Operational Amplifiers David Lomax Azeem Meruani Gautam Jadhav What is an Op-Amp Low cost integrating circuit consisting of: Transistors Resistors Capacitors Able to amplify a signal due to an external power supply Name derives from its use to perform operations on a signal. Applications of Op-Amps Simple Amplifiers Summers Comparators Integrators Differentiators Active Filters Analog to Digital Converters Symbol for an Op-Amp +V Inverting Input Terminal Non-Inverting Input Terminal -V IC Circuit What do they really look like? Ideal Op-Amps I- V- Vout I+ V+ Infinite input impedance I+ = I - = 0 Infinite gain V+ = V- Zero output impedance Output voltage is independent of output current Inverting Amplifier RF iout iin R C Vin Vout RF Vin R Non-Inverting Amplifier RF iout iin R C Vin Vout RF 1 Vin R Summing Circuits • Used to add analog signals • Voltage averaging function into summing function Calculate closed loop gain for each input Rf Rf Rf ACL1 ACL1 ACL1 R1 R2 R3 Rf Rf Rf Vo Vin ACLn Vo V1 V2 V3 R1 R2 R3 If all resistors are equal in value: Vo V1 V2 V3 Difference Circuit • Used to subtract analog signals • Output signal is proportional to difference between two inputs V 2 R3 R1 R4 V1 R3 Vout ( R4 R2 ) R1 R1 If all resistors are equal: Vout V2 V1 Integrating Circuit • Replace feedback resistor of inverting op-amp with capacitor • A constant input signal generates a certain rate of change in output voltage • Smoothes signals over time Differentiating Circuit • Input resistor of inverting op-amp is replaced with a capacitor • Signal processing method which accentuates noise over time • Output signal is scaled derivative of input signal Filters Low Pass Filters High Pass Filters Band Pass Filters Low Pass Filter • Used to filter out signals above a specified frequency • Example: Noise Frequency range is governed by: 1 f 2 R C Where R = R2 C = C2 High Pass Filter • Filters out frequencies below a specified frequency • Reverse locations of resistors and capacitors in a low pass filter Band Pass Filter • Created by combining a high and low pass filter • Only allows signals within frequency ranges specified by the low and high pass filters to pass Comparator Circuit V1 is Vref V2 is Vin • Determines if one signal is bigger than another • No negative feedback, infinite gain and circuit saturates • Saturation: output is most positive or most negative value OR Gate If U1 or/and U2 = 5V, U3 = 5V If U2 and U1 = 0V, U3 = 0V Offset Comparator R2 If U2 .U1 R1 R2 U3 = 0V 5.R1 U1.R2 If U2 R1 R2 U3 = 5V Real Vs Ideal Op Amp Parameters Ideal Typical Input ∞ 106Ω Impedance Output 0Ω 100-1000Ω Impedance Voltage Gain ∞ 105 - 109 Common Mode 0 10-5 voltage Non-Ideal Op-Amps Gain Bandwidth Falloff Frequency Slew Rate (ΔV/ΔT) Rise Time Gain Bandwidth Gain Bandwidth Product (GBP)- is the product of the open-loop gain and the bandwidth at that gain. For practical purposes the actual gain should only be 1/10 to 1/20 of the open loop gain at a given frequency to ensure that the op-amp will operate without distortion. Open and Closed Loop Response Important Parameters for Op-Amps Input Parameters Voltage (Vicm) Offset voltage Bias current Input Impedance Output Parameters Short circuit current Voltage Swing Open Loop Gain Slew Rate Where to buy Op-Amps Newark Electronics Radio Shack DigiKey Jameco References David G Alciatore & Michael B. Histand, Introduction to Mechatronics and Measurement Systems http://www.elexp.com/t_gain.htm http://allaboutelectronics.com Electronics book Questions ?