CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS

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# CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS - PowerPoint PPT Presentation

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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Presentation Transcript
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

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.

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.
Continue…
• The overall closed-loop gain of the instrumentation amplifier is:

where R1 = R2 = R.

• The external gain-setting resistor, RG:
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

• An external resistor must be used to achieve a voltage gain greater than unity.
• RG is connected between pins 1 and 8.
• Shows how the gain varies with frequency for gains 1, 10, 100 and 1000.
• The bandwidth decreases as the gain increases.
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.

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.
• 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.

### ISOLATION AMPLIFIERS

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.
• 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.
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:
Example 3

Determine the total voltage gain of the 3656KG isolation amplifier in figure above.

### LOG AND ANTILOG AMPLIFIERS

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

Example 4

Determine the output voltage for the log amplifier in figure above. Assume IR = 50 nA.

Answer : VOUT = -0.150 V

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

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

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.
Example 6

For the antilog amplifier in figure below, find the output voltage. Assume IEBO = 40 nA.