Electronic Fundamentals
Core content
Electronic fundamentals covers semiconductor materials, PN junctions, diodes, rectifiers, Zener diodes, LEDs, bipolar transistors, field-effect transistors, operational amplifiers, filters, oscillators, power supplies and decibel notation. You should know that a diode conducts when forward biased above its threshold and blocks in reverse until breakdown. A BJT is a current-controlled device; a FET is voltage-controlled. An ideal operational amplifier has very high input impedance and very high open-loop gain. With negative feedback, the input terminals are at nearly the same potential, and the output is set by the external components. Filters pass or reject bands of frequency, and oscillators require positive feedback with the correct phase.
Traps
Do not apply the virtual-short rule to an op-amp without negative feedback; that is a comparator. Remember that a real silicon diode has a forward drop, roughly 0.6 to 0.7 V, so a bridge rectifier has two diode drops in the conducting path. In decibels, voltage gain uses 20 log(Vout/Vin), while power gain uses 10 log(Pout/Pin); mixing these is a common error. Also check transistor bias: a transistor must be correctly biased to amplify rather than simply switch.
Worked example
An inverting op-amp has Rin = 10 kilohms and Rf = 100 kilohms. Voltage gain = -Rf/Rin = -100/10 = -10. An input of 0.2 V produces an output of -2.0 V, within the supply limits. Rule of thumb: the ideal op-amp draws no input current, so the same current flows through Rin and Rf.
Key points
- A diode conducts when forward biased above its threshold.
- A BJT is current-controlled; a FET is voltage-controlled.
- Virtual-short op-amp rules apply only with negative feedback.
- Voltage gain in dB is 20 log(Vout/Vin).
- Inverting amplifier gain is approximately -Rf/Rin.