TODAY 4-terminal linear amplifier Op-Amp Basics, Ch-28, 31 Op-Amp Golden Rules for operation Op-amp gain, impedance, frequency response Videos Lab-6 Overview 1
Review Semiconductors Semiconductors Resistivity (ρ), R= ρl/a a material property Si, GaAs, InN, CdTe <valence>=4 bonding electrons II III IV V VI B C N O Al Si P S Zn Ga Ge As Se Cd In Sn Sb Te Hg Doping in Semiconductors Donor, n-type conduction to the right in periodic table Acceptor, p-type conduction to the left in periodic table ρ=1/neμ, μ=mobility, σ=1/ρ pn-junction Diode Depletion Region (DR) DR narrows in forward bias DR widens in reverse bias Transistors 2 diodes Bias the base input 2
Transistors Review Transistors +Vo Ro +Vo Ro Bipolar Junction Transistor npn, pnp Rb Vout Vout Base-controlled variable resistor RT Use as Voltage Divider circuit Regimes: cut-off, V be ~0, R large linear, V be ~finite, R finite saturated, V be ~large, R small R T VOUT V0 R T R 0 Any Transistor Base-controlled VARIABLE RESISTOR Use in a VOLTAGE DIVIDER circuit 3
Review Transistors FET Field Effect Transistor 99.99999999+ % of all transistors MOSFET - Metal Oxide Semiconductor FET CMOS - Complementary MOS CMOS One FET is always off, the other FET is always on. No current flows to ground when off. 4
Operational Amplifier (Op-Amp) Op-Amp Basics 4-terminal linear amplifier Op-Amp Golden Rules for operation Gain, impedance, frequency response Videos Lab-6 Overview Why Op-Amps? - Linear amplification - - Much easier to configure (change gain, etc.) - 5
Amplifier Uses 6
V Electronics - PHYS 2371/2 Linear Amplifier V in Amplifier 4-terminal device Negative or positive output Gain=out/in V out Voltage Gain G V V out / V in G V >> 1, e.g. G V ~10 5 G V = 1, unity gain (current gain) 0 < G V < 1, attenuator G V < 0, e.g. G V = -3, inverting 40 20 G = -3 V out = -3 V in Amplify Voltage, Current, Power 0 V In -20-40 0 5 10 15 time 7
Gain Decibels db units Voltage Gain G V = 10 db/20 db=20 log(g V ) Power=V 2 /R Power Gain G P = 10 db/10 db=10 log(g P ) G V db 10-2 -40 1 0 10 20 100 40 1000 60 10 4 80 10 5 100 10 6 120 8
Operational Amplifier (Op Amp) Op Amp Invented in 1966 at Fairchild Semiconductors 741 General purpose op-amp (1968) Integrated circuit contains 20 transistors 11 resistors 1 capacitor Many Youtube videos https://www.youtube.com/re sults?search_query=op+amp What does the output circuit look like? 9
Op Amp or Op-amp Two inputs Inverting (-) Noninverting (+) As V in increases Inverting: V out decreases Noninverting: V out increases inputs output Differential Inputs V out goes positive or negative V out =G(V + - V - ) For V + >V, V out >0 (pos) For V + <V, V out <0 (neg) Example, G=10 V =1 V, V + =0 V V out ~ 10 V V =0 V, V + =1 V V out = + 10 V 10
Op Amp Power Supply Dual Supply Needs a dual voltage supply +V o and V o (also called V cc ) -Vo V=0 +Vo The power supply is not usually shown in the circuit Cannot get more V out than the power supply V o Example, G=A=10 5 V =1 V, V + =0 V V out ~ V o (supply voltage) V =0 V, V + =1 V V out = + V o (supply voltage) 11
Example Circuit: Ideal inverting amplifier R f = feedback resistor - puts part of the output on inverting (-) input Question: What happens if you put part of the output on the noninverting (+) input? (positive feedback) Answer: output saturates to maximum voltage (microphone in front of speaker) R Rf Vin Vout 12
Op Amp Gain Rf Golden Rules 1. Virtual Ground Approximation - Put inputs at equal voltages Vin R V- V+ Vout 2. Infinite Impedance Approximation - Assume no current flows into inputs Vin Vout 3. The output adjusts automatically to make the 2 inputs equal Apply (1): V + =0 then V - =V + =0 (at ground) Apply (2): No currents flow into op-amp (ground) I R =I Rf =I assume current CCW Now, V in =I R R assume V in is positive and V out =-I Rf R f since I direction reversed I=V in /R =-V out /R f so V out = -V in (R f /R) and Gain=V out /V in G o = -R f /R R I Rf V=0 Note: current is toward V=0 for R current is away from V=0 for Rf This gives V in and V out opposite signs and G is negative NOMINAL GAIN G o = -R f /R REAL GAIN G = (-R f /R) A/(A+R f /R+1) where A=open loop gain (~10 5 ) Basics of Opamp circuits (0-7 min) 0https://www.youtube.com/watch?v=K03Rom3Cs28 Ideal Operational Amplifier (0-7:23 min) http://www.youtube.com/watch?v=gqxlsuakzig&feature=related 13
