Ic Op Amps

INTEGRATED CIRCUIT OPERATIONAL AMPLIFIER The trend toward the use of operational amplifiers as general-purpose analog building blocks began when modular, solid-state discrete-component designs became available to replace the older, more expensive vacuum-tube circuits that had been used primarily in analog computers. As cost decreased and performance improved, it became advantageous to replace specialized circuits with these modular operational amplifiers. This trend was greatly accelerated in the mid-1960s as low-cost monolithic integrated-circuit operational amplifiers became available. While the very early monolithic designs had sadly deficient specifications compared with discrete-component circuits of the era, present circuits approach the performance of the best discrete designs in many areas and surpass it in a few. Performance improvements are announced with amazing regularity, and there seem to be few limitations that cannot be overcome by appropriately improving the circuit designs and processing techniques that are used. No new fundamental breakthrough is necessary to provide performance comparable to that of the best discrete designs. It seems clear that the days of the discretecomponent operational amplifier, except for special-purpose units where economics cannot justify an integrated-circuit design, are numbered. In spite of the clear size, reliability, and in some respects performance advantages of the integrated circuit, its ultimate impact is and always will be economic. If a function can be realized with a mass-produced integrated circuit, such a realization will be the cheapest one available. The relative cost advantage of monolithic integrated circuits can be illustrated with the aid of the discrete-component operational amplifier used as a design example in the previous chapter. The overall specifications for the circuit are probably slightly superior to those of presently available general-purpose integrated-circuit amplifiers, since it has better bandwidth, d-c gain, and open-loop output resistance than many integrated designs. Unfortunately, economic reality dictates that a company producing the circuit would probably have to sell it for more than $20 in order to survive. General-purpose integrated-circuit operational amplifiers are presently available for approximately $0.50 in quantity, and will probably become cheaper in the future. Most system designers would find a way to circumvent any performance deficiencies of the integrated circuits in order to take advantage of their dramatically lower cost. The tendency toward replacing even relatively simple discrete-component analog circuits with integrated operational amplifiers will certainly increase as we design the ever more complex electronic systems of the future that are made economically feasible by integrated circuits. The challenge to the designer becomes that of getting maximum performance from these amplifiers by devising clever configurations and ways to tailor behavior from the available terminals. The basic philosophy is in fundamental agreement with many areas of design engineering where the objective is to get the maximum performance from available components. Prior to a discussion of integrated-circuit fabrication and designs, it is worth emphasizing that when compromises in the fabrication of integrated circuits are exercised, they are frequently slanted toward improving the economic advantages of the resultant circuits. The technology exists to design monolithic operational amplifiers with performance comparable to or better than that of the best discrete designs. These superior designs will become available as manufacturers find the ways to produce them economically. Thus the answer to many of the "why don't they" questions that may be raised while reading the following material is "at present it is cheaper not to."

Op-amp Integrator Circuit

As its name implies, the Op-amp Integrator is an operational amplifier circuit that performs the mathematical operation of Integration that is we can cause the output to respond to changes in the input voltage over time as the op-amp integrator produces an output voltage which is proportional to the integral of the input voltage. In other words the magnitude of the output signal is determined by the length of time a voltage is present at its input as the current through the feedback loop charges or discharges the capacitor as the required negative feedback occurs through the capacitor. Related Products: OP Amp, SP Amplifier When a step voltage, Vin is firstly applied to the input of an integrating amplifier, the uncharged capacitor C has very little resistance and acts a bit like a short circuit allowing maximum current to flow via the input resistor, Rin as potential difference exists between the two plates. No current flows into the amplifiers input and point X is a virtual earth resulting in zero output. As the impedance of the capacitor at this point is very low, the gain ratio of Xc/Rin is also very small giving an overall voltage gain of less than one, ( voltage follower circuit ). As the feedback capacitor, C begins to charge up due to the influence of the input voltage, its impedance Xc slowly increase in proportion to its rate of charge. The capacitor charges up at a rate determined by the RC time constant, ( τ ) of the series RC network. Negative feedback forces the opamp to produce an output voltage that maintains a virtual earth at the op-amp’s inverting input. Since the capacitor is connected between the op-amp’s inverting input (which is at earth potential) and the op-amp’s output (which is negative), the potential voltage, Vc developed across the capacitor slowly increases causing the charging current to decrease as the impedance of the capacitor increases. This results in the ratio of Xc/Rin increasing producing a linearly increasing ramp output voltage that continues to increase until the capacitor is fully charged. At this point the capacitor acts as an open circuit, blocking any more flow of DC current. The ratio of feedback capacitor to input resistor ( Xc/Rin ) is now infinite resulting in infinite gain. The result of this high gain (similar to the op-amps open-loop gain), is that the output of the amplifier goes into saturation as shown below. (Saturation occurs when the output voltage of the amplifier swings heavily to one voltage supply rail or the other with little or no control in between).

The rate at which the output voltage increases (the rate of change) is determined by the value of the resistor and the capacitor, “RC time constant“. By changing this RC time constant value, either by changing the value of the Capacitor, C or the Resistor, R, the time in which it takes the output voltage to reach saturation can also be changed for example.

