OPAMP parameters and configuration

Types of OPAMP:

1. Voltage feedback opamp
2. Current feedback opamp

NOTE: Mostly we use voltage opamps

Configurations:

1. Voltage Amplifier: Voltage input, Voltage output 
2. Current Amplifier: Current input, Current output
3. Trans-impedance Amplifier: Current input, Voltage output
4. Trans-conductance Amplifier: Voltage input, Current output

Parameters:

1. Supply voltage range: (Offseted or Rail-to-rail): 
• Rail to rail are preferred for better range of output voltage
• This helps in using full range of ADC to which opamp output is connected
• Otherwise ADC needs to be more precise i.e. more bits of ADC
2. Single / Dual supply operation supported/ NOT:
3. Input common mode voltage range
4. Input impedance
5. Output Voltage range
6. Large signal voltage gain
7. Gain Bandwidth Product:
• Concept of Gain bandwidth product remaining the same, applies only to voltage feedback opamps
• For current feedback opamps, feedback resistor is fixed
8. Slew rate
9. Output current Sink and source
10. Input Bias current:
• Very low for FETs and CMOS input stages
• Compensation resistor can be added to reduce the bias current
• But, it has a trade-off. It increases the johnson noise which in turns increases the noise voltage in the circuit
11. Input offset current
12. Input offset voltage
13. Input noise voltage (nV/sqrt(Hz)): 
• For calculating noise voltage introduced in the circuit = Input noise voltage x (sqrt (bandwidth of opamp))
• Noise in the circuit also increases with increase in resistors outside the circuit
14. Common Mode Rejection Ratio (CMRR)
15. Power Supply Rejection Ratio (PSRR)
16. Total harmonic Distortion

For precision opamps, following parameters are very important:

• Input bias current (Ib) = ((Ib+) + (Ib-))/2
• Input offset current (Iio) = ((Ib+) - (Ib-))
• Input offset voltage (Vio)
• Input noise voltage (Vn) = sqrt(4KTB)
• Effect of temperature on Input offset voltage
• Trade of between, Input bias current compensation v/s Input noise voltage