Special-Purpose Op-Amp Circuits

1. Chapter 20 Special-Purpose Op-Amp Circuits

2. Objectives Analyze and explain the operation of an instrumentation amplifier Analyze and explain the operation of an isolation amplifier Analyze and explain the operation of an OTA Analyze and explain the operation of active diode circuits Analyze and explain several special types of op-amp circuits

3. Instrumentation Amplifiers A basic instrumentation amplifier is shown Op-amps A1 and A2 are noninverting, providing high input impedance and voltage gain Op-amp A3 is a unity gain differential amplifier Resistor RG sets the gain

4. Instrumentation Amplifiers Instrumentation amplifiers are normally used to measure small differential signal voltages that are superimposed on a common-mode voltage often larger than the signal voltage Devices such as remote pressure or temperature transducers may have long lead lengths that will pick up common-mode electrical noise The instrumentation amplifier at the end of the line must amplify the small signal from the remote sensor and reject the large common-mode voltage

5. Isolation Amplifiers The difference between an isolation amplifier and an instrumentation amplifier is that the isolation amplifier has an input stage, an output stage and and power supply section that are all electrically isolated from each other Transformer coupled isolation is commonly used An isolation amplifier is capable of operating with three independent grounds, for the input signal, the output signal, and the power supply

6. Isolation Amplifiers

7. Isolation Amplifiers The isolation amplifier is used in applications that require no common ground between a transducer and the process circuits where interfacing to sensitive equipment is required In chemical, nuclear and metal-processing industries, millivolt signals many exist in the presence of large common-mode voltages in the kV range The isolation amplifier can amplify small signals from very noisy equipment and provide a safe output to sensitive equipment such as computers

8. Operational Transconductance Amplifiers (OTAS) The OTA is primarily a voltage-to-current amplifier in which the output current equals the gain times the input voltage Like the conventional op-amp, the OTA has: Two differential input terminals High input impedance High CMRR

9. Operational Transconductance Amplifiers (OTAS) The double circle symbol at the output represents an output current source that is dependent on a bias current

10. Operational Transconductance Amplifiers (OTAS) Unlike the conventional op-amp, the OTA has: A bias-current input terminal A high output impedance No fixed open-loop voltage gain By definition, the transconductance of an electronic device is the ratio of the output current to the input voltage Voltage is the input variable Current is the output variable Voltage-to-current gain is the transconductance, gm

11. Operational Transconductance Amplifiers (OTAS) The transconductance of the amplifier is determined by the amount of bias current, which is set by the dc supply voltages and the bias resistor RBIAS

12. Operational Transconductance Amplifiers (OTAS) One feature of an OTA is that the voltage gain can be controlled by the amount of bias current By changing the resistance, you can produce a change in IBIAS, which changes the transconductance A change in the transconductance changes the voltage gain Variations in the bias voltage, applied to RBIAS, will cause a change in the bias current

13. Operational Transconductance Amplifiers (OTAS) Amplitude Modulator The voltage gain is varied by applying a modulation voltage to the bias input

14. Active Diode Circuits A clamping circuit or clamper is used to add a dc level to a signal voltage Clampers are referred to as dc restorers because they are used to restore a dc level to a signal that has been processed through capacitively coupled amplifiers

15. Active Diode Circuits Diode limiters cut off or limit voltage above or below specified voltage levels

16. Active Diode Circuits When the input voltage is less than the reference voltage, the op-amp differential input voltage is positive The positive output of the op-amp forward biases the diode The op-amp operates as a voltage-follower When the input is greater than the reference voltage, the op-amp output is negative The negative output of the op-amp reverse biases the diode, so the diode is effectively open

17. Active Diode Circuits Peak Detector Used to detect the peak input voltage and store that peak voltage on a capacitor

18. Active Diode Circuits The basic operation of the peak detector is: Input voltage is applied to the noninverting input through Ri The high-level output voltage of the op-amp forward-biases the diode and charges the capacitor The capacitor charges until its voltage reaches a value equal to the input voltage and thus both inputs are at the same voltage When the capacitor voltage is greater than or equal to the input voltage, the diode is reverse biased, and the capacitor holds the peak voltage

19. Other Op-Amp Circuits Constant-Current Source Delivers a load current that remains constant when the load resistance changes As long as VIN and Ri remain constant, IL will be constant IL = VIN/Ri

20. Other Op-Amp Circuits Current-to-Voltage Converter Converts a variable input current to a proportional output voltage Vout = IiRf

21. Other Op-Amp Circuits Voltage-to-Current Converter Input voltage controls output current (load) IL = Vin/R1

22. Summary A basic instrumentation amplifier is formed by three op-amps and seven resistors, including the gain-setting resistor, RG An instrumentation amplifier has high input impedance, high CMRR, low output offset, and low output impedance The voltage gain of a basic instrumentation amplifier is set by a single external resistor

23. Summary An instrumentation amplifier is useful in applications where small signals are embedded in large common-mode noise A basic isolation amplifier has three electrically isolated parts: input, output, and power Most isolation amplifiers use transformer coupling for isolation Isolation amplifiers are used to interface sensitive equipment with high-voltage environments

24. Summary The operational transconductance amplifier (OTA) is a voltage-to-current amplifier The output current of an OTA is the input voltage times the transconductance In an OTA, transconductance varies with the bias current; therefore, the gain of an OTA can be varied with a bias voltage or a variable resistor Diode clampers add a dc level to an ac signal Diode limiters cut off voltage above and below specified levels. Limiters are also called clippers