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Module 6.4
Op amp = Characteristics
=20 =20- After studying this section, you = should be able=20 to:
- Understand commonly published op amp characteristics.
- =E2=80=A2 Power suppliy requirements.
- =E2=80=A2 Open loop voltage gain.
- =E2=80=A2 Large signal voltage gain.
- =E2=80=A2 Gain bandwidth product.
- =E2=80=A2 Input offset current.
- =E2=80=A2 Maximum differential input.
- =E2=80=A2 Input resistance, Temperature = coefficient.
- =E2=80=A2 Slew rate, Power = bandwidth.
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Supply Voltage
Two types of supply are used for op amps, the dual and single supply. = Many op=20 amps, especially older types use a dual supply (+VS and=20 -VS) often in the 12 to 18V range. This allows a zero = diffence=20 between the input terminals to produce a 0V output and an output signal = to swing=20 both positive and negative with respect to ground.
Single voltage supplies have grown in popularity with the increase = in=20 portable (battery operated and/or automotive) devices where dual = supplies, using=20 multiple batteries are more expensive to implement. A zero difference = between=20 the input terminals on these devices will produce an output at half = supply,=20 allowing an output signal to swing between supply and ground.
Frequency Response
An important characteristic of any amplifier is its frequency = response, in=20 the ideal amplifier its frequency response should be infinite, it will = amplify=20 any and all frequencies equally. In practical amplifiers this is=20 difficult/impossible to achieve, and not always desirable, but op amps = have=20 extremely wide, and easily variable bandwidths.
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Fig.6.4.1 Typical Op Amp Frequency Response
Open Loop Voltage Gain
Fig. 6.4.1 shows the frequency response of a typical op amp (LMC660), = which=20 confirms that the open loop gain (with no feedback) at very low = frequencies is=20 huge. When Open loop Gain is quoted it refers to the maximum AC gain at = very low=20 frequencies.
It can be seen from Fig. 6.4.1 that the LMC660 for example has an = open loop=20 voltage gain of about 126dB (a voltage gain of nearly 2 million), but at = frequencies above a few Hz, gain begins to fall rapidly at 20dB/decade = until, at=20 1.4MHz the gain has reduced to 0dB, a voltage gain of x1.
Large Signal Voltage Gain
The large signal voltage gain is usually quoted in preference to the = open=20 loop voltage gain. This is the open loop voltage gain measured at DC = with the=20 amplifier producing a large (just less than maximum) voltage output, = usually=20 quoted in V/mV. Figures for large signal voltage gain can cover a wide = range for=20 a given op amp, depending on design variant and factors such as minimum = or=20 maximum operating temperature.
Closed Loop Voltage Gain
In practise the huge gain of an op amp is greatly reduced by applying = an=20 appropriate amount of negative=20 feedback. In this way an impressively level response can be = achieved,=20 extending from DC (0Hz) to any frequency up to about 1MHz or more, as = well as=20 the added benefits of reduced noise and distortion. The blue dotted line = shows=20 the response of the op amp with negative feedback. The gain has been = reduced to=20 20dB, a closed loop voltage gain (Acl) of x10, which has = produced a=20 flat response from 0Hz to about 140kHz.
Gain Bandwidth Product
As the closed loop gain and the small signal bandwidth of an op amp = are=20 closely related, the parameter =E2=80=98Gain Bandwidth=20 Product=E2=80=99 is often used to better describe the possible = combinations of=20 gain and bandwidth. The graph of the open loop frequency response in = Fig. 6.4.1=20 therefore, plots all those points where voltage gain x bandwidth =3D 1.4 = million(Hz). For example, 140kHz bandwidth multiplied by a voltage gain = of 10=20 also gives a Gain Bandwidth Product of:
10 x 140kHz =3D = 1.4MHz
Note that bandwidth indicated by the Gain Bandwidth Product applies = to small=20 signals, but when large AC signals are involved, especially signals with = fast=20 rising and falling edges, the bandwidth may be further reduced by the Slew=20 Rate. Then the Power=20 Bandwidth becomes more relevant.
Maximum Differential Input
This is the maximum voltage that can be applied between the two = inputs, on=20 some devices this can be equal to the supply voltage, but on others it = can be=20 considerably less.
