IFB270 Advanced Electronic Circuits - Mohaisen · IFB270 Advanced Electronic Circuits ... dc leakage or equipment failure could be fatal ... Solution: BIAS BIAS BIAS ()1.4V 503 A

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


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


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= −


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= − = −

⎛ ⎞


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 ,


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= −


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


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


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


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


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


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


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



● 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



● 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



● 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= = =


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=


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

− −− −= =



● 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



● 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


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 =


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.



● 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=



● 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


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


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


Documents

IFB270 Advanced Electronic Circuits - Mohaisen · IFB270 Advanced Electronic Circuits ... dc leakage or equipment failure could be fatal ... Solution: BIAS BIAS BIAS ()1.4V 503 A