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IFB270Advanced Electronic Circuits
Chapter 14: Special-purpose op-amp circuits
Prof. Manar MohaisenDepartment of EEC Engineering
Korea University of Technology and Education (KUT)
Review of the Precedent Lecture● Introduce the level detection op-amp circuits
● Introduce the op amp comparators and their applications● Introduce the op-amp comparators and their applications
● Introduce the summing op-amp amplifiers and some applications
● Introduce the integrator and differentiator op-amp circuits
● Keywords
2Korea University of Technology and Education (KUT)
Lecture Objectives● Introduce the instrumentation amplifier
● +Applications
● Introduce the isolation amplifiers● +Applications
● Introduce the operational transconductance amplifier● +Applicationspp
● Introduce the op-amp converters and other circuits
● Keywords
3Korea University of Technology and Education (KUT)
Instrumentation Amplifiers● Instrumentation amplifier
● It amplifies the difference between its two inputs● The main objective is to amplify a small signal that might be riding on a large● The main objective is to amplify a small signal that might be riding on a large
common-mode voltages
● Characteristics of the instrumentation amplifier● High input Z low output Z high common mode rejection low output offset● High input Z, low output Z, high common-mode rejection, low output offset
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Instrumentation Amplifiers – contd.● Instrumentation amplifier
● A1 and A2 op-amps are non-inverting amplifiers with both high input impedance and gaing
● A3 is a unity gain differential amplifier with equal resistors (R3 = R4 = R5 = R6)● RG is set to control the closed-loop voltage gain● R1 = R2 = R1 2
221clG
RA R= +
2R21G
cl
RR A= −
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Instrumentation Amplifiers – contd.● Instrumentation amplifier
● Derivation of the formula of the voltage gain● Stage I
1 cminV V+
● Stage I
2 cminV V+2 2 1 2
2
( ) ( ) 0cm cm cmout in in in
G
V V V V V V VR R
− + + − ++ =
1 R RV V V V⎛ ⎞⎜ ⎟= + + 1R
■ Same way, we find that
2 2 11 cmout in inG G
V V V VR R⎜ ⎟⎜ ⎟⎝ ⎠
= + − +
1 R RV V V V⎛ ⎞⎜ ⎟= + +
62 2
4 6
12x out out
RV V VR R
= =+
● Stage II
1 1 21 cmout in inG G
V V V VR R⎜ ⎟⎜ ⎟⎝ ⎠
= + − +
1 0xxout outV VV V −− + = 2V V V V V= =
● Overall gain
1
5 30xout out
R R+ = 1 2 12 xout out out outV V V V V= − = −
⎛ ⎞
6Korea University of Technology and Education (KUT)
2 121 ( )out in in
G
RV V VR⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠
= + −
Instrumentation Amplifiers – contd.● Applications
● These amplifiers are generally used to measure the difference between two small signalsg
● These small signals are superimposed on the common-mode signal● Application include situations where quantities are measured by a remote device
● Measure temperature, pressure, etc.p , p ,
7Korea University of Technology and Education (KUT)
Instrumentation Amplifiers – contd.● A specific instrumentation amplifier
● The AD622 instrumentation amplifier
Note that gain is inversely prop. to the BW.Relation between gain and RRelation between gain and RG
25 25k
50.5 k1G
v RR A = Ω
Ω= −
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25.25kv R = Ω
Isolation Amplifiers● A basic capacitor-coupled isolation amplifier
● A device that consists of two electrically isolated stages; input and output stages● The two stages are isolated by an isolation barrier so that a signal must be processed● The two stages are isolated by an isolation barrier so that a signal must be processed
at the first stage before being coupled to the second stage● Isolation amplifiers use optical coupling (electronics I), transformer coupling, or
capacitive coupling● The two stages use different voltage sources and grounds
9Korea University of Technology and Education (KUT)
Isolation Amplifiers – contd.● A basic capacitor-coupled isolation amplifier
● Modulation: An information signal modulates (changes) a characteristic of a carrier signalg● Characteristic: Amplitude, phase, frequency
● The isolation barrier is a small-value capacitor● Demodulation: Recovers the original modulating signalg g g
10Korea University of Technology and Education (KUT)
