Detailed, exam-oriented notes covering every syllabus unit.
Aim: to plot the forward and reverse bias V-I characteristics of a p-n junction diode and determine its cut-in voltage, static and dynamic resistance.
Theory: forward current follows the diode equation I = I₀(exp(V/ηV_T) − 1); beyond the cut-in (threshold) voltage, current rises steeply. Reverse current stays very small (I₀, the reverse saturation current) until breakdown.
Key parameters extracted: cut-in voltage (~0.6–0.7 V for Si, ~0.2–0.3 V for Ge), static resistance (V/I at operating point), dynamic/AC resistance (ΔV/ΔI from the slope of the curve).
BJT (CE configuration): input characteristics plot I_B vs V_BE at constant V_CE; output characteristics plot I_C vs V_CE for several fixed I_B, revealing the active, saturation and cut-off regions and allowing extraction of current gain β = ΔI_C/ΔI_B.
FET: drain characteristics (I_D vs V_DS at fixed V_GS) show the ohmic, saturation (pinch-off) and breakdown regions; transfer characteristics (I_D vs V_GS at fixed V_DS) let you extract the pinch-off voltage V_P and saturation drain current I_DSS.
MOSFET: similar to FET but the gate is insulated (oxide layer), giving very high input impedance; characteristics distinguish depletion-mode and enhancement-mode operation and allow extraction of the threshold voltage V_T.
Colpitts oscillator: an LC feedback oscillator using a tapped capacitive voltage divider (two capacitors in series) with a single inductor to set the feedback fraction and resonant frequency f = 1/(2π√(L·C_eq)), where C_eq is the series combination of the two capacitors.
Hartley oscillator: the dual of the Colpitts — uses a tapped inductive divider (two inductors, or a single tapped inductor) with a single capacitor. Frequency f = 1/(2π√((L₁+L₂)C)).
Common measurements: observe the output waveform on a CRO, measure oscillation frequency and compare with the theoretical value from component values; note the effect of loading on amplitude and frequency stability.
Phototransistor: light falling on the base-collector junction generates a photocurrent that is then amplified by the transistor's current gain, giving much higher sensitivity than a simple photodiode. The characteristic curves (I_C vs V_CE at different illumination levels) resemble BJT output curves but with illumination replacing base current as the controlling parameter.
Fibre-optic communication experiment: demonstrates transmission of an analog or digital (on-off keyed) signal through an LED/laser diode transmitter, plastic optical fibre link, and photodiode/phototransistor receiver, illustrating the basic building blocks of an optical communication link.
Clipper circuits use diodes to remove (clip) part of a waveform above or below a reference level — positive clippers remove the positive portion, negative clippers the negative portion, and biased clippers clip at an adjustable reference level set by a battery/potential divider.
Fig. 1.1B (Animated) — A half-wave rectifier diode conducts only during the positive half-cycle of the AC input, so the output waveform retains only the positive lobes of the sine wave and is clamped to zero during each negative half-cycle.
Clamper circuits shift a waveform up or down without changing its shape, using a diode and capacitor to add a DC level — useful for restoring DC levels lost in AC-coupled systems.
Wien-bridge oscillator (IC 741): an RC feedback oscillator producing a sine wave, frequency f = 1/(2πRC), widely used as a reference for audio-frequency signal generation with a op-amp providing gain.
555 timer IC: in astable mode it free-runs as a square-wave generator with frequency set by two resistors and a capacitor; in monostable mode it produces a single output pulse of duration T = 1.1RC when triggered — the building block for countless timing and pulse-generation applications.
Fig. L1.7 (Animated) — In the 555 timer's astable mode, the timing capacitor repeatedly charges and discharges between the internal 1/3 Vᴄᴄ and 2/3 Vᴄᴄ thresholds (top trace), while the output (bottom trace) switches high and low each time a threshold is crossed — generating a continuous square wave.
Faculty Course Portal · 2026–27