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Differential Amplifier: An Introduction
- 2. Introduction of Amplifier • An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. • It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply to make the output signal stronger than the input signal. An amplifier is effectively the opposite of an attenuator, while an amplifier provides gain, an attenuator provides loss. • An amplifier can either be a separate piece of equipment or an electrical circuit within another device
- 3. Introduction • The differential amplifier can measure as well as amplify small signals that are buried in much larger signals. • There are two input terminals, labeled (−) input, and (+) input.
- 4. HISTORY • Differential amplifiers are usually implemented with a basic two-transistor circuit called a long-tailed pair or differential pair. This circuit was originally implemented using a pair of vacuum tubes. The circuit works the same way for all three-terminal devices with current gain. The long-tail resistor circuit bias points are largely determined by Ohm's Law and less so by active component characteristics.
- 5. • The long-tailed pair was developed from earlier knowledge of push-pull circuit techniques and measurement bridges. An early circuit which closely resembles a long-tailed pair was published by British neurologist Bryan Matthews in 1934,and it seems likely that this was intended to be a true long-tailed pair but was published with a drawing error. The earliest definite long-tailed pair circuit appears in a patent submitted by Alan Blumlein in 1936.By the end of the 1930s the topology was well established and had been described by various authors including Frank Offner (1937), Schmitt (1937)and Jan Friedrich Toennies (1938) and it was particularly used for detection and measurement of physiological impulses.
- 6. • The long-tailed pair was very successfully used in early British computing, most notably the Pilot ACE model and descendants, Maurice Wilkes' EDSAC, and probably others designed by people who worked with Blumlein or his peers. The long-tailed pair has many attributes as a switch: largely immune to tube (transistor) variations (of great importance when machines contained 1,000 or more tubes), high gain, gain stability, high input impedance, medium/low output impedance, good clipper (with not-too-long tail), non-inverting (EDSAC contained no inverters!) and large output voltage swings.
- 7. • One disadvantage is that the output voltage swing (typically ±10–20 V) was imposed upon a high DC voltage (200 V or so), requiring care in signal coupling, usually some form of wide- band DC coupling. Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex (ENIAC: 18,000 tubes for a 20 digit calculator) or unreliable. DC-coupled circuitry became the norm after the first generation of vacuum tube computers.
- 8. Superposition • If E1 is replaced by a short circuit, E2 sees an inverting amplifier with a gain of −m. – Therefore, the output voltage due to E2 is − mE2. − mE2
- 9. • Now let E2 be short-circuited: – E1 divides between R and mR to apply a voltage of E1m/ (1+ m) at the op amp’s (+) input. • This divided voltage sees a no inverting amplifier with a gain of (m + 1). – The output voltage due to E1 is the divided voltage: • E1m/(1 + m) times the noninverting amplifier gain, (1 + m), which yields mE1.
- 10. • Therefore, E1 is amplified at the output by the multiplier m to mE1. When both E1 and E2 are present at the (+) and (−) inputs, respectively. Vo is mE1 − mE2. • The output voltage of the differential amplifier, Vo, is proportional to the difference in voltage applied to the (+) and (−) inputs. • Multiplier m is called the differential gain and is set by the resistor ratios. • When E1 = E2 the output voltage is 0. – To put it another way, when a common (same) voltage is applied to the input terminals, Vo = 0
- 11. Differential Amplifier • The gain of the amplifier below can be determined using the Superposition Principle. ' ' ─ + 2.2 kΩ 4.7 kΩ 22 kΩ RS Ri Rf VOUT RD
- 12. • The output of the non-inverting amplifier is: ' ' ─ + 2.2 kΩ 22 kΩ RTh Ri Rf VOUT DS D RR R V + 2 += i f OUT R R VV 122 α += i f OUT R R VV 122 α
- 13. • The total output is the sum: • To balance the circuit, we set the coefficients to add to zero. −+ +=+ i f i f OUTOUT R R V R R VVV 1212 αα i f i f R R R R =+αα
- 15. ( ) ( ) ( ) ( ) ( ) ( ) ( )1212 12 1 2 2 12 VV R R V RR RR V R R V RR R RR R V R R R R V RRR R RRR RR V R R V R R RR R RR R VV i f fi fi i f fi f fi i i f i f fii f fii fi i f i f fi f fi f OUT − = − + + = −+ + + + = −+ + + + = −+ + + + =
- 16. • So the balanced condition yields – and the differential gain Ad is ( )12 VV R R V i f OUT − = = i f D R R A
- 17. • Common-mode rejection of 60 cycle power line interference in medical instrumentation which measures difference potentials on the body is a fundamental problem. – Power-line interference may exceed the level of the signal being measured. – This bad news is often cancelled by the fact that the interfacing signal appears equally intense at both input terminals of the diff amp, and is therefore called a common-mode signal. – The common-mode rejection ratio CMRR is defined as the magnitude of the ratio of the differential voltage gain Ad to the common-mode voltage gain Ac. VVwhen groundedisVwhen 12 2 = = OUT OUT V V CMRR
- 18. • In practice the CMRR is measured in the following steps: 1. Ground V2, and apply a voltage V1 to the upper terminal. 2. Measure the resulting VOUT. 3. Lift V2 from ground and short the two input leads, then apply the same value of V1. 4. Measure the resulting VOUT. 5. To compute CMRR, divide the results of step 2 by the result of step 4, and take the magnitude. 6. The CMRR is a voltage ratio, and therefore in decibel units we may define CMRRdb as CMRRCMRRdb log20=
- 19. Simulation of Differential Amplifier
- 20. Explanation…..
- 21. Graph/ Result Dc voltage graph is always state line.
- 22. Common-Mode Voltage • The simplest way to apply equal voltages is to wire inputs together and connect them to the voltage source.
- 23. • For such a connection, the input voltage is called the common-mode input voltage, ECM. • Now Vo will be 0 if the resistor ratios are equal (mR to R for the inverting amplifier gain equals mR to R of the voltage-divider network.)
- 24. • The potentiometer is trimmed until Vo is reduced to a negligible value. – This causes the common-mode voltage gain, Vo/ECM to approach 0. – It is this characteristic of a differential amplifier that allows a small signal voltage to be picked out of a larger noise voltage. • It may be possible to arrange the circuit so that the larger undesired signal is the common-mode input voltage and the small signal is the differential input voltage. – Then the differential amplifier’s output voltage will contain only an amplified version of the differential input voltage.