Difference Amplifier

3 Key excerpts on "Difference Amplifier"

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  eBook - ePub

  Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation

  • Robert B. Northrop(Author)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  3 Differential Amplifier

  3.1 INTRODUCTION

  The differential or Difference Amplifier (DA) is a cornerstone element in the design of most analog signal conditioning systems used in biomedical engineering applications, as well as in general instrumentation applications. Nearly all instrumentation and medical isolation amplifiers are DAs; also, nearly all operational amplifiers are DAs. Why are DAs so ubiquitous? The answer lies in their inherent ability to reject unwanted DC levels, interference, and noise voltages common to both inputs. An ideal DA responds only to the so-called difference-mode signal at its two inputs. Most DAs have a single-ended, complex output voltage, V o , given by the phasor relation:

  where

  V1

  is the complex (phasor) AC voltage at the DA’s noninverting input terminal, and

  V1

  ′ is the complex AC voltage at the DA’s inverting input terminal.

  Ideally, it is desired that the complex gains

  A1

  and

  A1

  ′ be exactly equal over as wide a frequency range as possible. In reality, this does not happen, and

  Vo

  is more generally given by the relation

  where

  and

  AD

  is the complex difference mode gain , and

  AC

  is the complex common-mode gain .

  From Equation 3.1 , it can easily be shown by vector summation that

  Clearly, if

  A1

  =

  A1

  ′, then

  AC

  → 0.

  3.2 DA CIRCUIT ARCHITECTURE

  Figure 3.1 illustrates a simplified circuit of the Burr-Brown, OPA606, JFET-input op amp (OA). (The circles with arrows in them represent BJT transistor DC current sources.) Note that a pair of p- channel JFETs connected as a DA is used as a differential input headstage in the OA. The single-ended signal output from the left-hand (inverting input) JFET drives the base (input) of a BJT emitter-follower which drives a second BJT connected as a grounded-base amplifier. Its output, in turn, drives the OA’s output stage.

  To appreciate how the differential headstage works, consider the simple JFET DA circuit of Figure 3.2 . Note that in the op amp schematic, the resistors Rs , Rd , and Rd ′ are shown as active DC current sources and thus can be assumed to have very high Norton resistances on the order of megohms. (See Northrop 1990, Section 5.2, and Section 2.3.5 in this text for a description on active current sources and sinks used in IC DA designs.) Figure 3.3 a illustrates the midfrequency, small-signal model (MFSSM) of the JFET DA. To make the circuit bilaterally symmetric so that the bisection theorem (cf. this text, Glossary; Northrop 1990, Chapter 2 ) can be used in its analysis, we put two 2Rs resistors in parallel to replace the one Rs in the actual circuit. Note that all DC voltage sources are represented by small-signal grounds in all SSMs.

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  eBook - ePub

  Op Amps: Design, Application, and Troubleshooting

  • David Terrell(Author)
  • 1996(Publication Date)
  • Newnes

    (Publisher)

  In spite of all the improvements, however, the high-performance, integrated operational amplifier of today is still based on the fundamental differential amplifier. Although the individual components in the amplifier are not accessible to you, it will enhance your understanding of the op amp if you have some appreciation for the internal circuitry.

  1.1.2 Review of Differential Voltage Amplifiers

  You will recall from your basic electronics studies that a differential amplifier has two inputs and either one or two outputs. The amplifier circuit is not directly affected by the voltage on either of its inputs alone, but it is affected by the difference in voltage between the two inputs. This difference voltage is amplified by the amplifier and appears in the output in its amplified form. The amplifier may have a single output, which is referenced to common or ground. If so, it is called a single-ended amplifier. On the other hand, the output of the amplifier may be taken between two lines, neither of which is common or ground. In this case, the amplifier is called a double-ended or differential output amplifier.

  Figure 1.1 shows a simple transistor differential voltage amplifier. More specifically, it is a single-ended differential amplifier. The transistors have a shared emitter bias so the combined collector current is largely determined by the −20-volt source and the 10-kilohm emitter resistor. The current through this resistor then divides (Kirchhoff’s Current Law) and becomes the emitter currents for the two transistors. Within limits, the total emitter current remains fairly constant and simply diverts from one transistor to the other as the signal or changing voltage is applied to the bases. In a practical differential amplifier, the emitter network generally contains a constant current source.

  FIGURE 1.1 A simple differential voltage amplifier based on transistors.

  Now consider the relative effect on the output if the input signal is increased with the polarity shown. This will decrease the bias on Q 2 while increasing the bias on Q 1 . Thus a larger portion of the total emitter current is diverted through Q 1 and less through Q 2 . This decreased current flow through the collector resistor for Q 2

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  eBook - ePub

  Electronics

  • David Crecraft(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  power supply. In the case of an electronic amplifier, the power supply is usually a d.c. voltage power supply (which gets its power from the a.c. supply mains) or a battery.

  The input signal causes the amplifier to control the flow of current from this voltage supply to the load. Thus more power may be delivered to the load than is taken from the input signal source. In practice, amplification usually means increasing the voltage amplitude of the signal into a given load. The opposite of amplification is called attenuation, and usually refers to a decrease in signal voltage.

  An electronic system which is designed primarily to give an output voltage proportional to the input signal voltage, without taking a significant amount of signal current, is called a voltage amplifier. Its voltage gain is specified, but its current gain is not, so it may give an increase in signal current if the load impedance is low enough.

  Voltage amplifier

  An example of this is the unity-gain buffer. It is designed so that its output voltage is almost equal to its input voltage, but the output current may be much larger than the input current. So the voltage gain can be specified as nearly 1, but the current gain cannot be specified.

  Current amplifier

  An amplifier designed primarily to give an output current proportional to the input signal current, without requiring a significant input signal voltage, is called a current amplifier.

  power amplification

  All these examples involve power amplification of course.

  Passive components

  In electronics a distinction is made between two types of component: those which can only absorb or transfer signal power, such as resistors and transformers, which are called passive components, and those, such as transistors, which can accept power from an extra power source and amplify signal power. These are called active components, or active devices.

  Active components or devices

  Throughout this chapter, there are occasional references to bipolar transistors and field-effect transistors (FET). You are not expected to know anything about transistors at this stage except that they are active devices. They are explained in Chapter 9

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