Input Resistance
This is the resistance looking into the input terminals with the = amplifier=20 operating without feedback (open loop). Typical resistances for bipolar = devices=20 are in the range of 1M=CE=A9 to 10M=CE=A9. For FET and CMOS types, = resistances are much=20 higher, and range up to 1012=CE=A9 or more.
Input Offset Current
The currents flowing into the two inputs should ideally both be zero, = but for=20 practical op amps, although the input currents are still extremely = small, they=20 do exist and may also be different. Unequal currents cause different = voltages at=20 the inputs, and when this small difference in voltage is amplified, it = causes=20 the output to be other than zero. To overcome this effect an Input=20 Offset Voltage can be applied between the inputs to correct the = output=20 voltage to zero. Typical values for bipolar op amps would be =C2=B11mV = ranging up to=20 15mV for FET types.
Temperature Coefficients
Both the input offset current and input offset voltage are affected = by=20 changes in temperature, and tend to drift higher as temperature = increases. The=20 temperature coefficient of input offset current is measured in nA or pA = / =C2=B0C=20 while the temperature coefficient of input offset voltage is usually = measured in=20 =C2=B5V/=C2=B0C.
=20Slew Rate
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Fig.6.4.2 Output Compensation
The Slew Rate of an op amp describes how fast the output voltage can = change=20 in response to an immediate change in voltage at the input. The higher = the value=20 (in V/=C2=B5s) of slew rate, the faster the output can change and the = more easily it=20 can reproduce high frequency signals.
If a square wave is applied to the input of the op amp, the output = should=20 also be a square wave. However the fast rising and falling edges of the = square=20 wave can tend to cause the amplifier to oscillate for a short time after = the=20 rise or fall. To prevent this effect, the op amp=E2=80=99s internal = circuitry contains a=20 small amount of compensation capacitance that slows down the rate of = change by=20 acting as a CR= =20 time constant so that very fast transient voltages do not trigger=20 oscillation, but this compensation also limits the slew rate of the op = amp, as=20 shown in Fig. 6.4.2.
In some op amps, because this compensation is internal, there is no = way of=20 altering the slew rate, but others use an external compensation = capacitor, and=20 therefore provide the means to control slew rate to some degree.
The slew rate also affects sine = wave (and=20 audio) signals as well as square waves. The rate of change of voltages = in a sine=20 wave is continually varying, it is changing at its fastest rate as the = signal=20 voltage crosses zero, and the rate of change falls momentarily to zero = (no=20 change) at both the positive and negative peaks of the wave. If the slew = rate of=20 the amplifier cannot keep up with the fastest rate of change of the = signal, some=20 distortion will be produced. Therefore, to be sure of amplifying large = amplitude=20 signals that are most likely to produce large (and fast) rates of = voltage=20 change, an op amp needs to have a sufficiently high value of slew rate = to cope=20 with the greatest possible rate of voltage change. If the largest = possible=20 voltage swing and the highest frequency of the signal are known, the = minimum=20 required slew rate for the op amp can be calculated using the = formula:
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Where f =3D the highest = signal=20 frequency (Hz) and Vpk =3D the maximum peak voltage of the = signal.
For example if an op amp is to amplify a signal with a peak amplitude = of 6=20 volts at a frequency of 40kHz, an op amp with a slew rate of at least = 2=CF=80 x 40=20 exp3 x 6 =3D 1.5V/=C2=B5s would be required.
=20Power Bandwidth
Once the slew rate calculated as above for the large signal becomes = equal or=20 greater than the amplifier=E2=80=99s slew rate, =E2=80=98Slew Rate = Limiting=E2=80=99 starts to occur,=20 causing reduced gain and distortion of the signal. The highest frequency = that=20 can be used to amplify a full amplitude signal before =E2=80=98Slew Rate = Limiting=E2=80=99 is=20 the highest frequency limit of the Power=20 Bandwidth. For example, in an op amp operating from a =C2=B115V = supply the=20 power bandwidth would be specified as the frequency range in which a = =C2=B110V swing=20 can be measured at the output with a total harmonic distortion of less = than 5%.=20 The Power Bandwidth is usually less than the small signal bandwidth = indicated by=20 the graph=20 of the closed loop gain, and in many cases the major factor in = specifying=20 the amplifier bandwidth.
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