. The op amp is basically a high gain amplifier that is used inconjunction with feedback networks to make up a circuit whose properties aredetermined by linear passive components, such as resistors, capacitors, inductors, aswell as nonlinear components (diodes, varistors, thermistors, etc). The term“operational amplifier” comes from the use of these devices in analog computersthat were used decades ago to perform mathematical operations (addition,multiplication, differentiation, integration, summation, etc) on input quantities. Theterm has stuck and is still used, even though analog computers have largelydeparted the scene, having been replaced by digital computers long ago. Theoperational amplifier of today is a sophisticated device, being composed of manytransistors, diodes, and resistors, all in a chip, and packaged in variousconfigurations. There are thousands of types of op amps available, from fleapowered microwatt units to units capable of handling a few hundred watts of power,from a few cents to many dollars in cost. As you may imagine, the specs andperformance requirements, as well as reliability, temperature range, and packaging,all affect cost. Op amps that can do many ordinary jobs very well are available forunder 50 cents, owing to low cost plastic packages and large scale integration, andhigh volume production. Technologies commonly used are bipolar, FET, CMOS andcombinations. Some large or high power op amps are made using monolithicfabrication methods.From a circuit viewpoint, for the purposes of explanation, an ideal amplifieris used to represent an op amp. An ideal amplifier has the following properties:Infinite forward gain, bandwidth and input impedance, with zero outputimpedance, noise voltage, DC offset, bias currents, and reverse gain. Inpractice, all op amps have some bias current that flows in the inputs, this beingalmost negligible for JFET and CMOS types, but more significant in bipolar types.This current must be considered in high impedance circuits, and in DC andinstrumentation amplifiers, and in circuits that must operate over a widetemperature range. In addition, even if you were to short the op amp inputs togetheryou may not get zero output voltage, but some random DC level. This DC voltagecan be considered as an equivalent DC input offset voltage present at the input. DCoffset can also be produced from equal input bias currents flowing through unequalresistances in the inverting and noninverting input circuits. This will produce a DCinput voltage differential at the input. Some op amps have external pins to which apotentiometer can be connected to balance out or otherwise cancel this voltage,bringing the DC output to zero under zero signal input conditions. These are widelyused in instrumentation amplifiers and related applications where nulling or zeroadjustments are required. All amplifiers generate some noise, which is due tothermal and semiconductor junction effects, and can be considered as an equivalentinput noise voltage. Amplifiers are available with low noise characteristics for thoseapplications where noise must be kept to a minimum. A real world op amp has a lotof gain (>1000X voltage gain) and a fairly high input impedance (>100K). Generallythere are two inputs shown, an inverting and a non inverting input, and one outputreferenced to ground (but not always, differential outputs are sometimes used incertain applications). One of the inputs may be grounded in many commonapplications where a single ended signal source is present. This is a commonsituation. There are limitations on the DC levels allowable on the inputs, andlimitations on the available output voltage swing. Op amps are available that allow afull output voltage swing between the positive (Vcc) supply and negative (Vdd)supply. These are sometimes referrred to as “rail to rail” capable. In addition, if theexact same voltage is present on the inverting and non-inverting inputs, ideally theoutput voltage should be zero. This is not always so, and the degree of imperfectionis called the common mode rejection ratio. This is usually 60 dB or better, with 70-80 dB as a minimum. Note that this may vary with input voltage levels to somedegree. Also, variations of power supply voltage may show up as equivalent inputsignals. The degree to which the op amp rejects this is called the supply voltagerejection ratio. It is usually better than 60 dB and typically 70 to 80 dB or better.After all, nothing is perfect in life.Op amp power supply connections are sometimes shown in diagrams, especially ifdecoupling capacitors and resistors are necessary, but more often shown elsewherein the schematic, as they play no part in the primary circuit function other than topower the amplifier. Many general-purpose op amp chips have two or four separateoperational amplifiers in one package, with common power supply connections. Inpractice the ideal amplifier criteria requirements are met only approximately, butas will be shown, close enough for most purposes. Practically, an op amp will have again of 10,000 or more, an input impedance of megohms, and a 3 dB bandwidth ofseveral tens of hertz or more. If an amplifier has a 3 dB bandwidth of 40 Hz and again of 100,000 times, this is a gain bandwidth product of 4 million hertz, or 4 MHz.(40 x 100,000). It is advantageous in many feedback applications to have the gainfalling at 6 dB per octave or 20 dB per decade at frequencies beyond the cornerfrequency (that frequency at which the amplifier gain has fallen 3 dB or 70.7percent of its DC value). Since the op amp is used in mainly in feedback circuitshaving much lower closed loop gain, these performance figures are good enough inmany cases. In fact, even a single high gain (100X) common emitter transistoramplifier stage can be treated as an op amp if feedback is employed, withsurprisingly little error. In many cases a single transistor will work almost as well asa more expensive op amp device. One example is a simple audio amplifier stagefrom which a moderate gain (5-20X) is required. This will be shown in an examplelater.One of the most popular op amps of all time is the venerable LM741, its dualversion LM747 and their many descendents. The JFET input TLO8X series is alsovery popular, coming in single (TLO81), double (TLO82) and quadruple (TLO84)units. The TLO81 and TLO82 come in 8 pin DIP packages, while the TLO84 comesin a 14-pin DIP package. These op amps operate well from 5 to 12 volt experimentersupplies, and require both a plus and a minus supply. These are also cheap andwidely available. Other general purpose types are the LM324 and LM1458 (bipolar)and LM3900, and all their variations and flavors. There are many others, but thesetypes mentioned are easily obtained by the hobbyist wishing to experiment withthem, and are cheap and in plentiful supply. Many manufacturers make them, soobsolescence should not be a problem for a long time. We will use the TLO8X seriesfor circuit examples, as they are general purpose JFET types, allowing the use ofhigher resistance values and therefore smaller capacitor values, which is often moreconvienient from a design standpoint. The TLO8X series have an open loop (nofeedback used, the full gain the amp can deliver) voltage gain of over 10000 andhaving JFET inputs, an input impedance of a million megohms. The gain bandwidthproduct (obtained by measuring frequency where gain falls to unity) is rated at at 4MHz for the TLO8X series. Op amps are available with gain bandwidth products toseveral hundred MHz and even higher, and these are used in video and RF applications.