Read the inverting op-amp in figure E7-3, calculate its gain and output, and explain what real op-amps do differently from ideal ones.
Syllabus: Operational amplifiers: characteristics and applications
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
An operational amplifier is a high-gain, direct-coupled differential amplifier. It amplifies the difference between its two inputs.
RememberInput impedance very high, output impedance very low.
The + input is grounded, so feedback holds the − input at about 0 V. All the input current (Vin ÷ R1) is forced through RF, and the output swings the opposite way. Gain = −RF ÷ R1. The minus sign means the output is inverted.
For an absolute gain, drop the sign and divide: RF ÷ R1. Ask "how many times bigger is RF than R1?"
RememberFeedback resistor on top, input resistor on the bottom, output is inverted.
A capacitor's reactance falls as frequency rises. Across RF, it shunts the feedback resistor at high frequencies, so the gain falls: a low-pass filter.
Filter circuits with high gain and high Q ring and become unstable. To prevent unwanted ringing and audio instability, restrict both gain and Q.
RememberCapacitor across RF = low-pass. Keep both gain and Q down.
An ideal op-amp has the same gain at every frequency. A real one loses open-loop gain as frequency rises. The frequency where open-loop gain falls to one is its gain-bandwidth.
Input offset voltage: ideally both inputs at 0 V give 0 V out. In a real op-amp they don't. The differential input needed to bring the output to zero is the offset voltage.
RememberGain-bandwidth = frequency where open-loop gain is 1. Offset = input difference that nulls the output.