The Bode Plot That Lost 20 dB in One Decade
A DC gain of 100 gives 40 dB only in the low-frequency region. A first-order pole changes the slope above its corner.
Archives: Case Files
The Gain-10 Loop That Only Reached 0.909
A forward-path gain of 10 does not mean a closed-loop gain of 10. Reduce the feedback loop before predicting output.
The 8 Ohm Load That Became 128 Ohms
Voltage scales with turns ratio, but impedance does not. An 8 ฮฉ load can appear as 128 ฮฉ through a 4:1 transformer.
The Feeder That Delivered 228 V Instead of 240 V
A feeder calculation can miss half the resistance when it counts only the outgoing conductor and forgets the return path.
The Motor Load That Drew More Current Than the kW Suggested
A kW-only three-phase current calculation can look correct and still undersize a feeder when power factor is below unity.
The Op-Amp That Tried to Output -10 V on ยฑ5 V Rails
A correct inverting-gain calculation can still request an impossible output. Check the supply rails before accepting the result.
The Transistor That Refused to Amplify 100ร
An NPN switch has VCC = 5 V, Rc = 100 ฮฉ, ฮฒ = 100, and Ib = 1 mA. ฮฒIb predicts 100 mA, but with VCE(sat) โ 0.2 V the collector circuit can supply only about
The Protection Diode That Starved the 5 V Sensor
A 5 V source drives a forward-biased silicon diode and a 1 kฮฉ resistor in series. Using a 0.7 V silicon forward-drop model, the measured current is about 4
The Digital Filter That Lagged a Perfect Step
Unity DC gain does not mean instant response. Step through a recursive digital filter and see why its output rises gradually.
The 1 kHz Filter That Only Passed 70.7%
Cutoff does not mean fully passed. A first-order low-pass is already at 0.707 amplitude at fc.