Operational Amplifier or OpAmp is a DC coupled amplifier with differential inputs and usually with a single output. An OpAmp can deliver an output voltage hundreds or thousand times greater than the voltage difference between its inputs. OpAmps may be single, dual, quad etc. In single OpAmps only one OpAmp is present while in dual type two OpAmps with separate inputs and outputs are present. In quad OpAmp, four OpAmps are present in the single package. Some of the OpAmps like CA3130, CA3140, TL0 71, LM311 etc have excellent performance with very low input current and voltage.
A typical OpAmp has two inputs and a single output. The inputs are names as Inverting ( – ve) and Non inverting (+ ve) inputs. The output depends on the voltage difference between these inputs. When the OpAmp is used as a voltage comparator, one input is set at fixed voltage through a potential divider or Zener while the other input gets a variable voltage. In the Comparator mode, the output goes high when the Non inverting input gets a higher voltage than the Inverting input. In the reverse situation, the output remains low. In amplifier design, Schmitt trigger etc, a Feedback resistor will be provided from the output to the input.
Besides the commonly used pins, the OpAmp has three more pins.
Offset Null pins
Pins 1 and 5 of OpAmp are called as Offset Null pins. These pins are used for Offset Nulling. When the inverting and Non inverting inputs are at same voltage, the output should be Zero. To keep the output zero, Offset nulling is used by connecting pins 1 and 5 in a preset (usually 10K ) with its wiper to the ground. By adjusting the preset, output can be set at Zero voltage.
Pin 8 is called Strobe pin. It is used for Phase compensation and Gain control to optimize frequency response. For this a capacitor (usually 47 pF) is connected between pin 8 and pin1
IC 741 is the most common Operational Amplifier used in circuits. It is designed as the Military grade IC to works in harsh environment. It is designed for a wide range of analog applications and is featured with wide range of input voltage. IC 741 has features like no latch up, high gain, short circuit protection. It can be used as Summing amplifier, Voltage follower, Integrator, Function generator etc. The high gain and wide range of operating voltages provide superior performance in integrator, summing amplifier and general feedback applications. It has very good stability in closed loop circuits. But one important aspect is its output never becomes Zero in the off state in normal function and gives around 0.6 – 1 volt. This may switch on a transistor connected to its output even in the off state of IC. This excess voltage can be removed by connecting a diode at the output of IC.
CA3130 and CA3140
CA 3130 and CA 3140 are excellent Operational Amplifiers for Signal detection circuits. The most important characteristics of these Op Amps are low input current requirements and the zero output in the off state. Some of the general purpose Op Amps will not give a zero output in the off statewhich will affect the working of the remaining circuitry. More over the input current requirement is also high in most Op Amps so that these devices cannot be used in circuits based on Photo diodes and Infrared sensors.
IC CA3130 is a 15 MHz BiMOS Operational amplifier with MOSFET inputs and bipolar output. The inputs contain MOSFET transistors to provide very high input impedance and very low input current as low as 10pA. It has high speed of performance and suitable for low input current applications.
CA3130A and CA3130 are op amps that combine the advantage of both CMOS and bipolar transistors. Gate-protected P-Channel MOSFET (PMOS) transistors are used in the input circuit to provide very-high-input impedance, very-low-input current and exceptional speed performance. The use of PMOS transistors in the input stage results in common-mode input-voltage capability down to 0.5V below the negative-supply terminal, an important attribute in single-supply applications.
A CMOS transistor-pair, capable of swinging the output voltage to within10mV of either supply-voltage terminal (at very high values of load impedance), is employed as the output circuit.
The CA3130 Series circuits operate at supply voltages ranging from 5V to 16V.They can be phase compensated with a single external capacitor, and have terminals for adjustment of offset voltage for application requiring offset-null capability. Terminal provisions are also made to permit strobing of the output stage. The CA3130A offers superior input characteristics over those of the CA3130.
CA3140 is the 4.5MHz BiMOS Operational Amplifier with MOSFET inputs and Bipolar output. This Op Amp combines the advantage of PMOS transistors and high voltage bipolar transistors.CA3140 has gate protected MOSFETs (PMOS) transistors in the input circuit to provide very high input impedance typically around 1.5T Ohms. The IC requires very low input current as low as 10pA to change the output status, high or low. The IC has very fast response and high speed of performance. The output stage of the IC uses bipolar transistors and includes built in protection against damage from load terminal short circuiting to either supply rails or to ground.
The use of PMOS FET in the input stage results in common mode input voltage capabilities down to 0.5 volts below the negative supply terminals. These operational amplifiers are internally phase compensated to achieve stable operation in unity gain follower operation, and additionally, have access terminal for a supplementary external capacitor if additional frequency roll-off is desired. Terminals are also provided for use in applications requiring input offset voltage Nulling.
Is the low noise JFET input operational Amplifier operates in wide common mode and differential voltage ranges. It has low power consumption and low input bias and offset currents. Output of TL071 is short circuit protected while the input is JFET. It has high slew rate of 13 V/us and exhibits latch free operation.TL071 is suited for high fidelity and audio preamplifier applications.TL0 71 and TL0 72 are single Op Amp while Tl0 74 is Quad Op Amp.
TL0 72 Pins
TL0 74 Pins
Is the high speed JFET input dual operational amplifier incorporating well matched high voltage JFET and Bipolar transistors. It has high slew rate, low input bias and offset current and low offset voltage temperature. Its input can be biased with very low current and output is short circuit protected. It also has internal frequency compensation and latch free operation.
Op Amp LM 311 is capable of driving DTL, RTL, TTL, or MOS logic circuits etc. The output can switch voltages to 50 volts at currents to 50 mA. IC LM311 operates between 5 and 30 volts single supply so it can be used to drive Relays, Lamps, Solenoids etc. Compared to other general purpose Op Amps, the pin connection of LM311 is different.
LM 311 Inputs
Like all other single Op Amps, LM 311 also has two inputs – Inverting input and Non inverting input. In typical Op Amps, the inverting input (negative) is pin 2 and Non-inverting (positive) input is pin3. But is LM311, the condition is just reversed. Pin 2 is non inverting input andpin3 is inverting input.
Output stage of LM 311 is different from a typical Op Amp like 741. This modification enhances the flexibility of LM 311. It has two outputs. Pin 7 is the positive output and forms the collector of the NPN output transistor. Pin 1 is the negative output that forms the emitter of the output transistor. Usually pin 1 is grounded to allow collector-emitter flow of current. Since the positive output is the collector of the output transistor, it is current sinking. Normally in the off state, the collector of output transistor is pulled to Vcc .The output transistor saturates when its base gets more than 0.7 volts. When this happens, the output is pulled to emitter voltage. That is the load turns on when the output goes low. In typical Op Amps, the output is current sourcing and gives current when the output becomes high.