|Internal Frequency Compensation|
|Short Circuit Protection|
|Offset voltage null capability|
|Excellent temperature stability|
|High input voltage range|
- Input Offset Voltage (Voi)
This is the voltage that
must be applied to one of the input pins to give a zero output voltage.
Remember, for an ideal op-amp, output offset voltage is zero!
- Input Bias Current (Ib)
This is the average of the
currents flowing into both inputs. Ideally, the two input bias currents are
- Input Offset Current (Ios)
This is the difference of
the two input bias currents when the output voltage is zero.
- Input Voltage Range (Vcm)
The range of the
common-mode input voltage (i.e. the voltage common to both inputs and
- Input Resistance (Zi)
The resistance 'looking-in' at
either input with the remaining input grounded.
- Output Resistance (Zoi)
The resistance seen 'looking
into' the op-amp's output.
- Output Short-Circuit Current (Iosc)
This is the
maximum output current that the op-amp can deliver to a load.
- Output Voltage Swing (Vo max)
Depending on what the
load resistance is, this is the maximum 'peak' output voltage that the op-amp
can supply without saturation or clipping.
- Open-Loop Voltage Gain (Aol)
The output to input
voltage ratio of the op-amp without external feedback.
- Large-Signal Voltage Gain
This is the ratio of the maximum
voltage swing to the charge in the input voltage required to drive the ouput
from zero to a specified voltage (e.g. 10 volts).
- Slew Rate (SR)
The time rate of change of the ouput voltage with
the op-amp circuit having a voltage gain of unity (1.0).
- Supply Current
This is the current that the op-amp will draw
from the power supply.
- Common-Mode Rejection Ratio (CMRR)
A measure of the ability of
the op-amp' to reject signals that are simultaneously present at both inputs.
It is the ratio of the common-mode input voltage to the generated output
voltage, usually expressed in decibels (dB).
- Channel Seperation
Whenever there is more than one op-amp in
a single package, like the 747 op-amp, a certain amount of "crosstalk" will be
present. That is, a signal applied to the input of one section of a dual
op-amp will produce a finite output signal in the remaining section, even
though there is no input signal applied to the unused
Open-Loop Gain &
the ideal op-amp (Fig. 5-1), the op-amp that is used in more realistic circuits
today, does not have infinite gain and bandwidth. Look at Open-loop gain in Fig.
4 above, it is graphed for a type 741 op-amp as a function of frequency. At very
low frequencies, the open-loop gain of an op-amp is constant, but starts to
taper off at about 6Hz or so at a rate of -6dB/octave or -20dB/decade (an
octave is a doubling in frequency, and a decade is a ten-fold increase in
frequency). This decrease continues until the gain is unity, or 0 dB. The
frequency at which the gain is unity is called the unity gain frequency or
Maybe the first factor in the consideration of a specific op-amp is its
"gain-bandwidth product" or GBP. For the response curve of Fig. 4,
the product of the open-loop gain and frequency is a constant at any point on
the curve, so that: GBP =
Graphically, the bandwidth is the point at which the
closed-loop gain curve intersects the open-loop curve, as shown in Fig. 5 for a
family of closed-loop gains. For a more practical design situation, the actual
design of an op-amp circuit should be approximately 1/10 to 1/20 of the
open-loop gain at a given frequency. This ensures that the op-amp will function
properly without distortion. As an example, using the response in Fig. 4, the
closed-loop gain at 10Khz should be about 5 to 10, since the open-loop gain is
One additional parameter is worth mentioning, the Transient
Response, or rise time is the time that it takes for the output signal
to go from 10% to 90% of its final value when a step-function pulse is used as
an input signal, and is specified under close-loop condistions. From electronic
circuit theory, the rise time is related to the bandwidth of the op-amp by the
relation: BW = 0.35 / rise
Lets have a look how the 'ideal' amplifier would look
like in Fig. 5-1. The search for an ideal amplifier is, of course, a futile
exercise. The characteristics of the operational amplifier are good enough,
however, to allow us to treat it as ideal. Below are some amplifier
properties that make this so. (Please realize that these ratings are next to
impossible to achieve).
2. Input impedance--infinite
3. Output impedance--zero
5. Voltage out--zero (when voltages into
each other are
6. Current entering the amp at either
In general op-amps are designed to
be powered from a dual or bipolar voltage supply which is typically in the range
of +5V to +15Vdc with respect to ground, and another supply voltage of
-5V to -15Vdc with respect to ground, as shown in Fig. 7. Although in certain
cases an op-amp, like the LM3900 and called a 'Norton Op-Amp', may be powered
from a single supply voltage.
Electrical characteristics for op-amps are usually
specified for a certain (given) supply voltage and ambient temperature. Also,
other factors may play an important role such as certain load and/or source
resistance. In general, all parameters have a typical minimum/maximum value in
6 - The two most common types are shown in the diagram on the right.
Depending on the application, the 8-pin version is used the most, worldwide.
Actually, there is a third type in the form of a metal-can but is obsolete and,
by my knowledge, no longer used. I have two of these metal-can types and keep
them as a 'gone-by' memory.
of 741-pin functions: (Refer to the internal 741 schematic of Fig.
Pin 1 (Offset
Null): Offset nulling, see Fig. 11. Since the op-amp is
the differential type, input offset voltage must be controlled so as to minimize
offset. Offset voltage is nulled by application of a voltage of opposite
polarity to the ofset. An offset null-adjustment potentiometer may be used to
compensate for offset voltage. The null-offset potentiometer also compensates
for irregularities in the operational amplifier manufacturing process which may
cause an offset. Consequently, the null potentiometer is recommended for
critical applications. See 'Offset Null Adjustment' for method.
