Operational Amplifiers and Linear Integrated Circuits
What this domain tests
Ideal op-amp parameters, inverting and non-inverting configurations, virtual ground, linear applications (adder, integrator, differentiator, active filter) and comparison with open-loop use as a comparator. Non-ideal specifications such as CMRR, slew rate and offset are also examined.
Core rules
- Ideal op-amp: infinite open-loop gain, infinite input impedance, zero output impedance, infinite CMRR and bandwidth, zero offset and zero drift.
- Inverting amplifier: Av = −Rf/Rin, with the inverting input at virtual ground. Non-inverting amplifier: Av = 1 + Rf/R1.
- Virtual ground holds only when negative feedback is present and the op-amp is not saturated; the input terminals are at the same potential but carry no current.
- Voltage follower (Av = 1) is used as a buffer to overcome loading.
- CMRR = Adm/Acm, usually expressed in dB; slew rate = dVo/dt in V/µs, limiting high-frequency large-signal response.
- Integrator uses a capacitor in the feedback path, differentiator a capacitor at the input; a comparator uses no feedback and drives the output into saturation.
- The 741 is an 8-pin IC: pin 2 inverting input, pin 3 non-inverting input, pin 4 −Vcc, pin 6 output, pin 7 +Vcc, pins 1 and 5 offset null.
Worked example
A non-inverting amplifier has Rf = 100 kΩ and R1 = 10 kΩ with Vin = 0.2 V. Av = 1 + 100/10 = 11, so Vo = 11 × 0.2 = 2.2 V. With a ±12 V supply the output stays in the linear range; had the calculated value exceeded about 10.5 V, the op-amp would have saturated and the formula would no longer apply.
Traps that cost marks
- Applying virtual ground to a comparator or a positive-feedback circuit.
- Dropping the leading 1 in the non-inverting gain formula.
- Ignoring output saturation limits near the supply rails.
- Confusing slew rate (V/µs) with unity-gain bandwidth (MHz).