Electronic Engineering

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The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme.

The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

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Electronic Engineering

  1. 1. Electronic Engineering © University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License .
  2. 2. <ul><li>The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year undergraduate programme. </li></ul><ul><li>The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments. </li></ul><ul><li>Contents </li></ul><ul><li>Circuit calculations using active non-linear devices. </li></ul><ul><li>Equivalent Circuits </li></ul><ul><li>Bipolar Transistor. </li></ul><ul><li>The Hybrid Parameter Network. </li></ul><ul><li>The Hybrid  Model </li></ul><ul><li>The electronic equivalent circuit is drawn in the followi... </li></ul><ul><li>Model at High Frequencies </li></ul><ul><li>The Miller Effect </li></ul><ul><li>Field Effect Transistor FET </li></ul><ul><li>Credits </li></ul><ul><li>In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see: </li></ul><ul><li>Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes  </li></ul><ul><li>James M, 2004, Higher Electronics, Newnes </li></ul>Electronic Engineering
  3. 3. Circuit calculations using active non-linear devices. <ul><li>Circuit theorems such as: </li></ul><ul><li>Kirchhoff’s Laws </li></ul><ul><li>Thévenin’s Theorem </li></ul><ul><li>Superposition </li></ul><ul><li>work only if the circuit components are linear i.e. if you double the voltage, you double the current. Components such as resistors, capacitors and inductors are, on the whole linear in nature. When we come to analysing circuits with non-linear components such as diodes, bipolar transistors and field effect transistors we must adopt one of two techniques: </li></ul>Electronic Engineering
  4. 4. <ul><li>Graphical </li></ul><ul><li>Equivalent Circuits </li></ul><ul><li>The graphical method uses plots of the input and output characteristics to determine the characteristics of the created amplifier or circuit. This requires a large amount of graphical information to be available especially if a design is being formulated with a wide range of possible devices to be considered. </li></ul>Electronic Engineering
  5. 5. Electronic Engineering
  6. 6. X I B
  7. 7. Vsupply (Vs) Vs/Rc i B Electronic Engineering
  8. 8. Equivalent Circuits <ul><li>The other approach is to use a circuit comprising linear components, which responds in the same way as the non-linear active device. The equivalent circuit may not be perfect but will often give us a starting point when designing. Note that electronics on the whole is far from exact as we are working with components with relatively high tolerance: </li></ul><ul><li>Resistors typically 5% and Capacitors typically 10% as well as active devices which can vary dramatically in terms of their characteristics from one device to another. </li></ul>Electronic Engineering
  9. 9. <ul><li>Diode </li></ul><ul><li>The diode can be modelled using a resistor R, a voltage source E and an ideal diode </li></ul><ul><li>The diode D only conducts when the anode is positive with respect to the cathode. The supply E ensures that the anode of D only goes positive when the applied voltage reaches a certain positive level. The resistor R controls the current once the diode is conducting. </li></ul>Electronic Engineering R E D Anode Cathode
  10. 10. <ul><li>The result is a characteristic that looks like: </li></ul>E This type of model is called a small signal model as it has good approximation for a small range of inputs. This approximates the characteristic for a simple diode where E for Silicon is about 0.6v. Of course this is not exact but is fairly good over the limited range where the diode is conducting. Electronic Engineering Applied voltage V Current flow Slope = 1/R
  11. 11. 1A 0.05V 0.53V R = 0.05 
  12. 12. Bipolar Transistor. <ul><li>The models for both NPN and PNP are the same. The models vary subtly for different configurations. We will examine the most common configuration – that of the common emitter. </li></ul>Input Output The input on the left is between the base and the emitter and the output on the right is between the collector and the emitter. The emitter is therefore common to input and output which gives the configuration the name. There are a number of models that exist for this device. We will look in detail at two of these.
  13. 13. The Hybrid Parameter Network. <ul><li>This replaces the input and the output sides by conventional circuit theory equivalent circuits. </li></ul>The input is replaced by a Thévenin equivalent i.e. a resistor in series with a voltage source. The output part of the transistor is replaced by a Norton equivalent circuit comprising a current source in parallel with a resistor, as shown
