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Analog Electronics

Analog Electronics. Lecture 7 . Op-amp Circuits and Active Filters. Muhammad Amir Yousaf. Lecture:. Op-amp Circuits. Comparators. A comparator is a specialized nonlinear op-amp circuit that compares two input voltages and produces an output state that indicates which one is greater .

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Analog Electronics

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  1. Analog Electronics Lecture 7 Op-amp Circuits and Active Filters Muhammad Amir Yousaf

  2. Lecture: • Op-amp Circuits

  3. Comparators A comparator is a specialized nonlinear op-amp circuit that compares two input voltages and produces an output state that indicates which one is greater. Comparators are designed to be fast and frequently have other capabilities to optimize the comparison function.

  4. Comparator with Hysteresis Noise contaminated signal may cause an unstable output. To avoid this, hysteresis can be used. Hysteresis is incorporated by adding regenerative (positive) feedback, which creates two switching points: The upper trigger point (UTP) and the lower trigger point (LTP). After one trigger point is crossed, it becomes inactive and the other one becomes active.

  5. Schmitt Trigger A comparator with hysteresis is also called a Schmitt trigger. The trigger points are found by applying the voltage-divider rule: and What are the trigger points for the circuit if the maximum output is ±13 V? =2.28 V By symmetry, the lower trigger point =-2.28 V.

  6. Output Bounding Some applications require a limit to the output of the comparator (such as a digital circuit). The output can be limited by using one or two Zener diodes in the feedback circuit. The circuit shown here is bounded as a positive value equal to the zener breakdown voltage.

  7. Comparator Applications Simultaneous or flash analog-to-digital converters use 2n-1 comparators to convert an analog input to a digital value for processing. Flash ADCs are a series of comparators, each with a slightly different reference voltage. The priority encoder produces an output equal to the highest value input. In IC flash converters, the priority encoder usually includes a latch that holds the converter data constant for a period of time after the conversion.

  8. Comparator Applications • Over temperature sensing circuit: • R1 is temperature sensing resistor with a negative temperature coefficient. • R2 value is set equal to the resistance of R1 at critical temperature • At normal conditions R1 > R2 driving op-amp to low At R1=R2, balance bridge creates high op-amp output, energizes relay, activates alarm. An over temperature sensing circuit

  9. Summing Amplifier A summing amplifier has two or more inputs; normally all inputs have unity gain. The output is proportional to the negative of the algebraic sum of the inputs.

  10. Example Summing Amplifier What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW? 10 kW VOUT = -(VIN1 + VIN2 + VIN3) = -(+5.0 V - 3.5 V + 4.2 V) =-5.7 V

  11. Averaging Amplifier An averaging amplifier is basically a summing amplifier with the gain set to Rf /R = 1/n (n is the number of inputs). The output is the negative average of the inputs. What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V? Assume R1 = R2 = R3 = 10 kW and Rf = 3.3 kW? 3.3 kW VOUT = -⅓(VIN1 + VIN2 + VIN3) = -⅓(+5.0 V - 3.5 V + 4.2 V) =-1.9 V

  12. Scaling Adder A scaling adder has two or more inputs with each input having a different gain. It is useful when one input has higher weight than the other. The output represents the negative scaled sum of the inputs. Assume you need to sum the inputs from three microphones. The first two microphones require a gain of -2, but the third microphone requires a gain of -3. What are the values of the input R’s if Rf= 10 kW? 10 kW 5.0 kW 3.3 kW

  13. Scaling Adder: D/A Converter An application of a scaling adder is the D/A converter circuit shown here. The resistors are inversely proportional to the binary column weights. Because of the precision required of resistors, the method is useful only for small DACs.

  14. R/2R Ladder DAC A more widely used method for D/A conversion is the R/2R ladder. The gain for D3 is -1. Each successive input has a gain that is half of previous one. The output represents a weighted sum of all of the inputs (similar to the scaling adder). An advantage of the R/2R ladder is that only two values of resistors are required to implement the circuit.

