Unlike 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 fT.
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 = AolBW
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 100 (40dB).
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 time
Open-Loop Gain:
Lets have a look how the 'ideal' amplifier would look like. 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).
1. Gain--infinite
2. Input impedance--infinite
3. Output impedance--zero
4. Bandwidth--infinite
5. Voltage out--zero (when voltages into
each other are equal)
6. Current entering the amp at either
terminal--extremely small
Power Supply:
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 Ratings:
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 most cases.
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.
Definition of 741-pin functions: (Refer to the internal 741 schematic of Fig. 3)
Pin 1 (Offset Null): Offset nulling, see Fig. ???. 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):
Pin 3 (Non-Inverted Input):
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. ???
Pin 6 (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 (+V): 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.
Pin 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.
Experiments:
You are given the opportunity to play with and analyze experiments to demonstrate the principles, concepts, and
applications of a couple of these basic configured op-amps.
If you have already a dual-voltage power supply (positive/negative), that would make things alot easier for you.
If not, build this Dual Volt Powersupply here to get you started.
This power supply has two non-adjustable voltages; +12Vdc and -12Vdc. However, in general, a very simple and cheap
power supply can be rigged up with two 9 Volt alkaline batteries and does the job in most, if not all, cases.
Bread Board Module:
A bread board module, or just 'breadboard', is a board manufactured of plastic with a couple 100 tiny holes with tiny
sockets in them connected electrically together and in the center of the breadboard a grove to hold a plastic panel
for leds, pots and switches.
They measure about 6 by 2 inches and come in white and blue. The blue kind is called 'BimBoard' and made in the UK.
I purchased mine back in 1980 from ElectroSonic in Toronto Canada and its
still working fine.
Radio Shack and the European Tandy are both selling their own version and they work fine too.
The Bread Board Design System is also available, if you can afford it, and would be preferred if you intend to do a
lot more experimenting in the future. This system contains everything you need already build-in, like the
powersupply, jacks, switches, leds, function generator and lots more goodies. Kindah nice to have everything in one
place.
The Norton Op-Amp:
It is not my intention to confuse you with a different type of op-amp so I will just mention a couple of points about
this op-amp.
The Norton op-amp, or current-differencing amplifier, is designed to operate from a single ended supply. Wow, that is
truly fantastic. You can use a voltage anywhere from +4V to a whopping +36V! The Norton op-amp referred to here is
the LM3900 and is the best known type made by National Semiconductor. This chip contains four op-amps in a single
14-pin package. The picture in Fig. 8-b below shows the symbol for the Norton op-amp. As you may notice it is
somewhat different than the normal op-amp symbol. Fig. 8-c shows the major parameters of relevance between the 741
and the 3900 op-amps.
The Peak Detector
The peak detector is a circuit that "remembers" the peak value of a signal. As shown in Fig. 9-a (left), 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 input.
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 in Fig. 9-b (right). Momentarily shorting the capacitor to ground will immediately
set the output to zero.
The Comparator
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. ??????????, 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.
There are many types of op-amps who are designed for a specific purpose like the Instrumentation Amplifier from
Burr-Brown.(see Fig.????????) 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.
Keep this in mind as a rule-of-thumb:
An operational amplifier circuit will not work at all unless:
1. External feedback limits the gain or desired responce to a design value.
2. Both inputs have direct-current return path to ground of a similar reference.
3. The input frequencies and required gain are well within the performance limitations of the op-amp
used.
Offset Null Adjustment Procedure for the µA741:
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.
2. Connect the potmeter outside leads between pins 1 and 5 of the op-amp.
Make sure that the power is applied to the design application.
3. Connect the wiper of the potmeter to the negative supply voltage.
4. Ensure that input signals are zero or null and that pins 2 and 3 have a
dc return to ground.
5. Measure the output with a dc meter and obtain zero null by adjusting the
potentiometer.
This just one method and is the recommended nulling procedure for the µA741 type (1968) op-amp. Always look for,
and follow the particular procedure as specified by a certain chip manufacturer. Procedures may become obsolete or may
be updated and changed when improved op-amps of the same type and brand come on the market.
Planning Your Prototype or Experiment:
Planning the layout of your experiments could be important, especially with large circuits. Use this
[lay-out sheet] to plan your components layout on the bread board, if you wish.
Remove every component and all wires from previous experiments.
Important: Before starting to insert components into the breadboard, make sure
all power and signal connections are removed and the power source disconnected. And if required, take the glue/dirt
of the components' legs before inserting them into the sockets, it is very hard if not impossible to get it cleaned
out.
A couple circuit examples:
Below are a couple circuit examples you can play and experiment with to understand the working of a 741 op-amp hands
on. If you are serious about electronics I strongly recommend buying a book or two about Op-Amps for your
experimenting pleasures.
Fig. 10 - 741 Light Sensor
This is a really nice circuit to play with. When there is NO light falling on the sensor, the relay closes.
When light falls on the LDR, the relay opens. To reverse the situation just exchange LDR and R1. Example uses for
this circuit: Dark-room, Automatic door-lock, closet, Shed Alarm, etc.
Fig. 11 - Smart Continuity Tester
Occasionally you need a continuity test between two points in an electronic circuit. Unfortunately, most continuity
testers are prone to "lie". They don't do that deliberately, but if they see a small resistance, they still
tell you that you have continuity. They just don't know any better. This unit is different. If you have continuity
it will tell you so. And if you're reading even a low resistance through a component, the unit will tell you that
as well. The unit uses two 741 op-amps. It offers a short-circuit test current of less than 200uA. It detects
resistance values of less than 10 ohms. Nicest of all, it will not break down a PN junction. The device has come in
handy in my own shop for debugging electronic circuits.
Fig. 12 - 12V Battery Monitor
The 100 Kilo-ohm trimmer pot controls the set point of this circuit to a 'high' point like 12.5 or something, so
adjust it to the point you wish the LED to light up. To monitor a 'low' point, and have the led light up when the
battery has drained to a certain point, connect the led (via the 330 ohm resistor) to ground (in the blue-boxed area).
The led is the high-brightness or ultra-bright type for maximum visibility. The trimpot in the schematic is a 10-turn
type for better accuracy, but basically any type will do.
Supply Voltage (- + Vs):
The maximum voltage (positive and negative) that can be safely used to feed the op-amp.
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