Forward biased diode:
Varactor is a silicon diode optimized for its variable capacitance when reversedbiased. Used for tuning frequencydependent equipment. Zener Diode is designed to operate in the breakdown region; used for voltage regulation.
An ideal diode acts like a closed switch when forward biased and an open switch when reverse biased. 1st approximation calculations assume an ideal diode. 2nd approximation calculations take into account the voltage drop across the diode. 3rd approximation calculations additionally take into account bulk resistance. Voltage Drop silicon diode .7V germanium diode .3V Bulk Resistance rB = ∆E/∆I A digital multimeter won't measure the resistance on a diode due to insufficient voltage. The diode check function of a digital multimeter reads the knee voltage. The knee voltage is the voltage at which a forward biased diode begins to conduct.
Avalanche Effect Reverse voltage exceeds the breakdown voltage and the minority carriers are given enough energy to dislodge valence electrons from their orbits. These free electrons then dislodge others. Zener Effect The electric field becomes strong enough across the junction of a heavily-doped reverse-biased diode to pull valence electrons from their shells. For breakdown voltages below 5V, the Zener effect dominates, above 6V the avalanche effect dominates. Second Approximation for a Zener Diode V − Vz I z = in Rs + Rz Iz = zener current Vin = supply voltage Vz = zener voltage Rs = source resistance Rz = zener resistance
Zener Resistance is the small series resistance of a zener diode when it operates in the breakdown region. ∆Vout = ∆I z Rz ∆V = change in output voltage ∆Iz = change in zener current Rz = zener resistance
Diode Ratings: PIV Reverse Breakdown Voltage If Forward Current Limit IS Saturation Current - minority carrier current of a reverse-biased diode Rf Forward Resistance Vk Knee Voltage
Half-Wave Rectifier: diode reverse voltage:
Light Emitting Diode When forward-biased,
free electrons combine with holes near the junction. As they move from an area of higher energy to lower energy, they emit radiation. Assume 2V drop unless specified.
diode forward current:
π PIV = V p I diode = I dc
Vrms 2 π
Half-Wave Rectifier With Capacitor Filter:
PIV = 2V p Vdc = Vp = Vrms 2
Schottky Diode has almost no charge
storage, so can switch on and off much faster than an ordinary diode. Has metallic/silicon junction; low power handling; .25V offset voltage; used for high frequencies.
Full-Wave Rectifier: diode reverse voltage: diode forward current:
Vrms 2 π
Vp is the voltage across the full secondary winding)
PIV = V p
I diode = 1 I dc 2
Full-Wave Rectifier With Capacitor Filter:
1 1 Vdc = 2 Vp = 2 Vrms 2
Bias: difference in potential between base and emitter. DC Alpha: (slightly less than 1)
Bridge Rectifier: diode reverse voltage: diode forward current: Bridge Rectifier With Capacitor Filter: Further refined to include the effect of ripple voltage: Ripple Formula for a capacitor-input filter Vrip I = dc fC
V 2 2 Vdc = = rms π π PIV = V p I diode = I dc
I = C IE
β = DC β DC + 1
Base Bias VBB VCC RB RC IC
Vdc = Vp = Vrms 2 Vdc = V p − Vrip 2
DC Beta: (usually 50 300)
β DC =
VBE = .7V IE
hFE is the same as βDC, the collector
Vrip = peak-to-peak ripple Idc = dc peak load current f = ripple frequency (twice the input frequency for a full-wave rectifier) C = filter capacitance
to emitter current gain The four operating regions of a transistor are saturation, active, cutoff, and breakdown.
DC and AC Load Lines, Q Point IC V CEQ ICQ + r L IC(SAT)
loa dl ine DC load line AC