Diodes Waveform shaping Circuits Lecture notes: page 2-20 to 2-31 - PowerPoint PPT Presentation

Diodes Waveform shaping Circuits Lecture notes: page 2-20 to 2-31 Sedra & Smith (6 th Ed): Sec. 4.5 & 4.6 Sedra & Smith (5 th Ed): Sec. 3.5 & 3.6 F. Najmabadi, ECE65, Winter 2012

Two-port networks as building blocks  Recall: Transfer function of a two-port network can be found by solving this circuit once.  Concept of input resistance can be used to find v i /v sig (will be discussed in transistor amplifier section)!  We focus on finding transfer function, v o vs v i (circuit below) o “Open-loop” Transfer function ( R L → ∞ or i o = 0 ) F. Najmabadi, ECE65, Winter 2012

Rectifier Circuit = KCL : i i o D = + → = − KVL : v v v v v v i D o o i D Ω = Law : i v / R D o L = < Diode OFF : i 0 and v V D D D 0 = = v R i 0 o L D < → − = < v V v v v V D D 0 i o i D 0 = ≥ Diode ON : and 0 v V i D D 0 D = − = − v v v v V o i D i D 0 = ≥ → = − ≥ → ≥ i v / R 0 v v v 0 v V D o L o i D i D 0 ≥ = − For v V , Diode ON and v v V i D 0 o i D 0 < = For v V , Diode OFF and v 0 i D 0 o F. Najmabadi, ECE65, Winter 2012

Rectifier Circuit: v o is the positive portion v i ≥ = − For v V , Diode ON and v v V i D 0 o i D 0 < = For v V , Diode OFF and v 0 i D 0 o F. Najmabadi, ECE65, Winter 2012

Application of Rectifier Circuit: AC to DC convertor for power supply Half-wave rectifier (only converts half of AC input to DC value) Full-wave rectifier (converts all of AC input to DC value) F. Najmabadi, ECE65, Winter 2012

Each pair of diodes conduct only for half of the cycle F. Najmabadi, ECE65, Winter 2012

Clipper or Limiter Circuit (open-loop transfer function) = < = ≥ Diode OFF : i 0 and v V Diode ON : v V and i 0 D D D 0 D D 0 D = × + → = = v R 0 v v v v V i o o i o D 0 < → < = − ≥ → ≥ v V v V i ( v V ) / R 0 v V D D 0 i D 0 D i D 0 i D 0 ≥ = For v V , Diode ON and v V i D 0 o D 0 < = For v V , Diode OFF and v v i D 0 o i F. Najmabadi, ECE65, Winter 2012

Clipper Circuit does not allow v o > V D0 to go through ≥ = For v V , Diode ON and v V i D 0 o D 0 < = For v V , Diode OFF and v v i D 0 o i  Impact of R L is discussed as an exercise problem F. Najmabadi, ECE65, Winter 2012

Rectifier & clipper circuits are the same but v o is taken at different locations Half-wave Rectifier Clipper F. Najmabadi, ECE65, Winter 2012

Clipper circuit limits v o when the diode is ON  By adjusting “ V D0 ” we can adjust limiting voltage! F. Najmabadi, ECE65, Winter 2012

Limiting voltage can be adjusted v o limited to ≤ V D0 + V Z v o limited to ≤ V D0 + V DC F. Najmabadi, ECE65, Winter 2012

Bottom portion of signal can also be clipped v o limited to ≥ − V D0 − V DC v o limited ≥ − V D0 − V Z F. Najmabadi, ECE65, Winter 2012

Both top & bottom portions of the signal can be clipped simultaneously v o limited to ≤ V D0 + V DC1 and ≥ − V D0 − V DC2 v o limited to ≤ V D0 + V Z1 and ≥ − V D0 − V Z2 F. Najmabadi, ECE65, Winter 2012

“Ideal” Peak Detector Circuit  Because v c cannot change suddenly, the state of diode will depend not only on v i but also on the “history” of the circuit ( e.g., dv i /dt , v c at certain times,) = < Diode OFF : i 0 and v V D D D 0  Capacitor does not charge or discharge!  v c ( t ) = v c 0 where v c 0 is the capacitor voltage at the moment diode turned OFF! = = v v const. o c 0 = − < → < + v v v V v v V D i c D 0 i c 0 D 0 F. Najmabadi, ECE65, Winter 2012

“Ideal” Peak Detector Circuit (open-loop transfer function) = ≥ Diode ON : v V and i 0 D D 0 D = = − v v v V o c i D 0 − dv d ( v V ) dv = = = = c i D 0 i i i C C C D c dt dt dt dv = ≥ → ≥ i i i 0 0 D c dt ≥ = + = = − For dv /dt 0 & v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = For v v V : Diode OFF, v v const i c D 0 o c 0  Because state of diode depends on v c , we cannot produce a universal plot v o vs v i F. Najmabadi, ECE65, Winter 2012

