Review EEE118: Electronic Devices and Circuits Reviewed the - PDF document

b b EEE118: Lecture 10 Review EEE118: Electronic Devices and Circuits Reviewed the principle of operation of Zener diodes (impact Lecture X ionisation). Used the device (diode) characteristic to examine the James E Green operating point and linearity of a circuit. Introduced the Zener diode shunt regulator circuit. Department of Electronic Engineering University of Sheffield Provided a method for designing the component values of the j.e.green@sheffield.ac.uk regulator. Introduced the idea of small signals and large signals. Considered the effects of distortion that large signals experience due to the non-linear nature of the diode characteristic with an audio example. 2/ 22 1/ 22 EEE118: Lecture 10 EEE118: Lecture 10 Linearising Circuits Outline Circuit Linearisation 1 Linearising Circuits Diode Conducting: Diode Not Conducting: Internal Resistance Improved Diode Model 1 k Ω 1 k Ω 2 Example Small Signal Diode Application 3 How does it look on the Characteristics? + V o V o 10 V 0.7 V 10 V 4 The Transistor − 5 Bipolar Junction Transistor 6 Numbering Systems In this model the diode is a perfect voltage source (0.7 V) with no JEDEC internal resistance. The model can be improved by the addition of Pro Electron a resistance in series with the voltage source - remember 7 Review Th´ evenin... 8 Bear 3/ 22 4/ 22 EEE118: Lecture 10 EEE118: Lecture 10 Linearising Circuits Linearising Circuits Internal Resistance Improved Diode Model Diode with Internal Resistance Diode with Internal Resistance Model The diode has an internal series resistance, which is proportional to Adding a constant resistance in series with the voltage source the slope of its characteristic. improves the accuracy of the diode model, but the diode resistance 30 dV B changes with diode current so many different values of resistor may Dynamic resistance blue: 3 . 75 kΩ 25 be needed. We use a fixed resistor based on the operating point or Dynamic resistance red: 31 . 25 kΩ quiescent conditions. It is important that the signal is small with Current [ µ A] 20 respect to the quiescent conditions otherwise the use of a single dI B 15 value of resistor will not accurately represent the diode operation. 60 Dynamic resistance is a r d 10 i d 1N4148 50 function of opperating point Voltage Source Current [mA] Th´ evenin 5 40 dI R 30 + 0 v d 0.7 V 20 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 − 10 dV R V A − C [V] 0 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 The internal series resistance depends on the current flowing V A − C [V] through the diode. The series resistance is changing continuously , 5/ 22 6/ 22 but over a small region it is nearly constant.

EEE118: Lecture 10 EEE118: Lecture 10 Example Small Signal Diode Application Example Small Signal Diode Application Example Small Signal Diode Application The Components Problem Your friend is watching TV in the next room. You can hear the TV Name Purpose all the time but the adverts are louder than the normal programming. It’s the adverts that are disturbing your thoughts I B Sets the operating point of the diode. The TV audio output. while attacking a particularly difficult EEE118 problem sheet. Your v 1 friend is unwilling to turn the TV down, so you decide to build a A capacitor to block any DC voltage from the TV C 1 which might bias the diode. circuit to automatically control the volume of the TV to a constant level. R 1 The upper resistor in a potential divider. The lower (small signal) resistor in a potential divider. D 1 A capacitor to block the ∼ 0.7 V across the diode from C 1 C 2 R 1 22 µ F passing a current into the oscilloscope (CRO). 4.7 k Ω I B R 2 A simple approximation to an oscilloscope probe. 100 µ A C 2 to v 1 22 µ F 30 mA D 1 R 2 v o 0.2 V 1N4148 10 M Ω pk − pk I D 1 CRO 7/ 22 8/ 22 EEE118: Lecture 10 EEE118: Lecture 10 Example Small Signal Diode Application Example Small Signal Diode Application How Does It Work? Two Operating Points The diode dynamic or incremental or small signal resistance ( r d ) We will inspect two examples at different values of I B to observe varies according to the current flowing through the diode ( D 1 ). the effect on the value of r d and the output of the circuit. The The quiescent current in the diode is simply I B . We aim to make total diode current is the sum of the quiescent current ( I B ) and the the signal current small with respect to I B in order that r d will vary current flowing in the potential divider due to v 1 . only with I B . A voltage will appear across D 1 which is sufficient to sustain the current flowing in it. It will be approximately 0.7 V. The value of I B should be set by the average amplitude of the TV The linearisation of the circuit requires that the signal current due output (perhaps by using a peak detector with a long time to v 1 does not change the total current so much that the constant, but this is ignored, for now...). When the TV volume is exponential shape of the diode’s IV characteristic becomes “loud” I B will be larger and so r d will be smaller and will drop a significant. To ensure the Th´ evenin model of the diode holds the smaller share of the TV’s sound signal. Since r d is the lower leg of diode characteristic must approximate a straight line. the potential divider - across which the output is taken - the volume will be reduced. This is an example of feedback. 9/ 22 10/ 22 EEE118: Lecture 10 EEE118: Lecture 10 Example Small Signal Diode Application Example Small Signal Diode Application Example Diode Characteristic at Two Operating Points I B Small: Calculate Some Important Parameters We would like to know the small signal resistance of the diode, 10 . 06 10 . 04 ∆ V = 1 ∆ I (1) r d 10 . 02 Current [mA] 1 = 360 µ A − 271 µ A 10 . 00 (2) r d 538 mV − 525 mV 10 9 . 98 r d = 146 Ω (3) 8 9 . 96 Current [mA] 6 9 . 94 And the total signal current, 688 690 692 694 696 698 700 702 V A − C [mV] 4 r total = 4 . 7 k Ω + 146 Ω (4) 2 400 0 350 = 4846 Ω (5) Current [ µ A] 0 0 . 2 0 . 4 0 . 6 0 . 8 r = 0 . 2 V A − C [mV] 300 i = v (6) 4846 250 = 41 . 2 µ A pk − pk (7) 200 525 527 529 531 533 535 537 539 V A − C [mV] 11/ 22 12/ 22

