EEE118: Electronic Devices and Circuits Lecture X James E Green - PowerPoint PPT Presentation

EEE118: Electronic Devices and Circuits Lecture X James E Green Department of Electronic Engineering University of Sheffield j.e.green@sheffield.ac.uk 1/ 22

EEE118: Lecture 10 Review Reviewed the principle of operation of Zener diodes (impact ionisation). Used the device (diode) characteristic to examine the operating point and linearity of a circuit. Introduced the Zener diode shunt regulator circuit. Provided a method for designing the component values of the 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

EEE118: Lecture 10 Outline 1 Linearising Circuits Internal Resistance Improved Diode Model 2 Example Small Signal Diode Application 3 How does it look on the Characteristics? 4 The Transistor 5 Bipolar Junction Transistor 6 Numbering Systems JEDEC Pro Electron 7 Review 8 Bear 3/ 22

EEE118: Lecture 10 Linearising Circuits Circuit Linearisation Diode Conducting: Diode Not Conducting: 1 k Ω 1 k Ω + V o V o 10 V 0.7 V 10 V − In this model the diode is a perfect voltage source (0.7 V) with no internal resistance. The model can be improved by the addition of a resistance in series with the voltage source - remember Th´ evenin... 4/ 22

b b EEE118: Lecture 10 Linearising Circuits Internal Resistance Diode with Internal Resistance The diode has an internal series resistance, which is proportional to the slope of its characteristic. 30 dV B Dynamic resistance blue: 3 . 75 kΩ 25 Dynamic resistance red: 31 . 25 kΩ Current [ µ A] 20 dI B 15 Dynamic resistance is a 10 function of opperating point 5 dI R 0 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 dV R V A − C [V] The internal series resistance depends on the current flowing through the diode. The series resistance is changing continuously , 5/ 22 but over a small region it is nearly constant.

EEE118: Lecture 10 Linearising Circuits Improved Diode Model Diode with Internal Resistance Model Adding a constant resistance in series with the voltage source improves the accuracy of the diode model, but the diode resistance changes with diode current so many different values of resistor may be needed. We use a fixed resistor based on the operating point or quiescent conditions. It is important that the signal is small with respect to the quiescent conditions otherwise the use of a single value of resistor will not accurately represent the diode operation. 60 r d 1N4148 i d 50 Voltage Source Current [mA] Th´ evenin 40 30 + v d 20 0.7 V − 10 0 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7 0 . 8 V A − C [V] 6/ 22

EEE118: Lecture 10 Example Small Signal Diode Application Example Small Signal Diode Application Problem Your friend is watching TV in the next room. You can hear the TV all the time but the adverts are louder than the normal programming. It’s the adverts that are disturbing your thoughts while attacking a particularly difficult EEE118 problem sheet. Your friend is unwilling to turn the TV down, so you decide to build a circuit to automatically control the volume of the TV to a constant level. C 1 R 1 22 µ F 4.7 k Ω I B 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

EEE118: Lecture 10 Example Small Signal Diode Application The Components Name Purpose Sets the operating point of the diode. I B v 1 The TV audio output. A capacitor to block any DC voltage from the TV C 1 which might bias the diode. 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 2 passing a current into the oscilloscope (CRO). R 2 A simple approximation to an oscilloscope probe. 8/ 22

EEE118: Lecture 10 Example Small Signal Diode Application How Does It Work? The diode dynamic or incremental or small signal resistance ( r d ) varies according to the current flowing through the diode ( D 1 ). The quiescent current in the diode is simply I B . We aim to make the signal current small with respect to I B in order that r d will vary 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 output (perhaps by using a peak detector with a long time constant, but this is ignored, for now...). When the TV volume is “loud” I B will be larger and so r d will be smaller and will drop a smaller share of the TV’s sound signal. Since r d is the lower leg of the potential divider - across which the output is taken - the volume will be reduced. This is an example of feedback. 9/ 22

