Introduction to Op Amp Circuits ELEC 121

April 2004ELEC 121 Op Amps2 Basic Op-Amp The op-amp is a differential amplifier with a very high open loop gain 25k ≤ A VOL ≤ 500k (much higher for FET inputs) high input impedance 500k  ≤ Z IN ≤ 10M  low output impedance 25  ≤ R O ≤ 100 

April 2004ELEC 121 Op Amps3 Op-Amp Equivalent Circuit

April 2004ELEC 121 Op Amps4 Op-Amp Specifications – DC Offset Parameters Even though the input voltage is 0, there will be an output. This is called offset. The following can cause this offset: –Input Offset Voltage –Output Offset Voltage due to Input Offset Current –Total Offset Voltage Due to Input Offset Voltage and Input Offset Current –Input Bias Current See lm301.pdf or mc1741c.pdf for sample specification sheets

April 2004ELEC 121 Op Amps5 General Op-Amp Specifications V IO Input Offset Voltage V IO –The voltage that must be applied to the input terminals of an op amp to null the output voltage –Typical value is 2mV with a max of 6mV –When operated open loop, must be nulled or device may saturate

April 2004ELEC 121 Op Amps6 General Op-Amp Specifications I IO Input Offset Current –The algebraic difference between the two input currents –These are base currents and are usually nulled –Typical value I IO 20 nA with a max of 200nA

April 2004ELEC 121 Op Amps7 Technique to Null V O Short Input terminals to ground Connect potentiometer between compensation pins with wiper to V EE –Potentiometer is usually a 10 turn device Connect meter to output and adjust potentiometer for V O = 0

April 2004ELEC 121 Op Amps8 General Op-Amp Specifications CMRR Common Mode Rejection Ratio –The ratio of the differential voltage gain (A D ) to the common mode gain (A CM ) –A CM is the ratio between the differential input voltage (V INCM ) applied common mode, and the common mode output voltage (V OCM ) –it can exceed minimum is 70db with a typical value of 90 db –in properly designed circuit, it may exceed 110db

April 2004ELEC 121 Op Amps9 General Op-Amp Specifications Input Bias Current –The average of the currents that flow into the inverting and noninverting terminals –Typical values rage from 7nA to 80 nA Differential Input Resistance –Also know as the input resistance –Resistance seen looking into the input terminals of the device –Runs from a low of 2M  for an LM741 to a high of  for FET input devices Output resistance –Resistance between the output terminal ad ground –Typical values are 75  or less Input Capacitance –The equivalent capacitance measured at either the inverting or noninverting terminal with the other terminal connected to ground –May not be on all spec sheets –Typical value for LM741 is 1.4pF

April 2004ELEC 121 Op Amps10 General Op-Amp Specifications Power Supply Range –May be differential or single ended –Max is ± 22V Output Voltage Swing –Range of output voltage –Depends on power supply voltage used (typically about 85% to 90%) –Usually about ±13.5V for a power supply voltage of ±15V Slew Rate –The maximum rate of change in the output voltage in response to an input change –Depends greatly on device, higher is better (output resonds faster to input changes) –For LM741 it is.5V/  s while for the LM318 it is 70V /  s Gain Bandwidth Product –The bandwidth of the device when the open loop voltage gain is 1

April 2004ELEC 121 Op Amps11 Op Amp Equivalent Circuit

April 2004ELEC 121 Op Amps12 Op-Amp Gain Op-Amps have a very high gain. They can be connected open- or closed loop. Open-loop (A VOL ) refers to a configuration where there is no feedback from output back to the input A VOL may exceed 10,000 Closed-loop (A VCL ) configuration reduces the gain In order to control the gain of an op-amp it must have negative feedback Negative feedback will reduce the gain and improve many characteristics of the op-amp

April 2004ELEC 121 Op Amps13 Typical Op Amp Frequency Response

April 2004ELEC 121 Op Amps14 Change in A V with Feedback

April 2004ELEC 121 Op Amps15 Virtual Ground Since Z IN is very high, we assume no current can flow into any lead of the op amp When the non- inverting input pin is at ground, the inverting input pin is at 0V The equivalent circuit.

April 2004ELEC 121 Op Amps16 Practical Op-Amp Circuits Typical Op-amp circuit configurations include the: Unity Gain Buffer (Voltage Follower) Inverting Amplifier Noninverting Amplifier Summing Amplifier Integrator Differentiator Note: the integrator and differentiator are considered active filters

April 2004ELEC 121 Op Amps17 Unity Gain Buffer (Follower)

April 2004ELEC 121 Op Amps18 Inverting Op Amp The input is applied to the inverting (-) input the non-inverting input (+) is grounded R F is the feedback resistor, and is connected from the output to the inverting input This is called negative feedback

April 2004ELEC 121 Op Amps19 Inverting Op Amp We assume that no current enters the inverting terminal I I- < 100nA V D  0V

April 2004ELEC 121 Op Amps20 Inverting Op-Amp Gain Closed Loop Gain is controlled by the external resistors: R F and R 1 For Unity Gain: A V is -1 and R F = R 1 The minus sign denotes a 180 degree phase shift between input and output

April 2004ELEC 121 Op Amps21 Inverting Op Amp Compensated for I bias R is used to compensate for difference in I BIAS+ and I BIAS-

April 2004ELEC 121 Op Amps22 Inverting Op-Amp This configuration achieves high gain with a smaller range of resistor values than the basic inverter A V- V+

April 2004ELEC 121 Op Amps23 Inverting Amplifier with High Z in Use a Unity Gain Buffer to obtain a very high input resistance with an inverting amplifier

April 2004ELEC 121 Op Amps24 Inverting Amplifier for Low R L Use a Unity Gain Buffer to obtain a very high input resistance to drive a low impedance load

April 2004ELEC 121 Op Amps25 Noninverting Amplifier V- = V + = v i

April 2004ELEC 121 Op Amps26 Noninverting Op Amp Compensated for I BIAS R bias is used to compensate for difference in I BIAS+ and I BIAS-

April 2004ELEC 121 Op Amps27 Differential (Difference) Amplifier A A V1 V2

April 2004ELEC 121 Op Amps28 Differential Amplifier Output

April 2004ELEC 121 Op Amps29 Instrumentation Amplifier Buffered Input R 1 = R 2, R F1 = R F2

April 2004ELEC 121 Op Amps30 Instrumentation Amplifier R 1 = R 2, R F1 = R F2

April 2004ELEC 121 Op Amps31 Inverting Summing Amplifier By applying KCL to the multiple inputs, we can consider the contribution of each source individually I F + I - = I 1 + I 2 + I 3 but I -  0 \I F = I 1 + I 2 + I 3 V O = -I F R F

April 2004ELEC 121 Op Amps32 Non-inverting Summing Amplifier Perform a source transformation for each input Sum the current sources and find RTH for the resistances V IN+ = I T R TH