3-Handouts_Handout_6 - EE- 203Lectures on TTL and CMOSDr...

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Unformatted text preview: EE- 203Lectures on TTL and CMOSDr Sheikh Sharif Iqbal, KFUPM- In chapter 5, BJT operating in active mode is used to design Amplifier. Now we will concentrate on the other two modes. At one extreme BJT operates in cutoff region (BJT ÎOFF or IC ≈0) and at other extreme BJT operates in saturation region (BJT ÎON, maximum IC)- If vi< 0.5volt, the EBJ is reverse biased (R.B). Since CBJ is also reverse biased (as Vccis ‘+’)ÎBJT is OFFor operating in cutoff modeThus,IB=0 , IC=0 , IE=0 , VC=VCC- If vi≥0.7volt, the EBJ is forward biased. If CBJ is still reverse biased(if VC > 0.7)ÎBJT is ONbut in Active region (can’t be used as a switch)- With ↑vi;IB↑resulting an ↑VBand ↑IC. Thus, VCwill↓and when VC < VBby 0.5v, CBJ will be F.B. As EBJ is still F.BÎBJT is SaturatedThus,VBE= 0.7, VBC= 0.5, VCEsat= 0.2 , ICsat= (VCC - VCESat) / RCand IB= IC / βSat5.3.4 & 5.10: Bipolar Junction Transistor (BJT) Switches:Reference: “Microelectronic Circuits by Sedra/Smithand EE-203 Online course, KFUPM- Example:For the circuit shown, select a value for RBso that transistor saturates with an overdrive factor of 10. The BJT have a minimumβ=30 and VCEsat=0.2V. What is the resulted value of forced β?BJT Switches (cont’d.):- If due to some reason (i.e. temperature change), the base current is reduced than the transistor will enter the active mode from the edge of saturation. So the base current is designed to be higher than the edge of saturation value, by a factor of 2 to 10. This is called overdrive factor.IC = (5 – 0.2) / 1 k = 4.8 mA ; IB = IC/ β= 0.16 mA; So, 10IB = (VCC -VBE) / RB ; RB = 2.7 kand βF= IC/ IBF = 3- Solution:BJT Switches Parameters:- Internal capacitance of BJT introduces a time delay between the application of iBand the flow of iC. The measure of BJT's switching speed depends on its delay-time (td ,n-sec), rise-time (tr,n-sec), turn-on-time (ton), storage-time (ts) andturn-off-time (toff). Often larger tsconstitute the limiting factor on S/W- In this slide, logic level specification of digital circuits are presented. - Analog systems uses continuous property of electrical signals, whereas in digital systems, electrical signals represent numbers. Typically in binary digital circuits, it is customary to choose two predetermined DC voltage levels to represent logic high ‘1’ or low ‘0’. Chapter 10Î10.1 Digital Logic Circuits:- However, in order to allow for the inevitable component tolerances and other effects that change the signal voltage levels, two distinct voltage ranges are usually used instead of two voltage values- Thus, in digital circuits, if the signal voltage lies inthe range VL1to VL2, the signal is interpreted as logic low or '0'. But if signal voltage lies in the range VH1to VH2, the signal is interpreted as logic high or '1'. Note, the undefined region voltages are never used.Characteristics that are essential to successfully design digital circuits are;(a)Allowable rangeof input and output voltage/current at gates10.1: Characteristics of Digital Logic circuits:(b)...
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3-Handouts_Handout_6 - EE- 203Lectures on TTL and CMOSDr...

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