## The Differential Amplifiers in Instrumentation amplifier

Updated by: Mila

On: 12 Jul, 2019

Viewed:140 times - 1 day, 6 hour, 54 minute, 37 second ago

Downloaded: 0 times -

Category: Sensors

Most resistance sensor bridges are supplied by a grounded voltage or curreht source. Therefore the amplifier at the bridge's output should not have any of its input terminals grounded. In addition we will show later that it is best for input termi...

Most resistance sensor bridges are supplied by a grounded voltage or curreht source. Therefore the amplifier at the bridge's output should not have any of its input terminals grounded. In addition we will show later that it is best for input terminals to have high and similar impedances to ground. An amplifier having these characteristics is called a differential amplifier.

Figure 3.36 shows a very simple circuit to implement a differential ampli- fier. We assume that the op amp is ideal (Vr: V); then the output voltage is

To illustrate the differential properties of the circuit, it is convenient to write the output as a function of the differential input voltage Ed : Ez - Er. In order to do this, we must make the following substitutions in

where E" is the common mode voltage. Substitution of (3.48) and (3.49) in (3.47) yields an equation where there is one factor multiplying E" and an- other multiplying Ea. The first factor is called common mode gain, G", and the second factor differential mode gain, G6. That is,

Their expressions for the circuit in Figure 3.36 are

In a differential amplifier we wish to amplify the difference between the input voltages but not the common mode signal. Thus we must have G" : 0, which is obtained when

Then V. : kEo. Because the matching expressed by (3.53) is difficult to fulfill exactly, the circuit's ability to reject common mode signals will be limited rather than infinite. It is quantified by means of the Common Mode Rejection Ratio (CMRR), defined as the differential gain divided by the common mode gain. For Figure 3.36 it is given by

The CMRR is usually expressed in decibels. We obtain that by taking the decimal logarithm of the previous expression and multiplying the result by 20. If in Figure 3.36 the op amp is not ideal we must substitute the model in Figure 3.37, where the common mode gain for the op amp (A.) is obtained from the CMRR in the specification sheets. For the p.A74l, for example, Aa : 50,000 minimum at dc, and CMRR : 70 dB minimum. Therefore

Using this model for the op amp, the analysis of Figure 3.36 is more cumbersome. But we can follow the same steps that lead us before to equa- tions (3.47-3.50), now defining V6 and V" from Vl and V2. Fortunately, after simplifying and reordering, from the flnal equation we obtain a very simple rule,

That is, the CMRR for resistors, equation (3.54), and for the op aup add in "parallel"; that is, their reciprocals add. Each CMRR must be expressed as a fraction, not in decibels.

The circuit of Figure 3.36 can be directly applied to a sensor bridge, where E1 and E2are the voltages at the bridge output terminals. It is also possible to arrange connections in order to identify output bridge voltages with Vr and V2, zs shown in Figure 3.38a.

For Figure 3.36, note that by assuming an ideal op amp, the input imped- ances seen by sources E1 and E2are respectively R1 and R3 * Ra, implying that Rz and Ra will have to be very large resistors if high input impedance and high gain are required. A high input impedance is required in order to reduce loading effects in voltage measurements. The requirement for a high gain is due to the low amplitude for the bridge output. It would certainly be possible to arrange several gain stages in cascade in order to obtain the amplitude needed at the ADC input, but drifts and noise effects in amplifiers are lower when the gain is concentrated in the flrst amplifying stages (see chapter 7). Figure 3.38b shows the equivalent circuit for analyzing Figure 3.384. If, as usual, we want to have % : 0 when x : 0, then we must have Rz : Rl (: R). By applying (3.54), we obtain

Thus the CMRR for this circuit degrades when the bridge imbalance increases. If, for example, we want a differential mode gain of 100 when x : 0.01, the resulting CMRR will be approximately 86 dB. Therefore the actual common mode gain, G", is about 5 x 10-3. If the supply voltage for the bridge is 20 V, then the common mode voltage at the bridge output will be 2012: l0 V. The voltage contribution at the amplifier output will be 50 mV on a signal voltage of 5 v, even with an ideal op amp. This contribution is proportional to x, so the result is a small change in gain. For large values of x the nonlinearity is increased.

Both the circuit in Figure 3.38a and that in Figure 3.36 show the additional shortcoming of the need for modifying two resistors whenever the differen- tial mode gain is to be changed. Even more, this niodiflcation must be per- formed without degrading the matching required by equation (3.57). This lack of flexibility has lead to the development of better alternatives, imple- mented in circuits generally called instrumentation amplifiers.

Keywords related to Differential Amplifiers: #differential amplifier transistor #differential amplifier pdf #differential amplifier using mosfet #cmos differential amplifier #differential amplifier calculator #emitter coupled differential amplifier pdf #differential amplifier design #advantages of differential amplifier

Monolithic integration techniques allow a reduction of production costs for hybrid and modular circuits. For instrumentation ampliflers there are circuit alternatives to those in Figures 3.39 and 3...

The circuit in Figure 3.40 is the classic implementation for an instrumentation amplifier. Its analysis when the three op amps are ideal leads to.. By eliminatingV6, Vn, and Vc in the preceding equ...

An instrumentation amplffier is an electronic circuit that simultaneously yields: high input impedance; high common mode rejection; high stable gain that can be adjusted by a single resistor and wi...

**Definition** - Shunt calibration is the known, electrical unbalancing of a strain gage bridge, by means of a fixed resistor that is placed, or “shunted”, across a leg of ...

Resistance temperature detectors (RTDs) are commonly used in industrial and scien-tific temperature measurements. The most common types are pure platinum (Pt) formed into wire or evaporated in a th...

Wheatstone bridge circuits have been in the field for a very long time and still are among the first choices for front-end sensors. Whether the bridges are symmetric or asymmetric, balanced or unba...

As the fastest growing demand of circuit and wiring diagram for automotive and electronics on internet based on different uses such as electronic hobbyists, students, technicians and engineers than we decided to provide free circuit and wiring diagram base on your needed.

To find circuit and wiring diagram now a day its easy. E-learning through internet as a right place to search an exact circuit and wiring diagram of your choice and it's much fun and knowledgable. On internet you will find thousands of electronic circuit diagrams some are very good designed and some are not. So you have to modify them to make them according to your needs but some circuits are ready to make and require no changes.

There are many categories of circuit and wiring diagrams like automotive, audio circuits, radio & RF circuits, power supply circuits, light circuits, telephone circuits, timer circuits, battery charger circuits etc. There are many types of circuit and wiring diagrams some are very easy to build and some are very complicated, some are so small and some contain huge list of parts.

We provides free best quality and good designed schematic diagrams our diagrams are free to use for all electronic hobbyists, students, technicians and engineers. We also provides a full educational system to students new to electronics. If you are new to electronics you are a student or a electronic hobbyist and want to increase your knowledge in electronics or want to understand electronics in a very easy way so this is the right place for you we provide electronics beginner guide tutorials to easily understand complicated electronic theory. Our mission is to help students and professionals in their field.

## Related Posts