Isolation Amplifiers – contd.● A basic capacitor-coupled isolation amplifier
● Amplitude or pulse-width modulation is possible● (b) represents an isolation amplifier with a pulse-width modulator● (b) represents an isolation amplifier with a pulse-width modulator● Example: IS0124 with a unity gain
11Korea University of Technology and Education (KUT)
Isolation Amplifiers – contd.● A transformer-coupled isolation amplifier
● Texas Instruments (Burr-Brown) 3656KG● The gains of both stages are adjusted externally using the resistors● The gains of both stages are adjusted externally using the resistors
● AD208● The gain of the input stage can only be controlled● It requires an external square wave to drive an output stage power converterIt requires an external square wave to drive an output stage power converter
1 1fRA +1
11f
vi
A R= +
22 1fR
A = +3656KG 22
1vi
A R +
1 2( )A A A=
3656KG
12Korea University of Technology and Education (KUT)
1 2( ) v vv totA A A
Isolation Amplifiers – contd.● Applications
● In medical applications● Heart rate and blood pressure signals are monitored in presence of high common-● Heart rate and blood pressure signals are monitored in presence of high common
mode signals● In these applications, without isolation, dc leakage or equipment failure could be fatal
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Operational Transconductance Amplifiers (OTA)
● Conventional op-amp● A voltage amplifier where the output voltage is a scaled version of the input voltage
● OTA● A voltage-to-current amplifier where the output current equals the input voltage times
the gainthe gain
Th d bl i l t t th t i d d t bi t● The double circle represents a current source that is dependent on a bias current
● OTA has a high input impedance, high CMRR, bias-current input terminal, a high output impedance and no fixed open-loop voltage gainoutput impedance, and no fixed open-loop voltage gain
14Korea University of Technology and Education (KUT)
Operational Transconductance Amplifiers – contd.
● Transconductance● Transconductance of an electronic device is the ratio between the output current to
the input voltagep g● Transconductance is the gain of the OTA
● The voltage-to-current gain is given by outmIg V=The voltage to current gain is given by
● The transconductance is a function of the bias-current
min
g V
BIASmg K I=
● Therefore,
BIASmout in inI g V K I V= = BIASout in in
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Operational Transconductance Amplifiers – contd.
● Basic OTA circuits● The transconductance is defined by the amount of bias-current
● The bias-current depends on the bias voltage and bias resistorBIASmg K I=
● The bias current depends on the bias voltage and bias resistor● Therefore, the voltage gain can be controlled by the amount of bias-current
out out LV I R= out outv mL LV IA R g RV V= = =L L
in inV V
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Operational Transconductance Amplifiers – contd.
● A specific OTA● LM13700 is a representative device of the OTAs
● Bias-current( ) 1 4 VV V− − −
● The 1.4 V is due to the internal circuit ■ A base-emitter junction and a diode connect RBIAS to the negative supply voltage
BIASBIAS
BIAS
( ) 1.4 VV VI R=
■ A base emitter junction and a diode connect RBIAS to the negative supply voltage
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Operational Transconductance Amplifiers – contd.
● A specific OTA● Example 14-6: Find the gain
● VBIAS = +V = 9 V● VBIAS +V 9 V● K = 16 μS/μA● Solution:
BIASBIAS
BIAS
( ) 1.4 V 503 AV VI R μ− − −= =
8 05 SK I 8.05mSm BIASg K I= =
(8.05)(10) 80.5v m LA g R= = =
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Operational Transconductance AmplifiersApplications
● First application: Amplitude modulator● The input is the fixed-amplitude carrier (sinusoidal of fixed frequency)● The bias input is connected to the modulating signal (information signal)● The bias input is connected to the modulating signal (information signal)● The amplitude of the output voltage is a function of the bias voltage
MOD ( ) 1.4 VV VI − − −= MOD
BIASBIAS
I R=
19Korea University of Technology and Education (KUT)
Operational Transconductance AmplifiersApplications – contd.
