Analog Electronics

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Analog ElectronicsLecture 7

Muhammad Amir Yousaf

Op-amp Circuits and Active Filters



Lecture:

How to compare an analog signal with certain voltage level.Comparing a noisy signal with certain (reference) level.Binding an signal to fixed +/- max levels.Analog to digital converters with comparators.Adding two analog signals.Adding weighted signals.Averaging on analog signals. Digital to Analog Converter with weighted additions.Integrating an analog waveform.Differentiating analog waveform.Logarithm on analog signal.Antilog of analog signal.Multiplying and diving analog signals.Converters.Peak Detectors.Filters.



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.

Because of the high open-loop voltage gain, a very small difference voltage between the two inputs drives the amplifier into saturation.

Zero Level Detection:



Comparators

Non-Zero Level Detection:



Noise on Comparator

Noise contaminated signal may cause an unstable output.



Comparator with Hysteresis

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.

Vin 0 t

VUTP

VLTP

+Vout(max)

–Vout(max)



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.

+VZ

Ri

–

+

Vin

0 V0

–0.7 V



Comparator Applications

Flash analog-to-digital converters use 2n-1 comparators to convert an analog input to a digital value of n bits 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.

Vin(analog)

R

R

R

R

R

R

R

R

VREF

Op-ampcomparators

Binaryoutput

D2D1D0

(7)

Priorityencoder

(6)(5)(4)

(3)(2)(1)(0)

Enableinput

+

–

+

–

+

–

+

–

+

–

+

–

+

–



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.



Summing Amplifier

VOUT = -(VIN1 + VIN2 + VIN3)

= -(+5.0 V - 3.5 V + 4.2 V)–

+

VIN1

VIN2

VIN3

VOUT

R

R

1

3

Rf

R2

What is VOUT if the input voltages are +5.0 V, -3.5 V and +4.2 V and all resistors = 10 kW?

= -5.7 V

10 kW

Example



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.

VOUT = -⅓(VIN1 + VIN2 + VIN3)

= -⅓(+5.0 V - 3.5 V + 4.2 V)

–

+

VIN1

VIN2

VIN3

VOUT

R

R

1

3

Rf

R2

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?

= -1.9 V

3.3 kW



Scaling Adder

A scaling adder has two or more inputs with each input having a different gain.

–

+

VIN1

VIN2

VIN3

VOUT

R

R

1

3

Rf

R2



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.

–

+

8R

VOUT

R

20

23

Rf

4R

21

2R

22

+V



The Integrator

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.

–

+

Vin

R

Vout

C

IdealIntegrator

An op-amp integrator simulates mathematical integration, a summing process that determines total area under curve.

Ii

IC

dt

dVC

dt

dQI .

fi II -

dtdVC

RV outin .-

inout VRCdt

dV .1-

Vout



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

–

+

Vin

R

R

Vout

C

f

PracticalIntegrator



The 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.

–

+

C

Vout

Vin

R

An op-amp differentiator simulates mathematical differentiation, a process to determine instantaneous rate of change of a function. Ideal

Differentiator

I

fi II -

𝐶 . 𝑑𝑉 𝑖𝑛𝑑𝑡 =−𝑉 𝑜𝑢𝑡

𝑅

= -RC.



Instrumentation Amplifiers

An instrumentation amplifier (IA) amplifies the voltage difference between its terminals.

It is optimized for amplifying small differential signals that may be riding on a large common mode voltages.

o High input impedanceo High CMMRo Low output offseto Low output impedance

Input 1 R3+

–

–

+

R1

R5

–

+

R2

A1

A2

A3 Output

R4

R6

Gain set

Gain set

Input 2




IC of instrumentation amplifier is made up of three op amps and several resistors.

Input 1 R3+

–

–

+

R1

R5

–

+

R2

A1

A2

A3 Output

R4

R6

Gain set

Gain set

Input 2

The gain is set by a single resistor that is supplied by the user.

Vin1 + Vcm

RG

Vin2 + Vcm

Vout = Acl(Vin2 - Vin1)

The output voltage is the closed loop gain set by RG multiplied by the voltage difference in the inputs.



Instrumentation Amplifiers (IA)

Applications:

o IA at the end of line amplifies only the small differential signal and reject the common mode signal

o Used where a quantity is sensed by a remote sensor e.g. temperature, pressure transducer and sensed signal is sent over a long line.

o Electrical noise produces common-mode voltages in the line.