OpAmp Gain Prob. 28-2 Given R=10 kω and R f =1 MΩ, compute nominal gain G o, real gain G and error for open loop gains of A=10, 10 3, and 10 6. Use G o = -R f /R as nominal or closed loop gain. G o = - 1 MΩ/10 kω = -100 nominal gain Real gain is G = G o A/(A+R f /R+1) Real gain divided by nominal gain For A=10, G/G o = 10/(10+100+1) = 10/111 For A= 10 3, G/G o = 10 3 /(10 3 +100+1) = 1000/1101 For A=10 6, G/G o = 10 6 /(10 6 +100+1) = 1,000,000/1,000,101 Error = 100% [(G - G o )/G o ]= 100% [G/G o -1] Rf R V- Vin V+ Vout G o = -R f /R nominal gain G = (-R f /R) A/(A+R f /R+1) where A=open loop gain (~10 5 ) 14
741 OpAmp Device Characteristics Open Loop Gain A V = 10 5, 100 db V out = A ΔV = A (V + - V - ) ΔV Vout Input Impedance (draws some current) 741 ~ 2 MΩ Rf FET op-amp ~ 10 12 Ω Input Offset Voltage (741 ~2 mv) Vin=0 R V- V+ Vout ΔV required to make V out =0 Input Bias Current Output Impedance (741 ~0.1 μa) (741, R~75 Ω, 20mA) Vin +Vo R Slew Rate = dv/dt] max (741, ~½ V/μs) Vout How fast V out can be changed 15
741 Frequency Response Gain-Bandwidth Product G * f max = constant =10 6 Hz If gain is large, cannot amplify high f. If you want high f, need to keep gain low. Gain G V 10 6 10 4 10 2 10 0 open-loop gain closed-loop gain G = -R f /R=1E4 closed-loop gain G = -R f /R=1E2 Gain-Bandwidth Product 741 OpAmp GBW = 10 6 Hz 10 0 10 2 10 4 10 6 0 f (Hz) 140 120 100 80 60 40 20 db V 16
Lab Experiment 6 Exploring the Op-Amp Don t forget to power the chip with +15 V and 15 V Use the power supply ground for the input (V + or V ) and the output Please please ask more questions in the lab 17
Protoboard (breadboard) Wiring The 5-hole rows are connected horizontally. The long red/blue rows are connected vertically. Typical layout with voltage on the long vertical rows. 18
Lab-6, Exploring the Op-Amp Physics PHYS 2371/2372, Electronics for Scientists Don Heiman, Northeastern University, 10/14/2016 This experiment will acquaint you with the robust, not-so-fancy 741 op-amp. It was developed by Fairchild Semiconductor in 1966, yet is still much in use today. Op-amps have superior properties compared to equivalent circuits made from discrete components. Op-amps are inexpensive (as low as $0.25), far simpler to use, require less wiring, and occupy a smaller space. In this lab you will measure a number of op-amp properties. In future experiments you will use them in various applications. As always, before wiring a circuit, you should draw the circuit diagram. (To emphasize this point, the circuit diagram is deliberately omitted here.) The circuit diagram should show the relevant pin connections, with the pin numbers and designations indicated. Note that the pin diagram is for the TOP view, opposite to that for the transistor. For example, the pins for the op-amp power are often labeled V cc = +V o and V ee = -V o. To see what the op-amp requires, refer to the specifications (in textbook, or search the web). You will encounter the idea of feedback especially negative feedback for the first time. This is a powerful concept applied in electronics. (I might add that while in everyday life positive feedback is usually "good" and negative feedback is "bad," in electronics it is usually just the I. Preliminary Test Search for and list the 741 specifications: input offset voltage, input resistance, output short circuit (maximum) current, gain-bandwidth product (or simply called bandwidth ), slew rate, and the open loop gain (A, often called the large signal voltage gain). 1. Connect +V o, -V o (~ ±15 V) to power the 741; and connect a +5 V supply between the inverting and noninverting inputs of a 741. Don t mistake the power supply voltages (+V o and -V o ) with the inputs denoted as V + and V - in the textbooks. 19
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