Operational Amplifier Fundamentals Unit 1 – The Operational Amplifier
Exercise 3 – Basic Op Amp Characteristics
EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with fundamental op amp
characteristics. You will verify your results by performing offset null adjustments.
EXERCISE DISCUSSION
- Three fundamental op amp characteristics are high input impedance, high voltage gain, and
low output impedance. - Ideal op amps have infinite input impedance. Practical op amps have typical input
impedances of 1 MΩ or more. - Ideal op amps have infinite open-loop gains. Practical op amps have typical open-loop gains
of 200,000 or more. - Ideal op amps have zero output impedance. Practical op amps have typical output
impedances of 10Ω or more. - Practical op amps possess a small input current called the input bias current.
- Input bias currents generate output error voltages called output-offset voltage.
- Shifts in temperature change offset current, which changes offset voltage. These changes are
referred to as drift. - If the input bias current of an op amp is low, its offset voltage will be small.
- If the inputs of a practical op amp are at the same potential, output voltage should be zero.
However, due to input bias current, an output- offset voltage is generated. - The effects of input bias current can be reduced significantly by the introduction of an input-
offset current or an input-offset voltage. This process is referred to as offset nulling. - Offset nulling, or op amp trim adjust, uses an adjustable external voltage to set the op amp
output to near 0V. - High gain op amps, unless properly bypassed, may oscillate. To prevent this undesirable
condition, power supply connections must exhibit low impedance characteristics. - Bypass, or decoupling, capacitors are used to ensure op amp stability.