An IA that is based on the three op-amp design is the AD622. The formula for choosing RG is:

Example

G50.5 k

1v

RA

W

-(7)

(6)

(5)

(4)

(2)

(8)

(1)

(3)

+V

+IN

–INREF(Output signalcommon)

OutputAD622

–V

RG

What value of RG will set the gain to 35?

= 1.5 kW

G50.5 k 50.5 k

1 35 1v

RA

W W

- -



The Logarithmic Amplifier

Log and antilog amplifiers are used in applications that require:o Compression of analog input data.o Linearization of transducers that have exponential outputs.o Analog multiplication and division, etc

A logarithmic (log) amplifier produces an output that is proportional to the logarithm of the input







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:




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.

R1

–

+

Op-amp Vout

+ –VF

IFIin

Vin

0 V

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.




For the circuit shown, the equation for Vout is

Example

R 1

0.025 V ln inout

VV

I R- (IR is a constant for a given diode.)

R1

–

+

Op-amp Vout

+ –VF

IFIin

Vin

0 V

What is Vout? (Assume IR = 50 nA.)

11 V0.025 V ln

50 nA 1.0 koutV -W

= -307 mV

R1

–

+

Op-amp Vout

+ –VF

Vin

+11 V1.0 kW



The Antilog Amplifier

The antilogarithm of a number is the result obtained when the base is raised to a power equal to the logarithm of that number.

x

fi II -IR

= - . IR



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)

RLRi

Ii

–

+

–

+VIN

0 A0 V

IL = Ii

If RL changes, IL remains constant as long as Vin and Ri are held constant.



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.



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.

Vin +

–R1

Vout

Ri

C



Charge Sensitive Amplifier

It is used in Radiation detection

Charge on a photon is accumulated in the capacitor



Active Filters



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.

f

Gain

f

Gain

f

Gain

f

Gain

Low-pass High-pass Band-pass Band-stop



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.

f

BW

0 dB

–20 dB

10 fc

–40 dB

–60 dB0.1 fc fc0.01 fc 100 fc 1000 fc

Passband

–3 dB

Gain (normalized to 1)

Actual response of asingle-pole RC filter

Transitionregion

Stopbandregion

– 20 dB/decade

VoutR

Vs C

BW = fc




o The ideal response is not attainable by any practical filter.o Actual filter responses depend on the number of poles,o Pole, a term used with filters to describe the number of RC circuits

contained in the filter.o This basic RC filter has a single pole, and it rolls off at

-20db/decade beyond the critical frequency.

o 20db/decade means that at a frequency of 10fc the output will be -20dB(10%) of the input.

o This roll-off allows too much unwanted frequencies through the filter




o Actual filters do not have a perfectly flat response up to the cutoff frequency.

o More steeper response cannot be obtained by simply cascading the basic stages due to loading effect.

o With combination of op-amps, the filters can be designed with higher roll-offs

o 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.



Active Filters

General Active Filters

A single pole active filters

The number of filter poles can be increases with cascading



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.

f

0 dB

–20 dB

fc

–40 dB

–60 dB0.01 fc 0.1 fc0.001 fc 10 fc 100 fc

Passband

–3 dB

Gain (normalized to 1)

Actual responseof a single-poleRC filter

– 20dB/de

cade Vout

RVs

C



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.

VoutR

Vs C L

0.707

1

Vout (normalized to 1)

BW

fc1 f0 fc2

f

Center frequency fo= √ fc1 fc2

Quality Factor:In band pass filters it is ratio of center frequency to its bandwidth.Q = fo /B.W

Bandwidth B.W= fc2 – fc1



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.

Vout

RVs

C

L–3

Gain (dB)

fc1 f0 fc2f

0

BW



Ideal vs Real Filters

In comparison to ideal low pass filters, the real RC or RLC filters lack the following characteristics:

o Flat passbando Sharp transition regionoLinear phase response

–3

Gain (dB)

fc1 f0 fc2f

0

BW

0.707

1

Vout (normalized to 1)

BW

fc1 f0 fc2

f



Active Filters

Active filters include one or more op-amps in the design.

One of the three characteristic can be achieved with active filters:

Av

f

Butterworth: flat amplitude response

Chebyshev: rapid roll-off characteristic

Bessel: linear phase response

o Flat band pass with Butterworth

o Sharp roll-off rate with Chebyshev

o Linear phaseresponse.





ID (mA)

VD (V) VD (V)

ID (mA)

0.6 0.7 0.80.50.40.30.20.100

6.0

7.0

8.0

1.0

5.0

2.0

4.0

3.0

0.6 0.7 0.80.50.40.30.20.10

0.01

0.001

10

0.1

1.0

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Analog Electronics