  14. 14. <ul><li>If we now combine these we have the Hybrid Parameter Network. </li></ul>The reason for the choice of equivalent circuits is that the input is voltage driven whilst the output is associated with current flow. Electronic Engineering hie I B I C V BE V CE hoe hre x V CE hfe x I B
  15. 15. There are 4 parameters associated with this model, these being: <ul><li>hie – h ybrid i nput common e mitter </li></ul><ul><li>This is a measure of the input resistance of the transistor and is measured in ohms. It is given by: </li></ul><ul><li>hre – h ybrid r everse gain common e mitter </li></ul><ul><li>This is a measure of the effect of the output voltage on the input and is effectively a reverse voltage gain. It has no units. It is given by: </li></ul>Electronic Engineering
  16. 16. <ul><li>hfe – h ybrid f orward gain common e mitter </li></ul><ul><li>This is a measure of the effect of the input current on the output and is effectively a forward current gain. It has no units. It is given by: </li></ul><ul><li>hoe – h ybrid o utput common e mitter </li></ul><ul><li>This is a measure of the output conductance of the transistor and is measured in Siemens. It is given by: </li></ul>Electronic Engineering
  17. 17. <ul><li>The reason for the resistor on the output being expressed as a conductor will become apparent when we start to generate equations. </li></ul><ul><li>By looking at the input side and the output side we can generate two equations, these are </li></ul><ul><li>Input side: </li></ul><ul><li>VBE = hie x IB + hre x VCE </li></ul><ul><li>Output side </li></ul><ul><li>IC = hfe x IB + hoe x VCE </li></ul><ul><li>The symmetry between the equations can be seen – this would not be true if hoe were quoted as a resistor. </li></ul>
  18. 18. <ul><li>If we wish to measure the four parameters then we can see how this can be done using the equations: </li></ul><ul><li>VBE = hie x IB + hre x VCE </li></ul><ul><li>IC = hfe x IB + hoe x VCE </li></ul><ul><li>hie = VBE/IB as long as VCE is zero </li></ul><ul><li>this is written as: </li></ul><ul><li>hie = VBE/IB |VCE = 0 </li></ul><ul><li>What are the equations for the other three? </li></ul><ul><li>In any circuit containing a common emitter transistor, the transistor can now be replaced by the four interconnected components. </li></ul>
  19. 19. NOTES. <ul><li>This is a small signal model and only works effectively over a limited range of input conditions. </li></ul><ul><li>This is an a.c. model and cannot be used to set up the initial d.c. conditions around the transistor (i.e. biasing) </li></ul><ul><li>This model does not take into account variations in frequency and can only be used within the normal operating frequencies of the amplifier. </li></ul><ul><li>All capacitors in the transistor circuit are considered to be short circuits when constructing the equivalent circuit. </li></ul><ul><li>All d.c. power supplies act as large capacitors and can therefore also be thought of as short circuits. </li></ul>Electronic Engineering
  20. 20. <ul><li>We are now ready to start analysing transistor circuits but before we do here are some typical values for the parameters: </li></ul>This is for a BC107 – other transistor values can be found in manufacturer’s literature. NOTE – The values are typical values for that device and will vary considerably. Electronic Engineering Parameter Value hie 1 k  hre 3 x 10 -4 hfe 250 hoe 300  S
  21. 21. Example Rb1 = 68k  Rb2 = 27k  Rc = 1.8k  Re = 1k  Ce = 100  F Other caps = 0.1  F Vs = 9v Input is a voltage source with an internal resistance of 50  Output is a 120  loudspeaker Electronic Engineering Vs Rc Rb1 Vout Rb2 Re Ce Vin Ground
  22. 22. The Hybrid  Model <ul><li>This is model based on the physical construction of the transistor. A typical transistor has the following construction: </li></ul>Electronic Engineering Emitter Collector Base rbb’ rb’c rb’e rce Cb’c Cb’e b’
  23. 23. <ul><li>From the diagram: </li></ul><ul><li>rbb’ this is the resistance from the base connection to the centre of the base region. </li></ul><ul><li>rb’e this is the resistance from the centre of the base to the emitter connection. </li></ul><ul><li>rb’c this is the resistance from the centre of the base to the collector connection. </li></ul><ul><li>rce this is the resistance from the collector connection to the emitter connection. </li></ul><ul><li>Cb’e this is the junction capacitance of the base emitter junction. </li></ul><ul><li>Cb’c this is the junction capacitance of the base collector junction. </li></ul><ul><li>The current generator has a value given by g M x Vb’e. </li></ul><ul><li>g M is called the transistors transconductance. </li></ul>
  24. 24. <ul><li>Typical values for the device are: </li></ul>Electronic Engineering Parameter Value rbb’ 300  rb’e 2000  rb’c 1.5 M  rce 25 k  Cb’e 8 pF Cb’c 4 pF g M 0.125 S
