Operational amplifier and Opto-electronic devices

Operatinal amplifier

Operatinal amplifier or OPAMP are basic element in an analogue  circuit. Opamp is a linear device. When signal is applied to minus terminal of opamp then output is in opposite phase of input. Differential gain of opamp should be greater than common mode gain. Ideal value of common mode rejection ratio CMMR is infinity. Inverting amplifier is commonly used because of increased frequency stability. Unity follower doesn’t change phase polarity. Open loop gain of opamp is infinity

OPAMP comparator doesn’t use any feedback. BIFET is an opamp which combines BJT and FET. OPAMP is most commonly used linear IC. 741 OPAMP is industrial standard linear IC. Most power dissipated in OPAMP IC is 50mW. Approximate short circuit current in OPAMP is 25mA. Slew rate of 741 OPAMP is 0.5. Input bias current of OPAMP is average of two base current. Typical input resistance of OPAMP amplifier in open loop is 2M. Gain reduction in OPAMP is called roll off. Rate of gain reduction in OPAMP is 20 db/decade. Compensating capacitor is present in OPAMP to prevent oscillation. Maximum differential input to an OPAMP is 30 V

Slew rate is maximum rate at which output changes in volts per micro second. To prevent distortion in output rate of change should be less than slew rate. Dominant pole frequency compensation in an opamp increases slew rate and stability. At unity gain cross over frequency opamp gain becomes 0 db. Schmitt trigger is an application of opamp, it is also called regenerative comparator. Main use of Schmitt trigger is to convert very slowly varying input voltage into square wave. Speed up capacitors are used to reduce transition time of bistable multivibrator. Miller sweep have good linearity because of negative voltage feedback. Monostable circuit is used to generate pulse and flip flop. Asymmetric triggering is used for monostable

Opto electronic devices

Photodiode is reverse biased and it is a most commonly using photo-detector. Photo diode can turn on and off in nanoseconds. If intensity of light falls on photo-detector can be measured then it is called photo-conductive cell. PIN photo detector is current controlled attenuator. PIN have low output current. PIN is used as switching diode for frequencies up to GHz. Varactor diode is used in tuned circuits.

Avalanche photo diode is a photo detector. Avalanche photo diode gives more output current ( M times greater than photo diode). Visible LEDs are available in red, green and orange. Visible wavelength 0.45 to 0.7 micro meter (2.8 – 1.8 ev). GaAsP and GaP are the materials used for making LED. GaAs and GaAs with AlGaAs are used for making infrared LED. GaP LED gives red or green colour. GaAsP LED gives red or yellow (amber) colour. Gallium emits more light so it is used in LED. Gold is used as bottom substrate of LED to provide reflection and as cathode connection. LED are always encased to protect its wires. Operating voltage of LED is 1.5 – 3.3 V. Laser is highly directional, monochromatic, collimated and coherent.

Laser have a threshold voltage below this value it will act as an LED. To view the laser beam, filter or lens is necessary. Hetero structure laser have low threshold voltage than injection laser or diode laser. Photo transistor has greater sensitivity and current capacity. Optocoupler is an optoisolator. LAD (light activated diode) is a photodiode. Optoelectronic devices radiate and utilize power. Operating current of LED is 10 mA, voltage drop is 1.5V. Photodiffusion effect is also called dember effect. Photoconductive cell is a photoresistive device. In photodiode electrons generated is proportional to intensity of light inscident