  15. The Integrator An op-amp integrator simulates mathematical integration, a summing process that determines total area under curve. IC Ii The ideal integrator is an inverting amplifier that has a capacitor in the feedback path. The output voltage is proportional to the negative integral (running sum) of the input voltage. Ideal Integrator

  16. The Integrator Capacitor in the ideal integrator’s feedback is open to dc. This implies open loop gain with dc offset. That would lead to saturation. The practical integrator overcomes these issues– the simplest method is to add a relatively large feedback resistor. Rf should be large enough Practical Integrator

  17. Example The Integrator If a constant level is the input, the current is constant. The capacitor charges from a constant current and produces a ramp. The slope of the output is given by the equation: Sketch the output wave: 220 kW Vin 0.1mF 10 kW Vout

  18. The Differentiator An op-amp differentiator simulates mathematical differentiation, a process to determine instantaneous rate of change of a function. Ideal Differentiator The ideal differentiator is an inverting amplifier that has a capacitor in the input path. The output voltage is proportional to the negative rate of change of the input voltage.

  19. Example The Differentiator The output voltage is given by Sketch the output wave: Vin 10 kW 220 W 0.1mF Vout 10 kW

  20. Instrumentation Amplifiers An instrumentation amplifier (IA) amplifies the voltage difference between its terminals. It is optimized for amplifying smalldifferential signals that may be riding on a large common mode voltages. • High input impedance • High CMMR • Low output offset • Low output impedance

  21. Instrumentation Amplifiers IC of instrumentation amplifier is made up of three op amps and several resistors. The gain is set by a single resistor that is supplied by the user. The output voltage is the closed loop gain set by RG multiplied by the voltage difference in the inputs.

  22. Instrumentation Amplifiers Applications: • Used where a quantity is sensed by a remote sensor e.g. temperature, pressure transducer and sensed signal is sent over a long line. • Electrical noise produces common-mode voltages in the line. • IA at the end of line amplifies only the small differential signal and reject the common mode signal

  23. Example Instrumentation Amplifiers An IA that is based on the three op-amp design is the AD622. The formula for choosing RG is: Example: What value of RG will set the gain to 35? Solution: =1.5 kW

  24. The Logarithmic Amplifier A logarithmic (log) amplifier produces an output that is proportional to the logarithm of the input • Log and antilog amplifiers are used in applications that require: • Compression of analog input data • Linearization of transducers that have exponential outputs • Analog multiplication and division, etc

  25. The Logarithmic Amplifier

  26. The Logarithmic Amplifier A semiconductor pn-junction in the form of either a diode or the base-emitter junction of a BJT provides a logarithmic characteristic. Voltage across the diode is proportional to the log of the current in the diode. Compare data for an actual diode on linear and logarithmic plots:

  27. The Logarithmic Amplifier When a diode is placed in the feedback path of an inverting op-amp, the output voltage is proportional to the log of the input voltage. The gain decreases with increasing input voltage; therefore the amplifier is said to compress signals. Many sensors, particularly photo-sensors, have a very large dynamic range outputs. Current from photodiodes can range over 5 decades. A log amp will amplify the small current more than the larger current to effectively compress the data for further processing.

  28. Example The Logarithmic Amplifier For the circuit shown, the equation for Vout is (IR is a constant for a given diode.) Example: What is Vout? (Assume IR = 50 nA.) Solution: = -307 mV

  29. The Logarithmic Amplifier BJT is also used in designing log amplifier Log amplifiers are available in IC form with even better performance than the basic log amps shown here. For example, the MAX4206 operates over 5 decades and can measure current from 10 nA to 1 mA.

  30. The Antilog Amplifier An antilog amplifier produces an output proportional to the input raised to a power. IC antilog amps are also available. For example, the Datel LA-8048 is a log amp and the Datel LA-8049is its counterpart antilog amp. These ICs are specified for a six decade range.