EEE118: Lecture 10 Example Small Signal Diode Application Two Operating Points We will inspect two examples at different values of I B to observe the effect on the value of r d and the output of the circuit. The total diode current is the sum of the quiescent current ( I B ) and the current flowing in the potential divider due to v 1 . The linearisation of the circuit requires that the signal current due to v 1 does not change the total current so much that the exponential shape of the diode’s IV characteristic becomes significant. To ensure the Th´ evenin model of the diode holds the diode characteristic must approximate a straight line. 10/ 22

EEE118: Lecture 10 Example Small Signal Diode Application Example Diode Characteristic at Two Operating Points 10 . 06 10 . 04 10 . 02 Current [mA] 10 . 00 10 9 . 98 8 9 . 96 Current [mA] 6 9 . 94 688 690 692 694 696 698 700 702 4 V A − C [mV] 2 400 0 350 Current [ µ A] 0 0 . 2 0 . 4 0 . 6 0 . 8 V A − C [mV] 300 250 200 525 527 529 531 533 535 537 539 V A − C [mV] 11/ 22

EEE118: Lecture 10 Example Small Signal Diode Application I B Small: Calculate Some Important Parameters We would like to know the small signal resistance of the diode, ∆ V = 1 ∆ I (1) r d 1 = 360 µ A − 271 µ A (2) r d 538 mV − 525 mV r d = 146 Ω (3) And the total signal current, r total = 4 . 7 k Ω + 146 Ω (4) = 4846 Ω (5) r = 0 . 2 i = v (6) 4846 = 41 . 2 µ A pk − pk (7) 12/ 22

EEE118: Lecture 10 Example Small Signal Diode Application I B Small: Small Signal Equivalent Circuit The small signal equivalent circuit is a circuit diagram which shows only the circuit components that influence what happens to the signal. It is how the signal “sees” the circuit. 4.7 k Ω v i v o 146 Ω 146 v o = (8) 4700 + 146 v i ≈ 0 . 03 V (9) V 13/ 22

EEE118: Lecture 10 Example Small Signal Diode Application I B Large: Calculate Some Important Parameters We would like to know the small signal resistance of the diode, ∆ V = 1 ∆ I (10) r d 1 = 10 . 8 mA − 9 . 2 mA (11) 698 mV − 690 mV r d r d = 5 Ω (12) And the total signal current, r total = 4 . 7 k Ω + 5 Ω (13) = 4705 Ω (14) r = 0 . 2 i = v (15) 4705 = 42 . 5 µ A pk − pk (16) Note, making R 1 much larger than r d controls r total and so keeps the peak to peak value of i almost constant. 14/ 22

EEE118: Lecture 10 Example Small Signal Diode Application I B Large: Small Signal Equivalent Circuit The small signal equivalent circuit has a new value for r d . Note that the quiescent conditions don’t appear in the small signal circuit. Only linear components (R, L, C and Sources) appear in small signal circuits. 4.7 k Ω v i v o 5 Ω v o 5 = (17) v i 4700 + 5 ≈ 0 . 00106 V (18) V 15/ 22

EEE118: Lecture 10 How does it look on the Characteristics? Representing Everything on the Characteristic 12 11 Anode Current [mA] 10 9 9 Time [s] Anode Current [mA] 8 8 0.5 1.0 1.5 2.0 2.5 7 7 6 5 4 Anode Current [mA] 3 3 Time [s] 2 1.5 0.5 1.0 1.5 2.0 2.5 1 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 “large” compared to the DC (quiescent) 1.0 1.0 conditions. 1.5 1.5 2.0 2.0 2.5 2.5 Time [s] Time [s] 3.0 3.0 16/ 22

EEE118: Lecture 10 The Transistor Transistor Definition Definition A transistor is a three terminal semiconductor electronic device which is capable of power amplification. Different from a transformer or resonant circuit which can only increase the amplitude of current or voltage. Several different kinds of transistor exist (BJT, MOSFET, JFET) and Valves. BJT is the most common small signal amplifier. 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