Electronics Components: Diodes
SEMICONDUCTORS ELTE 1403 Forward biased diode: cathode anode 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. Varactor Zener N + P electron flow depletion layer 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. rd 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: Vdc = Vp 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: LED ? PIV = V p I diode = I dc Vrms 2 ? 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. Schottky Full-Wave Rectifier: diode reverse voltage: diode forward current: Vdc = Vp ? = Vrms 2 ? Vp is the voltage across the full secondary winding) PIV = V p I diode = 1 I dc 2 Tom Penick [email protected] com www. teicontrols. com/notes 06/12/98 Full-Wave Rectifier With Capacitor Filter: 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 ? ? PIV = V p I diode = I dc 1 2 2V p ? DC ?DC I = C IE ? = DC ? DC + 1 IB Base Bias VBB VCC RB RC IC Vdc = Vp = Vrms 2 Vdc = V p ? Vrip 2 DC Beta: (usually 50 300) ? DC = IC IB VBE = . 7V IE hFE is the same as ? DC, 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 V + ICQ rL CEQ Q V CE(CUTOFF) VCE A choke is an iron-core inductor with a large value of L in Henrys. The choke has an inductive reactance in ohms of: A capacitor has an inductive reactance in ohms of: The resonant frequency of an inductor and capacitor (or varactor) in parallel:
X L = 2 ? f L 1 XC = 2 ? f C 1 f = 2? LC Clipper: Removes either the positive or negative peaks of a sine wave by shorting through a diode. Clamper: Raises or lowers the sine wave so that it becomes mostly positive or mostly negative. Or Gate: Output goes high when any input is high. And Gate: Output goes high when all inputs are high The DC Load Line is a graph representing all possible dc operating points of the transistor for a specific load resistor. VCE is the x-axis and IC is the y-axis. The equation is VCE = VCC ? I C RC .
The horizontal intercept will be the supply voltage VCC and the vertical intercept will be the collector current when the transistor is saturated, i. e. the collector/emitter is considered a closed switch. The Q Point is the operating point of the transistor, usually located near the middle of the DC Load Line AC Load Line The Q point VCEQ ic ( sat ) = I CQ + moves along the AC load rL line. Steeper than the DC load line. vce ( cutoff ) = VCEQ + I CQ rL AC Compliance – maximum peak to peak AC output voltage without clipping. AC Compliance is alculated by finding the smaller of the following: Saturation Clipping: Cutoff Clipping: TRANSISTORS heavily doped lightly doped npn n p n Emitter Base Collector electron flow PP = 2 I CQ rL PP = 2VCEQ n IE forward biased p IB n pnp When the Q point is centered on the DC load line, cutoff clipping occurs first because the AC load line is always steeper than the DC load line. p IC reverse biased p n DC Compliance is the DC voltage range over which the transistor can operate; in other words VCC. (the n is next to the arrow) I E = IB + I C VCE = VCC ? IC RC Tom Penick [email protected] com www. teicontrols. com/notes 06/12/98
Voltage Divider Bias: The Base Bias circuit above is usually impractical in linear circuits because the Q point is unpredictable due to variations in ? DC. The Voltage Divider Bias shown at right solves this problem. When ? DC is known, IE may be calculated as: VCC R1 RC IC AC Emitter Resistance of a Transistor: re? = 25mV IE ?= VCC AC Beta: Called ? as opposed to ? dc (DC Beta). Referred to as hfe as opposed to hFE for DC Beta. CE Characteristics: Common Emitter Output is out of phase with input High voltage gain is possible May be used with a swamping resistor to stabilize the voltage gain In a matched load condition, RL = RC c ib VB ? VBE IE ? RE + ( R1 R2 ) / ? dc IE R2 RE R1 RC RL But when RE ;; R1 R2 , ? dc the equation may be reduced to: I E ? 1) 2) 3) 4) 5) V B ? VBE RE R2 RE Calculate the voltage at the base The emitter voltage is . 7 less than the base Calculate IE IC ? IE Calculate voltage drop across RC When designing the voltage divider bias amplifier, the current through the voltage divider should be at least 10 times the current through the base. To center Q on the DC load line, VCE will be ? VCC, VE will be about . 1VCC. To center Q on the AC VCC I CQ = load line, use the RC + RE + rL formula:
AC Input Impedance of CE Amplifier: AC Voltage Gain (CE) when the emitter is AC ground: Swamping Resistor To desensitize a CE amplifier to changes in r’e, a resistor rE is added between the emitter and ac ground. This stabilizes the amount of gain, but also reduces it. Heavy Swamping The value of rE is much larger than the value of r’e: zin = R1 R2 ? re? A= zin ( base ) Vout rL = Vin re? = ? ( rE + re? ) zin = R1 R2 ? ( rE + re? ) A= rL rE + re? rL rE Other Biasing Methods Emitter Feedback Bias VCC RB IB RC IC Collector Feedback Bias Emitter Bias VCC zin = R1 R2 ? rE A= VCC RB IB
IE RC IC IB RC IC AC Input Voltage when a source v in zin vb = resistor (a resistor in series with Rs + zin the input) is present. AC Load Resistance, rL, rc, or rLac, is the parallel combination of all AC paths from collector to ground. Remember the battery and capacitors are considered shorts. IE RB RE -VEE AC Power delivered to the load (class A amplifier): where VL is rms: using peak to peak volts: IE RE VCC ? VBE RE + RB / ? dc VCC ? VBE RC + R B / ? dc PL = VL 2 RL PL = IC ? IC ? IE ? VEE ? VBE RE AC Resistance of a Diode: where I is the dc current through the diode.
To a second approximation, consider the . 7V drop across the diode in calculating the value I. rac = 25mV I Quiescent Power Dissipation of a transistor: Efficiency of a stage: P PL is load power at AC ? = L (max) ? 100% PCC compliance Total Current Drain is the voltage I CC = I 1 + I CQ divider current plus the collector current: VPP 2 8 RL PDQ = VCEQ I CQ Cascaded Stages Gain: Cascaded A = A1 A2 A3 rL = RC zin Stages The AC load resistance of one stage is affected by the impedance of the following stage: Tom Penick [email protected] com www. teicontrols. com/notes 06/12/98
CC Characteristics: Voltage gain ; 1 High input impedance AC output is in phase Low-distortion Has power gain Can be placed at the output of a CE amplifier to reduce output loading and thereby increase the gain. Common Collector (Emitter Follower) Field Effect Transistors Junction Field Effect Transistor JFET N channel P channel VCC R1 G D G S R2 RE RL Creating a depletion region by reverse biasing the gate reduces (pinches) current between the drain and the source. Never forward biased D S Drain R1 R2 ? ( rL + re? ) AC Voltage Gain of a CC is slightly r A= L less than 1: rL + re?
AC Power Gain of a CC: r G = ? L = ? A ? ? rL + re? AC Output Power of a CC: The Darlington Amplifier consists of cascaded CC’s for a very large increase in input impedance. R1 Input Impedance (high) of a CC: Gate n p p n Source V GG V DD Metal Oxide Silicon Field Effect Transistors Enhancement-type MOSFET N channel P channel Pout = ie 2 rL Darlington Pair VCC G D B S G D B S G Gate D Drain B Substrate* S Source R2 RE RL Thin silicon dioxide layer Metal Drain Gate *usually connected internally to the source Source n p n Substrate The Zener Follower is a oltage regulator circuit that offers improved load handling over the zener regulator. Voltage output is . 7V less than the value of the zener diode. Zener Follower Depletion-type MOSFET + N channel RL P channel Vin – G D B S G D B S CB Characteristics: Low input impedance Large voltage gain AC output in phase Useful at high frequencies Not as popular as CE or CC A differential amplifier is a CB driven by a CE Depletion-type MOSFET Common Base VCC MOSFET’s do not have thermal runaway. Gate may be positive or negative Drain Gate RL n p n Substrate Source Tom Penick [email protected] com www. teicontrols. com/notes 06/12/98