전자회로 1 (Fundamentals of Microelectronics 1) Diode Models and Circuits

Transcription

1 전자회로 1 (Fundamentals of Microelectronics 1) Diode Models and Circuits Instructor: Prof. Jintae Kim Mixed-Signal Electronics Group Konkuk University

2 What we will learn Diode model as circuit elements - Ideal model - P-N junction as a diode (constant-voltage model) - Large-signal and small-signal analysis Applications - Regulators - Rectifiers - Limiting and Clamping circuits We skip

3 The Need for a Simplified Models We looked at (simplified) underlying physics, deriving the P-N junction current for different bias conditions The exponential model, however, is still complicated and does not lend itself to a quick circuit-level analysis Therefore, we need simplified models for the diode - Ideal Model - Constant-oltage Model 3

4 Ideal-Model In an ideal diode, if the voltage across it tends to exceed zero, current flows. It is analogous to a water pipe that allows water to flow in only one direction. 4

5 I/ Characteristics of an Ideal Diode R 0 I R I 0 R R If the voltage across anode and cathode is greater than zero, the resistance of an ideal diode is zero and current becomes infinite. However, if the voltage is less than zero, the resistance becomes infinite and current is zero. 5

6 Example: Anti-Parallel Ideal Diodes If two diodes are connected in anti-parallel, it acts as a short for all voltages. 6

7 Example: Diode-Resistor Combination The I characteristic of this diode-resistor combination is zero for negative voltages and Ohm s law for positive voltages 7

8 Application: Rectifier A rectifier is a device that passes positive-half cycle of a sinusoid and blocks the negative half-cycle or vice versa. When in is greater than 0, diode shorts, so out = in ; however, when in is less than 0, diode opens, no current flows thru R 1, out = I R1 R 1 = 0. 8

9 Example: OR Gate using Diodes The circuit above shows an example of diodeimplemented OR gate. out can only be either A or B, not both. 9

10 Application: Limiter The purpose of a limiter is to force the output to remain below certain value. The addition of a 1 battery forces the diode to turn on after 1 has become greater than 1. 10

11 Application: :Limiter with varying B Limiting fails if B is greater than the input amplitude. 11

12 Constant-oltage Model Slightly more realistic model: constant voltage model approximates an exponential behavior of PN junction with built-in potential. 12

13 Ideal vs. Constant-oltage model The circuit above shows the difference between the ideal and constant-voltage model; the two models yield two different break points of slope. 13

14 More Examples When using a constant-voltage model, the voltage drop across the diode is no longer zero but d,on when it conducts. 14

15 More Examples In this example, since in is connected to the cathode, the diode conducts when in is very negative. The break point where the slope changes is when the current across R1 is equal to the current across R2. 15

16 Large-Signal and Small-Signal Analysis I x out 3 3 D T ln I I X s out = 3 D,on is used to charge cell phones. However, if Ix changes, iterative method is often needed to obtain a solution, thus motivating a simpler technique. 16

17 Small-Signal Analysis I D T I D1 Small-signal analysis is performed around a bias point by perturbing the voltage by a small amount and observing the resulting linear current perturbation. 17

18 Small-Signal Analysis in Detail I D I D s T I D1 T di d D D I exp D1 T DD1 If two points on the I curve of a diode are close enough, the trajectory connecting the first to the second point is like a line, with the slope being the proportionality factor between change in voltage and change in current. 18

19 Small-Signal model rd I Since there s a linear relationship between the small signal current and voltage of a diode, the diode can be viewed as a linear resistor when only small changes are of interest. T D 19

20 Small Sinusoidal Input ( t) 0 p cost If a sinusoidal voltage with small amplitude is applied, the resulting current is also a small sinusoid around a DC value. I D ( t) 0 I0 I p cost I s exp I T T 0 p cost 20

21 Example: Adapter v out 3rd R 3r 1 d 11.5m v ad I (3r 0.5mA(3 4.33) 6.5m Large-Signal Model Bias calculation Small-Signal Analysis response to the input change out D d ) 21

22 Applications of Diode Half-Wave Rectifier Full-Wave Rectifier Clamping Circuits Regulators 22

23 Half-Wave Rectifier A very common application of diodes is half-wave rectification, where either the positive or negative half of the input is blocked. But, how do we generate a constant output? 23

24 Diode-Capacitor Circuit If the resistor in half-wave rectifier is replaced by a capacitor, a fixed voltage output is obtained since the capacitor (assumed ideal) has no path to discharge. 24

25 Diode-Capacitor With Load Resistor A path is available for capacitor to discharge. Therefore, out will not be constant and a ripple exists. 25

26 Peak to Peak amplitude of Ripple The ripple amplitude is the decaying part of the exponential. Ripple voltage becomes a problem if it goes above 5 to 10% of the output voltage. 26 in L on D p in L on D p R L on D p on D p L on D p out L on D p out f C R C T R C t R C R t t C R t t 1, 1, 1,, 1, 1, ) ( ) )(1 ( ) ( )exp ( ) ( 0 t T in

27 Full-Wave Rectifier A full-wave rectifier passes both the negative and positive half cycles of the input, while inverting the negative half of the input. As proved later, a full-wave rectifier reduces the ripple by a factor of two. 27

28 The Evolution of Full-Wave Rectifier Figures (e) and (f) show the topology that inverts the negative half cycle of the input. 28

29 Full-Wave Rectifier: Bridge Rectifier The figure above shows a full-wave rectifier, where D 1 and D 2 pass/invert the negative half cycle of input and D 3 and D 4 pass the positive half cycle 29

30 Complete Full-Wave Rectifier Since C 1 only gets ½ of period to discharge, ripple voltage is decreased by a factor of 2. Also (b) shows that each diode is subjected to approximately one p reverse bias drop (versus 2 p in half-wave rectifier). 30

31 Half vs. Full-wave Rectifiers Full-wave rectifier is more suited to adapter and charger applications. 31

32 oltage Regulator The ripple created by the rectifier can be unacceptable to sensitive load; therefore, a regulator is required to obtain a very stable output. Three diodes operate as a primitive regulator. 32

33 Limiting Circuit Using a Diode As was studied in the past, the combination of resistordiode creates limiting effect. 33

34 arious Clipping Circuits 34

35 General oltage Limiting Circuit Two batteries in series with the antiparallel diodes control the limiting voltages. 35