  25. 25. The electronic equivalent circuit is drawn in the following way: It is possible to draw some comparisons between the two models and in doing so we can equate certain components: I C V BE V CE rce rb’c rbb’ rb’e Cb’e Cb’c g M x Vb’e Vb’e b b’ c e Hybrid Hybrid -  hie rbb’ + rb’e hre rb’e/rb’c hfe g M x rb’e hoe 1/rce
  26. 26. <ul><li>When we are working at low to medium frequencies, the capacitors will have relatively high values: </li></ul><ul><li>Cb’e = 8 pF will have a reactance at 20 kHz of </li></ul><ul><li>1/2  fC = 995 k  </li></ul><ul><li>This is so large compared to the other resistors both of the capacitors and rb’c are removed giving us: </li></ul>This model is now very similar to the original H parameter model. Electronic Engineering I C V BE V CE rce rbb’ rb’e g M x Vb’e Vb’e b b’ c e
  27. 27. Model at High Frequencies <ul><li>At higher frequencies the reactance of the capacitors begins to drop and their effect increases. What they effectively do is reduce the current flow through Rb’e by allowing it to flow via other paths as shown. </li></ul>Electronic Engineering I C V BE V CE rce rb’c rbb’ rb’e Cb’e Cb’c g M x Vb’e Vb’e b b’ c e
  28. 28. <ul><li>We need to determine the flow through Cb’c </li></ul><ul><li>Note. The transistor amplifier inverts the signal as it amplifies which means that as the input goes positive the output goes negative. The upshot of this is that the voltage across Cb’c is given by: </li></ul><ul><li>Vb’e + Vce but Vce = gain x Vb’e so this gives us </li></ul><ul><li>Vb’e + Vb’e x gain = Vb’e (1 + gain) </li></ul><ul><li>an approximation for the gain is g M x RL so </li></ul><ul><li>Voltage across Cb’c = Vb’e (1 + g M x RL) </li></ul><ul><li>Which means the current is Vb’e (1 + g M x RL) j  Cb’c </li></ul><ul><li>This has the same effect as a capacitor from b’ to e whose value is: </li></ul><ul><li>Cb’c x (1 + g M x RL) </li></ul>Electronic Engineering
  29. 29. <ul><li>The input part of the circuit can therefore be redrawn as: </li></ul>The two capacitors in parallel can now be combined to given a single capacitor C IN given by Cb’e + Cb’c x (1 + g M x RL) What has effectively happened is that the value of the feedback capacitor has been amplified and applied across the input. This is called the Miller Effect. Electronic Engineering V BE rbb’ rb’e Vb’e b b’ Cb’e Cb’c x (1 + g M x R L )
  30. 30. The Miller Effect <ul><li>If we have an inverting amplifier with a capacitor connected between it’s input and output then this is equivalent to the amplifier with one capacitor connected from its input to ground and another between its output and ground. </li></ul><ul><li>The value of the input capacitor is C (A + 1) and the output capacitor is given by C (A + 1)/A </li></ul>Electronic Engineering A A C ~AC ~C
  31. 31. <ul><li>Going back to our amplifier; </li></ul><ul><li>it is important to know the frequency at which the amplifiers gain begins to reduce due to the effect of the capacitors. The point at which we define the amplifier to be beyond it’s working limit, i.e. outside its bandwidth, is when the resistance of rb’e equals the reactance of C IN . (This is the amplifiers -3dB point) </li></ul><ul><li>rb’e = 1/(2  fC IN ) from which we can say that the break frequency for the amplifier is: </li></ul><ul><li>f = 1/(2  C IN rb’e) </li></ul>Electronic Engineering
  32. 32. Field Effect Transistor FET <ul><li>The equivalent circuit of an FET is relatively simple compared to the bipolar transistor. The circuit below shows the complete model. </li></ul>Typical Values for the parameters are: rds 100K  Cgs 4 pF Cgd 1 pF g M 5mS Electronic Engineering I D V GS V DS r ds g M x V GS Gate Drain Source C gs C gd
  33. 33. <ul><li>At low frequencies the model simplifies to become: </li></ul>If we have load R L and R L << rds then: V DS = g M V GS R L Giving Gain = V DS /V GS = g M R L At high frequencies the Miller effect can once again be used to give us an equivalent input capacitance of: Cin = Cgs + Cgd (1 + g M R L ) Electronic Engineering I D V GS V DS r ds g M x V GS Gate Drain Source
  34. 34. <ul><li>If we have a Voltage Source connected to the input then the input circuit becomes: </li></ul>Reactance of the capacitor is therefore this gives us a gain of: This produces a gain that rolls off above a certain frequency. C in R S V S ~ V GS
  35. 35. This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. © 2009 University of Wales Newport This work is licensed under a Creative Commons Attribution 2.0 License . The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence.  All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. The name and logo of University of Wales Newport is a trade mark and all rights in it are reserved. The name and logo should not be reproduced without the express authorisation of the University. Electronic Engineering

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