  31. Constant-current source A constant-current source delivers a load current that remains constant when the load resistance changes. A basic circuit in which a stable voltage source (Vin) provides a constant current (Ii) through the input  resistor (Ri) If RL changes, IL remains constant as long as Vin  and  Ri are held constant.

  32. Current to Voltage Converter A current-to-voltage converter converts a variable input current to a proportional output voltage. A specific application of this circuit is where a photoconductive cell is used to sense changes in light level. As the amount of light changes, the cur-rent through the photoconductive cell varies because of the cell’s change in resistance. This change in resistance produces a proportional change in the output voltage.

  33. Peak Detector The circuit is used to detect the peak of the input voltage and store that peak voltage on a capacitor. The circuit can be used to detect and store the maximum value of a voltage surge.

  34. Charge Sensitive Amplifier It is used in Radiation detection Charge on a photon is accumulated in the capacitor

  35. Active Filters

  36. Basic filter Responses A filter is a circuit that passes certain frequencies and rejects all others. The passband is the range of frequencies allowed through the filter. The critical frequency defines the end (or ends) of the passband and is normally specified at the point where the response drops -3dB (70.7%) from the passband response. Following the passband is a region called the transition region that leads into a region called the stopband. Low-pass High-pass Band-pass Band-stop

  37. The Basic Low-Pass Filter The low-pass filter allows frequencies below the critical frequency to pass and rejects other. The simplest low-pass filter is a passive RC circuit with the output taken across C. BW = fc

  38. The Basic Low-Pass Filter • The ideal response is not attainable by any practical filter. • Actual filter responses depend on the number of poles, • Pole, a term used with filters to describe the number of RC circuits contained in the filter. • This basic RC filter has a single pole, and it rolls off at -20db/decade beyond the critical frequency. • 20db/decade means that at a frequency of 10fc the output will be -20dB(10%) of the input. • This roll-off allows too much unwanted frequencies through the filter

  39. The Basic Low-Pass Filter • Actual filters do not have a perfectly flat response up to the cutoff frequency. • More steeper response cannot be added by simply cascading the basic stages due to loading effect. • With combination of op-amps, the filters can be designed with higher roll-offs •  In general, the more poles the filter uses, the steeper its transition region will be. The exact response depends on the type of filter and the number of pole.

  40. The Basic High-Pass Filter The high-pass filter passes all frequencies above a critical frequency and rejects all others. The simplest high-pass filter is a passive RC circuit with the output taken across R.

  41. The Band-Pass Filter A band-pass filter passes all frequencies between two critical frequencies. The bandwidth is defined as the difference between the two critical frequencies fc1 and fc2. The simplest band-pass filter is an RLC circuit. Bandwidth B.W= fc2 – fc1 Center frequencyfo= √fc1fc2 Quality Factor: In band pass filters it is ratio of center frequency to its bandwidth. Q = fo/B.W

  42. The Band-Stop Filter A band-stop filter rejects frequencies between two critical frequencies; the bandwidth is measured between the critical frequencies. The simplest band-stop filter is an RLC circuit.

  43. Ideal vs Real Filters • In comparison to ideal low pass filters, the real RC or RLC filters lack the following characteristics: • Flat passband • Sharp transition region • Linear phase response

  44. Active Filters Active filters include one or more op-amps in the design. One of the three characteristic can be achieved with active filters: Chebyshev: rapid roll-off characteristic • Flat band pass with Butterworth • Sharp roll-off rate with Chebyshev • Linear phase • response. Butterworth: flat amplitude response Bessel: linear phase response

  45. Active Filters General Active Filters A single pole active filters The number of filter poles can be increases with cascading

  46. Active Filters Two poles filters Sallen-Key Configuration The Sallen-Key is one of the most common configurations for a second-order (two-pole)filter. There are two RC circuits that provides a roll-off of -40 dB /decade (Butterworth) For RA = RB = R and CA = CB = C

  47. Active Filters Two poles filters Sallen-Key Configuration

  48. Active Filters Two poles filters Multiple feedback

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