EE40 Lecture 6 Josh Hug 7 2 2010 EE40 Summer 2010 Hug 1 General Info HW2 due today at 5PM HW3 out due TUESDAY at 2PM There will be an optional pre midterm HW available Tuesday No lecture Monday Labs as usual on Tuesday No lab Wednesday Midterm next Friday in class 12 10 1 30 be on time No electronic devices One 8 5 x11 or A4 sheet of paper Handwritten anything you want both sides EE40 Summer 2010 Hug 2 Op Amps How Good Are They Exactly We ve been studying ideal op amps Of course real Op Amps aren t perfect For example you can t drive every device in the universe from a real op amp How do we precisely state the quality of a voltage source Look at its Thevenin equivalent Lower Thevenin resistance is better EE40 Summer 2010 Hug 3 Example Batteries Real voltage sources like batteries have a limit to how much current they can draw Called internal resistance This internal resistance often varies with charge status load attached temperature and more Just like Thevenin resistance EE40 Summer 2010 RTH e g a car battery supplies 12 Volts and can supply at most 200 VTH amps what is its internal resistance 12V 200A 0 06 Vout Hug 4 Measuring the Quality of a Source If you attach a resistive load then the output voltage is If you want Vout to be 99 of VTH then RTH VTH EE40 Summer 2010 RL Vout So basically for loads which are more than 99 times the Thevenin resistance you get 99 of the Thevenin voltage Lower RTH is better can handle smaller loads 5 Hug Source Quality Example Everyone s favorite resistive power supply again vo vi 1000 Vout Vin 2 3 RL a 1 1000V EE40 Summer 2010 b RL 99 2 3 66 66 load gets 99 of VTH6 Hug Thevenin Equivalents of Op Amp circuits Can find Thevenin equivalent of an op amp circuit at its output terminals RTH VTH vo Just like finding Thevenin equivalent of a simple resistor based voltage attenuator at its output terminals 2 3 a 1 1000V EE40 Summer 2010 Hug 7 Thevenin of Inverting Amplifier V TH RTH vo Assuming that the op amp here is IDEAL what s the best way to find the Thevenin equivalent circuit We ve already derived that it s a perfect voltage source EE40 Summer 2010 Hug 8 But if you really want to 0 0 EE40 Summer 2010 Technically you should take limits here but we are lazy Hug 9 What s wrong Our op amp model is missing something That s why it s the ideal op amp model We ll now introduce the resistive op amp model EE40 Summer 2010 Ideal Resistive Op Amp Model Hug 10 Resistive Op Amp model Takes in to account the fact that Some current flows into the input terminals The op amp cannot source all device in the universe output resistance is non zero EE40 Summer 2010 Hug 11 Output Resistance of Inverting Op Amp On board using resistive model of opamp Output Resistance Tells us how small our load can be before we start losing signal fidelity EE40 Summer 2010 Hug 12 Input Resistance Resistance at the input terminals of a device Tells us how much current will be generated for a fixed input voltage Useful for example to find power needed to power a device at that input EE40 Summer 2010 Hug 13 Input Resistance of Inverting Amplifier What is the input resistance of an inverting amplifier using ideal op amp model What is the input resistance of an inverting amplifier using resistive op amp model See sec 15 42 in book EE40 Summer 2010 Hug 14 Why are these quantities useful Input resistance tells us how much current power our input signal needs to provide Output resistance says how small of a load we can drive EE40 Summer 2010 Hug 15 Why are these quantities useful An iPod provides roughly 1V signal output with 20 internal resistance Speakers might be 4 resistance Connect iPod directly to such speakers Internal resistance dominates 4 1 0 17 20 4 0 172 4 0 0072 EE40 Summer 2010 Hug 16 Why are these quantities useful 20 iPod EE40 Summer 2010 Rs 1V 4 Speakers Hug 17 Bad Amplifier Small Rs 20 iPod Rs 1V 4 Speakers Very small Rs iPod must supply 50mW Output resistance is large can t drive speakers EE40 Summer 2010 Hug 18 Good Amplifier 20 Rs iPod 2 93V 1V 0 00133 4 Speakers Must provide 1 mW Rs 1000 Rf 3000 A 106 Rt 1000 EE40 Summer 2010 Hug 19 Op Amp Saturation Remember those power ports we ve been ignoring They specify the maximum and minimum voltage that our op amp can deliver If Op Amp output is If Op Amp output is EE40 Summer 2010 If Op Amp output is Hug 20 Op Amp Saturation Example If A 3 Vin Vo 5 V 12V 1V 3V 2V 6V 1 512 312V 12V EE40 Summer 2010 12V 4V 4V 12V Hug 21 Positive Feedback On the board EE40 Summer 2010 Hug 22 That s all forOp Ampsks No class Monday Enjoy weekend doing op amp problems EE40 Summer 2010 Hug 23
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