Pin 2 (Inverted Input): All input
signals at this pin will be inverted at output pin 6. Pins 2 and 3 are very
important (obviously) to get the correct input signals or the op amp can not do
Pin 3 (Non-Inverted
Input): All input signals at this pin will be processed
normally without invertion. The rest is the same as pin 2.
Pin 4 (-V): The V- pin (also
referred to as Vss) is the negative supply voltage terminal. Supply-voltage
operating range for the 741 is -4.5 volts (minimum) to -18 volts (max), and it
is specified for operation between -5 and -15 Vdc. The device will operate
essentially the same over this range of voltages without change in timing
period. Sensitivity of time interval to supply voltage change is low, typically
0.1% per volt. (Note: Do not confuse the -V with ground).
Pin 5 (Offset Null): See pin 1,
and Fig. 11.
(Output): Output signal's polarity will be the oposite of
the input's when this signal is applied to the op-amp's inverting input. For
example, a sine-wave at the inverting input will output a square-wave in the
case of an inverting comparator circuit.
Pin 7 (posV): The V+ pin (also referred to as
Vcc) is the positive supply voltage terminal of the 741 Op-Amp IC.
Supply-voltage operating range for the 741 is +4.5 volts (minimum) to +18 volts
(maximum), and it is specified for operation between +5 and +15 Vdc. The device
will operate essentially the same over this range of voltages without change in
timing period. Actually, the most significant operational difference is the
output drive capability, which increases for both current and voltage range as
the supply voltage is increased. Sensitivity of time interval to supply voltage
change is low, typically 0.1% per volt.
8 (N/C): The 'N/C' stands for 'Not Connected'. There is no
other explanation. There is nothing connected to this pin, it is just there to
make it a standard 8-pin package.
The peak detector is a circuit that "remembers" the peak
value of a signal. As shown in Fig. 9-a, when a
positive voltage is fed to the noninverting input after the capacitor has
been momentarily shorted (reset), the output voltage of the op-amp forward
biases the diode and charges up the capacitor. This charging last until the
inverting and noninverting inputs are at the same voltage, which is equal to the
input voltage. When the noninverting input voltage exceeds the voltage at the
inverting input, which is also the voltage across the capacitor, the capacitor
will charge up to the new peak value. Consequently, the capactor voltage will
always be equal to the greatest positive voltage applied to the noninverting
Once charged, the time that the peak detector "remembers" this
peak value is typically several minutes and depends on the impedance of the load
that is connected to the circuit. Consequently, the capacitor will slowly
discharge towards zero. To minimize this rate of discharge, a voltage follower
can be used to buffer the detector's output from any external load, as shown in
Fig. 9-b. Momentarily shorting the capacitor to ground
will immediately set the output to zero.
A 'comparator' is circuit that compares an input voltage
with a reference voltage. The ouput of the comparator then indicates whether the
input signal is either above or below the reference voltage. As shown for the
basic circuit in Fig. 9-c(1), the output voltage
approaches the positive supply voltage when the input signal is slightly greater
than the reference voltage, Vref. When the input is slightly less than the
reference, the op-amp's output approaches the negative supply voltage.
Consequently, the exact threshold is dominated by the op-amp's input offset
voltage, which should be nulled out. Fig. 9-c(2) shows a
Led indicator wich input is connected to the output Vout of the
9-d(left) The output polarity of the op-amp switches from positive to
negative, it is inconvenient to keep reversing the voltmeter leads to keep
polarity correct. One way to overcome that prorblem is to use an indicatior
light to tell the output state. The circuit show on the left uses a transistor
to switch a led on or off depending on the comparator's output state. When the
op-amp output is 8.5 volts, the transistor switches on the led via the 220 ohm
current-limiting resistor. When the output is -8.5 volts the transistor is
cut-off turning off the led. Transistor choice is not critical; it can be any
common type NPN device. Any type of silicon diode will protect the transistor.
Fig. 9-e(right). The output on pin 6 switches
(repeatedly) from positive to negative and so either bias Q1 (NPN) or Q2 (PNP
and activates RL which is the resistive
load. Just a basic circuit to show you what exactly a 'Boosted-Output' circuit
The Instrumentation Amplifier
There are many types
of op-amps who are designed for a specific purpose like the Instrumentation
Amplifier from Burr-Brown.(see Fig. 10) In this example we are talking about the
3660J type. It can be used in both balanced and unbalanced systems, like a
Wheastone Bridge circuit. This does not mean in any way that the instrumentation
amp cannot be used for other applications, on the contrary, it is in many a case
prefered because of the unique parameters of this device.
Keep this in mind as a rule-of-thumb:
operational amplifier circuit will not work at
1. External feedback limits the
gain or desired responce to a design value.
inputs have direct-current return path to ground of a similar
3. The input frequencies and required gain
are well within the performance limitations of the op-amp
Offset Null Adjustment Procedure for the
Offset null adjustments differ with the application
(e.i. Inverting or Non-Inverting Amplifier). Offset-null potentiometers are
not placed on design schematics as they would detract from a design. For
practice, perform the following Offset Null adjustment if you wish:
1. Adjust the 10K pot(entio)meter to its center position.
Connect the potmeter outside leads between pins 1 and 5 of the op-amp.
sure that the power is applied to the design application.
the wiper of the potmeter to the negative supply voltage.
that input signals are zero or null and that pins 2 and 3 have a dc return to
5. Measure the output with a dc meter and obtain zero null by
adjusting the potentiometer.
This is just one method and recommended
nulling procedure for the µA741 type op-amp. Always look for, and follow the
particular procedure as specified by that chip manufacturer. Procedures may
become obsolete or updated and changed when improved op-amp versions come on the
Below is the Dual Volt Power Supply to power the op-amps. Check the output
voltages when you are done. You may lower the 220µF caps to 100µF if need