Response of the “Ideal” Peak Detector (1) ≥ = + = = − For dv /dt 0 & v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = For v v V : Diode OFF, v v const i c D 0 o c 0  Start at t = 0 with v c = 0  When v i = v c 0 + V D0 = V D0 , diode  For t > 0, dv i /dt > 0. turns ON (since dv i /dt > 0)  For v i < v c 0 + V D0 = V D0 ,  Capacitor starts to charge and v c tracks v i diode remains OFF. v o = v c 0 = 0 v o = v c = v i - V D0 o o F. Najmabadi, ECE65, Winter 2012

Response of the “Ideal” Peak Detector (2) ≥ = + = = − For dv /dt 0 & v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = For v v V : Diode OFF, v v const i c 0 D 0 o c 0  Cap continue to charge until  Even when v i starts to increase ( dv i /dt > 0) v i = V + ( v c = V + - V D0 ) diode remains OFF as v o < v c0 + V D0  Afterward v i starts to v c 0 + V D0 = V + − V D0 +V D0 = V + ! o decrease ( dv i /dt < 0) and  Diode turns ON v i = V + and immediately diode turns OFF. turns OFF v i starts to decrease ( dv i /dt < 0) v o = v c0 = V + − V D0 o F. Najmabadi, ECE65, Winter 2012

Response of the “Ideal” Peak Detector (3)  v o is the “peak” value of input waveform ( V + – V D 0 ): “Peak Detector” o Note v o did not “drop” after the peak was decreased in the 3 rd cycle. Exercise: Show that if the diode direction is reversed, circuit detects the “negative” peak value, − V − (i.e., lowest voltage of the wave form which should be negative) F. Najmabadi, ECE65, Winter 2012

Practical Peak Detector Circuit (1)  A resistor is added in parallel to the capacitor! (It can be the load for the circuit) = < Diode OFF : i 0 and v V D D D 0 = = − τ v v ( t ) v exp[ - ( t t )/ ]  Capacitor discharges into the resistor o c c 0 0 = − < → < + with a time constant of τ = RC v v v V v v ( t ) V D i c D 0 i c D 0 F. Najmabadi, ECE65, Winter 2012

Practical Peak Detector Circuit (2) = ≥ Diode ON : v V and i 0 D D 0 D = = − v v v V o c i D 0 − dv d ( v V ) dv = = = = c i D 0 i i i C C C D c dt dt dt dv = ≥ → ≥ i i i 0 0 D c dt ≥ = + = = − For dv /dt 0 & , v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = − τ For v v V : Diode OFF, v v ( t ) v exp[ - ( t t )/ ] i c D 0 o c c 0 0 F. Najmabadi, ECE65, Winter 2012

Response of the Practical Peak Detector (1) ≥ = + = = − For dv /dt 0 & , v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = − τ For v v V : Diode OFF, v v ( t ) v exp[ - ( t t )/ ] i c D 0 o c c 0 0  Start at t = 0 with v c = 0  When v i = v c 0 + V D0 = V D0 , diode  For t > 0, dv i /dt > 0. turns ON (since dv i /dt > 0)  For v i < v c 0 + V D0 = V D0 ,  Capacitor starts to charge and v c tracks v i diode remains OFF. v o = v c 0 = 0 v o = v c = v i - V D0 o o F. Najmabadi, ECE65, Winter 2012

Response of the Practical Peak Detector (2) ≥ = + = = − For dv /dt 0 & , v v V : Diode ON , v v v V i i c D 0 o c i D 0 < + = = − τ For v v V : Diode OFF, v v ( t ) v exp[ - ( t t )/ ] i c D 0 o c c 0 0  Cap continue to charge until  Even when v i starts to increase ( dv i /dt > 0) v i = V + ( v c = V + - V D0 ) diode remains OFF as long as v o < v c + V D0  Afterward v i starts to decrease  Diode turns ON when v i = v c + V D0 and ( dv i /dt < 0) and diode turns charges capacitor until v i = V + is reached ) OFF. Capacitor discharges: = = − τ v v ( t ) v exp[ - ( t t )/ ] o c c 0 0 F. Najmabadi, ECE65, Winter 2012

Response of the Practical Peak Detector (3)  Shape of output signal depends on the ratio of τ /T  “ideal” peak detector: τ /T → ∞  “Good” peak detector: τ /T >> 1  As τ /T decreases, the circuit departs from a peak detector.  For τ /T << 1, capacitor discharges very fast and circuit resembles a rectifier circuit Decreasing τ /T F. Najmabadi, ECE65, Winter 2012

Peak detector is used in AM receivers Carrier wave amplitude is modulated with the sound data (sound signal is the “envelop” of the carrier wave) << τ = << T RC T carrier sound F. Najmabadi, ECE65, Winter 2012