ECE 3336 Introduction to Circuits & Electronics Lecture Set #13 Amplifiers & Operational Amplifiers Part 2 Dr. Han Le ECE Dept.

Outline Part 1 Introduction Amplifier basics Gain: power, voltage, current Other parameters of amplifiers Operational amplifier (Op Amp) basics Op amp technology Op amp basic circuit concepts Op amp with negative feedback

Outline Part 2 Op amp applications Limitation of op amp Voltage follower Transimpedance amplifier Summing amplifier Differential amplifier Instrumentation amplifier Periodic signal generator Active filter and other signal processing Limitation of op amp

Voltage Follower Advantages: High input resistance (for low power signal) Low output resistance (high power output signal It is a gain in power via current, but maintain the same voltage. This is known as impedance buffer for very low power voltage sensor signal, e. g. thermocouple.

Transimpedance Amplifier (TIA) Advantages: convert a non-ideal current source signal into voltage signal with effective zero input impedance optional Essential in optical signal detection with semiconductor photodiodes

Example Application

Summing Amplifier Advantages: Weight-summing and amplifying several voltage signals Ex: adding many sensor signals. Use in analog computing before the advance of digital electronics

Example: Music Synthesizing

Differential Amplifier Advantages: True measure of the difference between two signals and with amplification

Example: Automatic Control (proportional controller) We have +-15 V and 5 V DC power input. +15 V Temperature sensor input User preset (controllable) voltage input. -15 V

Instrumentation Amplifier See homework Advantages: Differential amplifier with: high input resistance high common mode rejection ratio (CMRR) for sensitive measurements

Square Wave Generator - + R vout C R1 R2 VS -VS Generate AC from DC Bi-stability of charging- reversal charging of the capacitor Period determined by RC time constant + - vout C R1 R2 R Let vout=VS If C is “+”charged until Then C then is reverse charged until Then System is intrinsically bi-stable, switching between two extrema in response to capacitor charging-reverse charging to opposite polarities

Square Wave Generator - + R vout C R1 R2 VS -VS Let vout switch from VS to – VS at t=0 v- v+ After time t=T/2 such that: Then v+ >= v- again and vout switches back to VS is the period

Square Wave Computer Simulation + - vout C R1 R2 R Frequency change via Maximum frequency is limited by op amp bandwidth

Example Low cost generic (analog) wave generators Audio signal generators General purpose oscillators

Level Shifter (DC Bias) Advantages: shift the signal by a DC bias level for ease of processing

Example Move the trace up and down on the screen

Active Filter For AC and harmonic signals, resistance is replaced with impedance. Hence: Inverting configuration Non-inverting configuration Response function H(w) for filtering can be designed with appropriate ZF(w) and ZS(w)

Review: Frequency Transfer Function (Frequency Response Function) For many linear RLC circuits, the frequency response function usually has the form: With op-amp: Advantages: filter with gain and transimpedance flexible filter design

Review: Example 1 Frequency Response or, Frequency Transfer Function R input vin[t] output vout[t] i(t) Frequency Response or, Frequency Transfer Function R C output vout[t] i(t) input vin[t]

Review: Example 2 RC band stop filter. RC bandpass filters

Op-amp RC Low-Pass Filter See Mathematica homework

Op-amp RC High-Pass Filter See Mathematica homework

Op-amp RC Band-Pass Filter See Mathematica homework

Op-amp Integrator Used in early days of analog computer Useful but not as accurate as digital Often for level detection and trigger-setting

Op-amp Differentiator Used in early days of analog computer Useful but not as accurate as digital Often for edge-detection and trigger-level setting

Outline Part 2 Op amp applications Limitation of op amp Voltage follower Transimpedance amplifier Summing amplifier Differential amplifier Instrumentation amplifier Periodic signal generator Active filter and other signal processing Limitation of op amp

Output Voltage Saturation The maximum voltage output is the supply voltage. Signal can be saturated Care must be taken to be sure that signals are within acceptable input amplitude range.

Bandwidth Op-amp, like any circuit, have finite bandwidth. It is like a built-in low-pass filter. Select appropriate op-amps (which come in a wide range of bandwidth) for applications.

Slew Rate Slew rate (also related to bandwidth and phase response) is caused by the fundamental gain-bandwidth product. Finite slew rate results in the sloping of sharp edge (step function signal)- causing signal distortion

Conclusion Op amp: the most versatile, ubiquitous device in many electronic circuits, especially linear circuits If there is one IC you learn, that should be the op amp The building block for many signal processing functions Essential to both analog and digital circuits (for analog output)

Homework helps

A transimpedance amplifier, as we learn, converts a current source signal into a voltage signal. This HW is the opposite: convert a voltage source signal into a current source control. i1 Answer RL iL vin - R1 + Other designs: non-inverting or ground-based load will be accepted.

- + - + vin vin VCC i1 iL RL R1 R2 iL RL i1 R1 Non-inverting design Answer + - RL vin R1 i1 iL Note: this can be used with a transistor (app: diode laser driver) R2 iL RL i1 - R1 + vin VCC