 # CHAPTER 3: SPECIAL PURPOSE OP-AMP CIRCUITS

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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

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 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.

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. Answer : Av(tot) = 62.7

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 = 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 = 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. Answer : Vout = -3V

~ End of Chapter 3 ~