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CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS

CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS. Objectives:. Explain and analyze the operation of an instrumentation amplifier. Explain and analyze the operation of an isolation amplifier. Explain and analyze the operation of log and antilog amplifiers. INSTRUMENTATION AMPLIFIER.

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CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS

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  1. CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS

  2. Objectives: • Explain and analyze the operation of an instrumentation amplifier. • Explain and analyze the operation of an isolation amplifier. • Explain and analyze the operation of log and antilog amplifiers.

  3. INSTRUMENTATION AMPLIFIER

  4. Instrumentation Amplifier A differential voltage-gain device that amplifies the difference between the voltages existing at its two input terminals. The main purpose is to amplify small signals that may be riding on large common-mode voltages. It is an integrated circuit that internally has three operational amplifiers and several resistors.

  5. Op-amps A1 and A2 are noninverting configurations that provide high input impedance and voltage gain. • Op-amp A3 is used as a unity-gain differential amplifier with high-precision resistors that are all equal in value (R3 = R4 = R5 = R6) • The gain-setting resistor, RG is connected externally.

  6. Continue… • The overall closed-loop gain of the instrumentation amplifier is: where R1 = R2 = R. • The external gain-setting resistor, RG:

  7. Example 1 Determine the value of the external gain-setting resistor RG for a certain IC instrumentation amplifier with R1 = R2 = 25 kΩ. The closed-loop voltage gain is to be 500. Answer : RG = 100 Ω

  8. AD622 Instrumentation Amplifier • An external resistor must be used to achieve a voltage gain greater than unity. • RG is connected between pins 1 and 8.

  9. Gain vs. Frequency for AD622 • Shows how the gain varies with frequency for gains 1, 10, 100 and 1000. • The bandwidth decreases as the gain increases.

  10. Example 2 Calculate the voltage gain and determine the bandwidth using the graph gain vs. frequency for AD622 for the instrumentation amplifier in figure below.

  11. Noise Effect in Instrumentation Amplifier Applications • Guarding is a technique to reduce the effects of noise on the common-mode operation of an instrumentation amplifier operating in critical environments by connecting the common-mode voltage to the shield of a coaxial cable. • The purpose is to eliminate voltage differences between the signal lines and the shield. • Virtually eliminating leakage currents and cancelling the effects of the distributed capacitances so that the common-mode voltages are the same in both lines.

  12. AD522 Instrumentation Amplifier • The AD522 is a low-noise IA that has a Data guard output, which is connected to the shield as shown. The AD522 has a programmed gain from 1 to 1000 depending on RG.

  13. ISOLATION AMPLIFIERS

  14. Capacitor-Coupled Isolation Amplifier • Is a device that consists of two electrically isolated stages. • The input and output stages are separated by an isolation barrier. • Each stage has separate supply voltages and grounds so that there are no common electrical paths between them. • It is used for the protection of human life or sensitive equipment in those applications where hazardous power-line leakage or high-voltage transients are possible.

  15. The input stage consists of an amplifier, an oscillator and a modulator. • The modulator uses a high-frequency square-wave oscillator to modify the original signal. • A small-value capacitor in the isolation barrier is used to couple the lower frequency modulated signal or dc voltage from the input to the output. • The output stage consists of a demodulator that extracts the original input signal from the modulated signal so that the original signal from the input stage is back to its original form.

  16. Transformer-Coupled Isolation Amplifier • 3656KG is an example which can have gain for both the input and output stages. • Gain of the input stage: • Gain of the output stage:

  17. Example 3 Determine the total voltage gain of the 3656KG isolation amplifier in figure above. Answer : Av(tot) = 62.7

  18. LOG AND ANTILOG AMPLIFIERS

  19. Log Amplifier with a Diode • When a diode is placed in the feedback path of an inverting op-amp, the output is at –VF when the input is positive. • Since VF is logarithmic, so is Vout which is limited to a maximum value of approximately -0.7 V because the diode’s logarithmic characteristic is restricted to voltages below 0.7 V. where IR is a constant for a given diode

  20. Example 4 Determine the output voltage for the log amplifier in figure above. Assume IR = 50 nA. Answer : VOUT = -0.150 V

  21. Log Amplifier with a BJT • The base-emitter junction of a BJT exhibits the same type of logarithmic characteristic as a diode because it is also a pn junction. • It is connected in a common-base form in the feedback loop. where IEBO is the emitter-to-base leakage current

  22. Example 5 What is Vout for a transistor log amplifier with Vin = 3 V and R1 = 68 kΩ ? Assume IEBO = 40 nA. Answer : Vout = -175.1 mV

  23. Basic Antilog Amplifier • Is formed by connecting a transistor (or diode) as the input element. • An antilog amplifier produces an output proportional to the input raised to a power. In effect, it is the reverse of the log amp.

  24. Example 6 For the antilog amplifier in figure below, find the output voltage. Assume IEBO = 40 nA. Answer : Vout = -3V

  25. ~ End of Chapter 3 ~

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