Operational Amplifier BasicsThe op-amp is basically a differential amplifier having a large voltagegain, very high input impedance and low output impedance. Theop-amp has a "inverting" or (-) input and "noninverting" or (+) inputand a single output. The op-amp is usually powered by a dual polaritypower supply in the range of +/- 5 volts to +/- 15 volts.
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Operational Amplier (Op-Amp) BasicsThe op-amp is basically a
dierential amplier having a large voltagegain, very high input
impedance and low output impedance. Theop-amp has a "inverting" or
(-) input and "noninverting" or (+) inputand a single output. The
op-amp is usually powered by a dual polaritypower supply in the
range of +/- 5 volts to +/- 15 volts. A simple dualpolarity power
supply is shown in the gure below which can beassembled with two 9
volt batteries.Inverting Amplier:The op-amp is connected using two
resistors RA and RB such that theinput signal is applied in series
with RA and the output is connectedback to the inverting input
through RB. The noninverting input isconnected to the ground
reference or the center tap of the dualpolarity power supply. In
operation, as the input signal movespositive, the output will move
negative and visa versa. The amount ofvoltage change at the output
relative to the input depends on theratio of the two resistors RA
and RB. As the input moves in onedirection, the output will move in
the opposite direction, so that thevoltage at the inverting input
remains constant or zero volts in thiscase. If RA is 1K and RB is
10K and the input is +1 volt then therewill be 1 mA of current
owing through RA and the output will haveto move to -10 volts to
supply the same current through RB and keepthe voltage at the
inverting input at zero. The voltage gain in thiscase would be
RB/RA or 10K/1K = 10. Note that since the voltage atthe inverting
input is always zero, the input signal will see a inputimpedance
equal to RA, or 1K in this case. For higher inputimpedances, both
resistor values can be increased.Noninverting Amplier:The
noninverting amplier is connected so that the input signal
goesdirectly to the noninverting input (+) and the input resistor
RA isgrounded. In this conguration, the input impedance as seen by
thesignal is much greater since the input will be following the
appliedsignal and not held constant by the feedback current. As the
signalmoves in either direction, the output will follow in phase to
maintainthe inverting input at the same voltage as the input (+).
The voltagegain is always more than 1 and can be worked out from
Vgain = (1+RB/RA).Voltage Follower:The voltage follower, also
called a buer, provides a high inputimpedance, a low output
impedance, and unity gain. As the inputvoltage changes, the output
and inverting input will change by anequal amount.
2nd Order Opamp FiltersThe gures below illustrate using opamps
as active 2nd order lters.Three 2nd order lters are shown, low
pass, high pass, andbandpass. Each of these lters will attenuate
frequencies outsidetheir passband at a rate of 12dB per octave or
1/4 the voltageamplitude for each octave of frequency increase or
decrease outsidethe passband.First order low or high pass cuto
frequency (-3dB point) =1/(2pi*R*C)2nd order low or high pass cuto
frequency (-3dB point) =1/2pi(R1*R2*C1*C2)^.5Example for 200 Hz
cuto frequency - R1=R2=7.95K, C1=C2=0.1uF
Single Op-Amp Bandpass FilterA bandpass lter passes a range of
frequencies while rejectingfrequencies outside the upper and lower
limits of the passband. Therange of frequencies to be passed is
called the passband and extendsfrom a point below the center
frequency to a point above the centerfrequency where the output
voltage falls about 70% of the outputvoltage at the center
frequency. These two points are not equallyspaced above and below
the center frequency but will look equallyspaced if plotted on a
log graph. The percentage change from thelower point to the center
will be the same as from the center to theupper, but not the
absolute amount. This is similar to a musicalkeyboard where each
key is separated from the next by the samepercentage change in
frequency, but not the absolute amount.The lter bandwidth (BW) is
the dierence between the upper andlower passband frequencies. A
formula relating the upper, lower, andcenter frequencies of the
passband is:Center Frequency = Square Root of (Lower Frequency *
UpperFrequency)The quality factor, or Q of the lter is a measure of
the distancebetween the upper and lower frequency points and is
dened as(Center Frequency / BW) so that as the passband gets
narroweraround the same center frequency, the Q factor becomes
higher. Thequality factor represents the sharpness of the lter, or
rate that theamplitude falls as the input frequency moves away from
the centerfrequency during the rst octave. As the frequency gets
more thanone octave away from center frequency the rollof
approaches 6 dBper octave regardless of Q value. Approximate rollo
rates fordierent Q values for a single octave change from center
frequencyare:Q = 1 = 6 dBQ = 5 = 18 dBQ = 10 = 24 dBQ = 50 = 40
dBFor a single op-amp bandpass lter with both capacitors the
samevalue, the Q factor must be greater than the square root of
half thegain, so that a gain of 98 would require a Q factor of 7 or
more.The example below shows a 1700 Hz bandpass lter with a Q of
8and a gain of 65 at center frequency (1700 Hz). Resistor values
forthe lter can be worked out using the three formulas below.
Bothcapacitor values need to be the same for the formulas to work
andare chosen to be 0.01uF which is a common value usable at
audiofrequencies. This same lter is used in the "Whistle On /
Whistle O"relay toggle circuit.
Low Power Op-Amp - Audio Amp (50 milliwatt)The example below
illustrates using an op-amp as an audio amplierfor a simple
intercom. A small 8 ohm speaker is used as amicrophone which is
coupled to the op-amp input through a 0.1uFcapacitor. The speaker
is sensitive to low frequencies and the smallvalue capacitor serves
to attenuate the lower tones and produce abetter overall response.
You can experiment with dierent valuecapacitors to improve the
response for various speakers. The op-ampvoltage gain is determined
by the ratio of the feedback resistor to theseries input resistor
which is around one thousand in this case (1Meg / 1K). The
non-inverting input (pin 3) to the op-amp is biased at50% of the
supply voltage (4.5 volts) by a couple 1K resistorsconnected across
the supply. Since both inputs will be equal whenthe op-amp is
operating within it's linear range, the voltage at theinverting
input (pin 2) and the emitter of the buer transistor(2N3053) will
also be 4.5 volts. The voltage change at the emitter ofthe
transistor will be around +/- 2 volts for a 2 millivolt change at
theinput (junction of 0.1 cap and 1K resistor) which produces a
currentchange of about 2/33 = 60 mA through the 33 ohm emitter
resistorand the speaker output. The peak output speaker power is
about I^2* R or .06 ^2 * 8 = 28 milliwatts. The 100 resistor and
47uFcapacitor are used to isolate the op-amp from the power supply
andreduce the possibility of oscillation. An additional 22uF cap is
used atthe non-inverting input to further stabilize operation.
These partsmay not be needed in such a low power circuit but it's a
good idea todecouple the power supply to avoid unwanted feedback.
The circuitdraws about 1.2 watts from a 9 volt source and is not
very eicientbut fairly simple to put together. The circuit was
tested using acouple 4 inch speakers located a few feet apart (to
reduce feedback)
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