Analog Electronics
Analog Electronics tests op-amp configurations, feedback, frequency response, filters, oscillators, and noise. You should be able to derive ideal closed-loop gain, check bandwidth and slew-rate limits, and identify stable feedback.
Core rules
- Ideal op-amp with negative feedback: input current is zero and differential input voltage is zero (virtual short).
- Inverting gain = -Rf/Rin; non-inverting gain = 1+Rf/Rg.
- Closed-loop bandwidth ≈ gain-bandwidth product / closed-loop gain for a dominant-pole op-amp.
- Slew rate limits large-signal dV/dt; full-power bandwidth = SR/(2πVpeak).
- Negative feedback improves linearity and bandwidth but can oscillate with excess phase shift.
Worked example
A non-inverting amplifier has Rf=9 kΩ, Rg=1 kΩ, Vin=0.5 V, and an op-amp with GBW=1 MHz and SR=0.5 V/µs. Ideal gain is 1+9/1=10, so Vout=5.0 V. Closed-loop bandwidth is about 1 MHz/10=100 kHz. For a 5 V peak sine, required SR is 2πfVpeak; at 20 kHz this is 2π×20,000×5≈0.628 V/µs, above the 0.5 V/µs limit, so the output slews and distorts. Reduce frequency or amplitude, or choose a faster op-amp.
Traps
Swapping inverting and non-inverting inputs; forgetting the input resistor in the non-inverting gain; using GBW at the signal frequency rather than closed-loop bandwidth; ignoring output swing limits; and assuming a virtual short in positive feedback or comparator circuits.