If we apply a constantly changing input signal such as a square wave to the input of an Integrator Amplifier then the capacitor will charge and discharge in response to changes in the input signal. This results in the output signal being that of a saw tooth waveform whose output is affected by the RC time constant of the resistor/capacitor combination because at higher frequencies, the capacitor has less time to fully charge. This type of circuit is also known as a Ramp Generator and the transfer function is given below. Op-amp Integrator Ramp Generator

We know from first principals that the voltage on the plates of a capacitor is equal to the charge on the capacitor divided by its capacitance giving Q/C. Then the voltage across the capacitor is output Vout therefore: -Vout = Q/C. If the capacitor is charging and discharging, the rate of charge of voltage across the capacitor is given as:

But dQ/dt is electric current and since the node voltage of the integrating op-amp at its inverting input terminal is zero, X = 0, the input current I(in) flowing through the input resistor, Rin is given as:

The current flowing through the feedback capacitor C is given as:

Assuming that the input impedance of the op-amp is infinite (ideal op-amp), no current flows into the op-amp terminal. Therefore, the nodal equation at the inverting input terminal is given as:

From which we derive an ideal voltage output for the Op-amp Integrator as:

To simplify the math’s a little, this can also be re-written as:

Where ω = 2πƒ and the output voltage Vout is a constant 1/RC times the integral of the input voltage Vin with respect to time. The minus sign ( – ) indicates a 180o phase shift because the input signal is connected directly to the inverting input terminal of the op-amp. The AC or Continuous Op-amp Integrator If we changed the above square wave input signal to that of a sine wave of varying frequency the Opamp Integrator performs less like an integrator and begins to behave more like an active “Low Pass Filter”, passing low frequency signals while attenuating the high frequencies. At 0Hz or DC, the capacitor acts like an open circuit blocking any feedback voltage resulting in very little negative feedback from the output back to the input of the amplifier. Then with just the feedback capacitor, C, the amplifier effectively is connected as a normal open-loop amplifier which has very high open-loop gain resulting in the output voltage saturating. This circuit connects a high value resistance in parallel with a continuously charging and discharging capacitor. The addition of this feedback resistor, R2 across the capacitor, C gives the circuit the characteristics of an inverting amplifier with finite closed-loop gain of R2/R1. The result is at very low frequencies the circuit acts as an standard integrator, while at higher frequencies the capacitor shorts out the feedback resistor, R2 due to the effects of capacitive reactance reducing the amplifiers gain.

The AC Op-amp Integrator with DC Gain Control

Unlike the DC integrator amplifier above whose output voltage at any instant will be the integral of a waveform so that when the input is a square wave, the output waveform will be triangular. For an AC integrator, a sinusoidal input waveform will produce another sine wave as its output which will be 90o out-of-phase with the input producing a cosine wave. Further more, when the input is triangular, the output waveform is also sinusoidal. This then forms the basis of a Active Low Pass Filter as seen before in the filters section tutorials with a corner frequency given as.

In the next tutorial about Operational Amplifiers, we will look at another type of operational amplifier circuit which is the opposite or complement of the Op-amp Integrator circuit above called the Differentiator Amplifier. As its name implies, the differentiator amplifier produces an output signal which is the mathematical operation of differentiation that is it produces a voltage output which is proportional to the input voltage’s rate-of-change and the current flowing through the input capacitor. IC 741 The most commonly used op-amp is IC741. The 741 op-amp is a voltage amplifier, it inverts the input voltage at the output, can be found almost everywhere in electronic circuits. Pin Configuration: Let’s see the pin configuration and testing of 741 op-amps. Usually, this is a numbered counter clockwise around the chip. It is an 8 pin IC. They provide superior performance in integrator, summing amplifier and general feedback applications. These are high gain op-amp; the voltage on the inverting input can be maintained almost equal to Vin.

It is a 8-pin dual-in-line package with a pinout shown above. Pin 1: Offset null. Pin 2: Inverting input terminal. Pin 3: Non-inverting input terminal. Pin 4: –VCC (negative voltage supply). Pin 5: Offset null. Pin 6: Output voltage. Pin 7: +VCC (positive voltage supply). Pin 8: No Connection. The main pins in the 741 op-amp are pin2, pin3 and pin6. In inverting amplifier, a positive voltage is applied to pin2 of the op-amp; we get output as negative voltage through pin 6. The polarity has been inverted. In a non-inverting amplifier, a positive voltage is applied to pin3 of the op-amp; we get output as positive voltage through pin 6. Polarity remains the same in non-inverting amplifier. Vcc is usually in the range from 12 to 15 volts. When two supplies (+Vcc/-Vcc) are used, they are the same voltage and of opposite sign in almost all cases. Remember that the operational amplifier is a high gain, differential voltage amplifier. For a 741 operational amplifier, the gain is at least 100,000 and can be more than a million (1,000,000). That’s an important fact you’ll need to remember as you put the 741 into a circuit. There are many common application circuits using IC741 op-amp, they are adder, comparator, subtractor, integrator, differentiator and voltage follower.

Below is some example of 741 IC based circuits. However, the 741 is used as a comparator and not an amplifier. The difference between the two is small but significant. Even if used as a comparator the 741 still detects weak signals so that they can be recognized more easily. A comparator is a circuit that compares two input voltages. One voltage is called the reference voltage and the other is called the input voltage. It is a circuit which compares a signal voltage applied at one input of an op-amp with a known reference voltage at the other input. The 741 op-amp has ideal transfer characteristics (output ±Vsat); and the output is changed by increment in the input voltage of 2mV.