● First application: Amplitude modulator – contd.● Example 14-7: Determine the output signal
● Input: 1MHz with peak-to-peak voltage 50 mV● Input: 1MHz with peak to peak voltage 50 mVMOD(max)
BIAS(max)BIAS
( ) 1.4314 A
V VI
Rμ
− − −= =
⇓
MOD(min)BIAS(min)
BIAS
( ) 1.4154 A
2 46 mS
V VI
R
g K I
μ− − −
= =
⇓= =
BIAS(max)
(max)
5.02 mS
50.2
m
m Lv
g K I
A g R
= =
⇓= =
⇓
BIAS(min)
(min)
2.46 mS
24.6
m
m Lv
g K I
A g R
= =
⇓= =
⇓
(max) 2.51 Vv inout peak to peakpeak to peakV A V
− −− −
⇓
= = (min) 1.23 Vv inout peak to peakpeak to peakV A V
− −− −= =
20Korea University of Technology and Education (KUT)
Operational Transconductance AmplifiersApplications – contd.
● Second application: Schmitt Trigger● It is a comparator with hysteresis● The input is large enough to drive the device into saturation● The input is large enough to drive the device into saturation
● When the input is larger than one threshold value, the device switches to one of its saturation outputssaturation outputs
● When the input is below another threshold value, the device switches to the second saturation output
21Korea University of Technology and Education (KUT)
Operational Transconductance AmplifiersApplications – contd.
● Second application: Schmitt Trigger● The maximum output current equals the bias-current (IBIAS)● The threshold is set by the current through R1● The threshold is set by the current through R1
● The upper trigger point is +IBIAS R1
● The lower trigger point is –IBIAS R1
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Converters and Other Op-Amp Circuits● Constant-current source
● The current in the load is defined by the input circuit components; V and R● The current in the load is defined by the input circuit components; VIN and Ri
● Independent of the value of the load resistor
INL
VI =
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Li
I R
Converters and Other Op-Amp Circuits – contd.
● Current-to-voltage converter
out i fV I R=
● An application is depicted in (b)● The current through Rf is controlled by the amount of light a photoconductive device is
exposed to.
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Converters and Other Op-Amp Circuits – contd.
● Voltage-to-current converter
● The input voltage = the voltage drop at the resistor R1
● The load current is equal to the current through R (input current = 0)● The load current is equal to the current through R1 (input current = 0)
1
inL
VI R=
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Converters and Other Op-Amp Circuits – contd.
● Peak detector
● The capacitor charges to a value = to the maximum input voltage● When the input is less than that charged value, the diode is reverse-biased
● The peak value is still saved in the capacitor and can be measured● The peak value is still saved in the capacitor and can be measured
● Application● Measure the maximum value of a surge voltage● Measure the maximum value of a surge voltage
26Korea University of Technology and Education (KUT)
Keywords● Keywords and terms
● Instrumentation op-amp amplifier● Applications● Applications
● Isolation op-amp amplifier● Applications
● Operational transconductance amplifier● Operational transconductance amplifier● Applications
● Op-amp constant-current source● Op-amp current-to-voltage converterOp amp current to voltage converter● Op-amp voltage-to-current converter● Op-amp peak detector
27Korea University of Technology and Education (KUT)
Lecture Summary● Introduced the instrumentation amplifier
● Introduced the isolation amplifiers● Introduced the isolation amplifiers
● Introduced the operational transconductance amplifier
● Introduced the op-amp converters and other circuits
28Korea University of Technology and Education (KUT)