Differential Amplifier : input. resistance. Differential amplifiers are widely used in engineering instrumentation

Transcription

1 Differential Amplifier : input resistance Differential amplifiers are widely used in engineering instrumentation

2 Differential Amplifier : input resistance v 2 v 1 ir 1 ir 1 2iR 1 R in v 2 i v 1 2R 1

3 Differential Amplifier: why useful Difference amplifier find application in many areas, in particular in the design of instrumentation systems. Example Consider a transducer that produces between its two output terminals a relatively small signal around 1mV, But between each of the two wires (transducer terminals) and the ground there may be a large interference around 1V.

4 Differential Amplifier: why useful The instrumentation amplifier must reject this large interference signal (a common-mode signal), and amplify the small difference (differential ) signal.

5 A difference Amplifier V cm is the common mode signal V d is the differential signal This Circuit has low input resistance and the gain cannot be easily varied

6 A Instrumentation Amplifier To overcome the drawbacks of the difference amplifier the following instrumentation amplifier has been proposed

7 A Instrumentation Amplifier The circuit consists of two stage 1- the first stage formed by op amp A1 and A2 with their associated resistors. 2- the second stage is formed by op amp A3 together with its four resistors.

8 Find v 0 A Instrumentation Amplifier

9 Solution: A Instrumentation Amplifier

10 Common Mode Rejection Ratio (CMRR) An ideal differential amplifier will not have any output that depends on the value of the common mode voltage; ( The circuit gain for common mode voltage, Acm will be zero.) The common mode rejection ratio (CMRR) of a differential amplifier is defined as the ratio of the gain to the common mode gain (Acm). The common mode rejection (CMR) is the CMRR expressed in db.

11 Common Mode Rejection Ratio (CMRR) Clearly, the larger these numbers, the better the differential amplifier. Typical values of CMR range from 80 to 100 db. Let, R1=R2 and Rf=R3, then A and R R f 1 is the differential gain v out A( v 2 v 1 ) Av diff

12 Common Mode Rejection Ratio (CMRR) Common mode signal v1 v2 Vcm 2 common mode gain A cm v V out cm Common mode rejection CMRR CMR 20log 10 CMRR A A cm V V cm diff

13 Common Mode Rejection Ratio (CMRR) Any signal common to both inputs is effectively canceled and free to put the ground anywhere, the noise is common. v out AV s A cm V Vn A(V s CMRR ) n

14 Op Amp Integrated Circuit IC A typical example of an op amp is a 741 integrated circuit IC. Compensation for input offset voltage can be provided as a variable resistor connected to two terminals (offset null).

15 The output voltage is given by: Differentiator Amp

16 The output voltage is given by: Integrator Amp

17 Example Develop a circuit to realize the following equation Vout 10V in 4 V in dt

18 Example Develop a circuit to realize the following equation( Solution)

19 Example The above circuit consists of three op-amp circuits. The first one at the top to left is used to realize the gain = (-10), while the second one is the integrator with gain =(-4). The last one at the top to right is a summing amplifier with inverse sign to produce the final output.

20 Example Develop a circuit to realize the following equation Vout 10V in 4 V in dt

21 Example Develop a circuit to realize the following equation( Solution)

22 Nonlinear (logarithmic) amplifier The op-amp can also implement a nonlinear relationship, this is achieved by placing a nonlinear element in the feedback of the op-amp. For example, a diode can be used as shown figure. The summation of currents provides

23 Nonlinear (logarithmic) amplifier Were I is the current passes through R and at the same time in the diode. Note that the current in the diode has a nonlinear relation as function of Vout. In the diode we have the relation Where Io= amplitude constant and α=exponential constant. The inverse of this relation is the logarithm, and thus

24 Nonlinear (logarithmic) amplifier Were I is the current passes through R and at the same time in the diode. Note that the current in the diode has a nonlinear relation as function of Vout. In the diode we have the relation Where Io= amplitude constant and α=exponential constant. The inverse of this relation is the logarithm, and thus which constitutes a logarithmic amplifier.

25 Analog signal conditioning Measurement systems are usually used for: Displaying data about some event or variable Inspection or testing, i.e to determine whether an item is to specification or not (calibration) Providing feedback information in the control loop

26 Example: temperature measurement The measurement system consists of basic three components: Sensor to transform the variations in the physical variable into a measured form (resistance, displacement, current or volt) Signal conditioning to change the sensor output signal either in its form or range to met the control loop requirements (signal processing) Display element to monitor the variations in the physical variable

27 Analog signal conditioning includes: Signal level change - Attenuation (gain<1) - Amplification (gain>1) Linearization, if the transducer gain is nonlinear, anon linear amplifier can be used to compensate this problem. Finally, the over all gain of transducer and amplifier is linear.

28 Note that, linearity is a very important characteristic in control loop that we have to maintain. Conversion of the nature of signal as - Passive change (resistance, capacitance or inductance) to active change (volt or current) - Voltage to current - Current to voltage - Electric current to pneumatic signal Filtering and impedance matching

29 Signal conditioning circuits could be implemented using: Passive circuits - Divider circuits - Bridges - RC filters Op-amps

30 Passive circuits 1- Voltage divider 2- Bridge circuit (Wheatstone bridge)

31 Active circuits 1- Current to voltage converter The current signal I (ma) is supplied from the transducer and R (KΩ), so the output voltage is: V IR 0

32 Active circuits To avoid the negative sign in the above relation the circuit may be modified as following The output voltage is given by: V IR 0

33 Example Develop a signal conditioning circuit to convert a transducer output (4 20 ma) into a voltage range (0 10 Volt) and draw the circuit diagram.

34 Example Solution: First step we have to change the current signal into voltage signal. selecting R = 100 Ω. Therefore the current range will be converted into a voltage range (0.4 2 volt).

35 Example: solution Second step, we will use an amplifier to obtain the required output voltage range in assuming linear relation as: (Vout = A Vin + offset). Substitute in the above relation by the values (Vout= 0 at Vin= 0.4 V) and (Vout = 10 V at Vin= 2 V). We will obtain the equations: 0 = 0.4 A +offset (1) 10 = 2 A +offset (2) Solving (1) and (2) we get: Offset = -2.5 and A 6.25

36 solution