EEE118: Lecture 10 EEE118: Lecture 10 Example Small Signal Diode Application Example Small Signal Diode Application I B Small: Small Signal Equivalent Circuit I B Large: Calculate Some Important Parameters We would like to know the small signal resistance of the diode, The small signal equivalent circuit is a circuit diagram which shows ∆ V = 1 ∆ I only the circuit components that influence what happens to the (10) signal. It is how the signal “sees” the circuit. r d 1 = 10 . 8 mA − 9 . 2 mA (11) 4.7 k Ω r d 698 mV − 690 mV r d = 5 Ω (12) v i v o 146 Ω And the total signal current, r total = 4 . 7 k Ω + 5 Ω (13) = 4705 Ω (14) 146 i = v r = 0 . 2 v o = (8) (15) 4705 v i 4700 + 146 = 42 . 5 µ A pk − pk (16) ≈ 0 . 03 V (9) V Note, making R 1 much larger than r d controls r total and so keeps the peak to peak value of i almost constant. 13/ 22 14/ 22 EEE118: Lecture 10 EEE118: Lecture 10 Example Small Signal Diode Application How does it look on the Characteristics? I B Large: Small Signal Equivalent Circuit Representing Everything on the Characteristic The small signal equivalent circuit has a new value for r d . Note 12 that the quiescent conditions don’t appear in the small signal 11 Anode Current [mA] 10 circuit. Only linear components (R, L, C and Sources) appear in 9 9 Time [s] Anode Current [mA] 8 8 small signal circuits. 0.5 1.0 1.5 2.0 2.5 7 7 6 4.7 k Ω 5 4 Anode Current [mA] 3 3 Time [s] 2 1.5 0.5 1.0 1.5 2.0 2.5 1 v i v o 5 Ω 0 0 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 Anode - Cathode Voltage [mV] In this diagram the signal currents V A − C [mV] V A − C [mV] are much larger than 41 µ A in 520 530 540 550 560 570 580 590 600 610 620 670 680 690 order that they can be seen easily this allows us to observe the distortion 0.5 0.5 that occurs when the signal is becomes 5 v o “large” compared to the DC (quiescent) 1.0 1.0 = (17) conditions. v i 4700 + 5 1.5 1.5 ≈ 0 . 00106 V 2.0 2.0 (18) 2.5 2.5 V Time [s] Time [s] 3.0 3.0 15/ 22 16/ 22 EEE118: Lecture 10 EEE118: Lecture 10 The Transistor Bipolar Junction Transistor Transistor Definition Bipolar Junction Transistor The bipolar transistor 1 was invented by John Bardeen, Walter Definition Brattain and William Shockley in the late 1940s at Bell Labs. A transistor is a three terminal semiconductor electronic device Read: http://dx.doi.org/10.1109/5.658752 . They shared which is capable of power amplification. the 1956 Nobel Prize. The BJT is a semiconductor device Different from a transformer or resonant circuit which can only composed of three semiconductor regions N-P-N or P-N-P named increase the amplitude of current or voltage. Several different Emitter (E), Base (B) and Collector (C). The NPN can be thought kinds of transistor exist (BJT, MOSFET, JFET) and Valves. of as two diodes with their anodes connected together. The PNP, BJT is the most common small signal amplifier. two cathodes connected. MOSFETs are more common in large signal applications such as switching power supplies. MOSFETs also find use in integrated circuits (producing them on a semiconductor wafer is easy c.f BJT). JFETs are found in ICs but are also used as discrete devices. Thermionic valves are limited to specialist applications (e.g. high power microwave generation, radio and RADAR transmission, specialist audio applications.) 17/ 22 18/ 22