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electronics fundamentals
circuits, devices, and applications
THOMAS L. FLOYD
DAVID M. BUCHLA
chapter 20
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Instrumentation amplifiers
An instrumentation amplifier (IA) is a special integrated
circuit designed to for applications where small signals are
in noisy environments. They have high input impedance and
a high CMRR for excellent noise rejection.
R3 to R6 are equal
Vin1 + Vcm
values. If R1 = R2,
then the closed-loop
gain is set by a single
R
external resistor, RG, G
supplied by the user.
The gain is:
2 R Vin2 + Vcm
Acl  1 
RG
Electronics Fundamentals 8th edition
Floyd/Buchla
R3
+
- R
1
-
R2
+
R5
R4
Vout =
Acl (Vin2-Vin1)
+
R6
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Instrumentation amplifiers
For R1 = R2 = 10 kW, what value of RG will set
the gain to 40?
2R
Acl  1 
RG
Solving for RG,
2R
RG 
Acl - 1
2 10 kW 
RG 
 513 W
40 - 1
Vin1
R3
+
- R
1
RG
-
R2
Vin2
+
R5
R4
Vout
+
R6
(510 W is the nearest standard value).
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Instrumentation amplifiers
The bandwidth of instrumentation amplifiers decreases
when higher gain is selected. The frequency response is
published by the manufacturer. For example the
frequency response of an INA333 is shown.
If the required frequency response is
less than about 300 Hz, a gain of 1000
can be selected. The INA333 can
operate on as little as 1.8 V, so is used
in portable medical, handheld
instrumentation, weigh scales and data
acquisition applications.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Isolation amplifiers
An isolation amplifier provides dc isolation for applications where
electrical separation between input and output is necessary. A block
diagram of a isolation amplifier is shown. This one uses pulse width
modulation (PWM) to send a signal across the isolation barrier.
+V -V
+V -V
Modulation is the
process of modifying
Input stage
Output stage
the input with another
waveform in order to Op-amp
Modulator
Demodulator
Op-amp
transmit the signal
across the barrier.
Notice the separate
Oscillator
power supplies and
Isolation barrier with
grounds for each stage.
capacitive coupling
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Isolation amplifiers
Signals for pulse width modulation are shown. This is the method
used by the ISO124, discussed in the text. After transmitting the
modulated signal across the barrier, the original signal is recovered.
+V -V
Original
input
+V -V
Input stage
Op-amp
Modulator
Output stage
Demodulator
Op-amp
AM
Oscillator
PWM
Modulated signal
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Isolation amplifiers
Transformer coupling can also be used in isolation amplifiers. The
Burr-Brown 3656KG is an example of a versatile IC that has gain
control as well as three-port isolation (with three separate grounds).
It can also supply isolated power on both the input and the output
side for external devices.
The 3656KG is suited to applications such as interfacing
the signals in an electrocardiogram, or to isolate the input signals
for fetal heart monitoring as shown here in in the text.
Electrode for
sensing fetal
heartbeat
Common
electrode
Electronics Fundamentals 8th edition
Floyd/Buchla
Shielded
cable
3656KG
Input
Output
Heart
monitor
Com
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
An operational transconductance amplifier is a voltage
to current amplifier. The symbol shows a current source
on the output which is dependent on bias current.
The gain of an OTA is given as a transconductance parameter
(similar to a FET): g  I out
m
Vin
IBIAS
The transconductance is
dependent on the bias current
(IBIAS) and a constant (K):
Inputs
OTA
+
gm = KIBIAS
Output
Iout = KIBIASVin
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
A typical curve is shown. Notice
that both axes are logarithmic.
For example, a transconductance
of 1000 mS (103), requires a bias
current of about 63 mA.
100 k
Transconductance, gm (mS)
The specification sheet shows a
graph of the relationship between
transconductance and bias current.
The user can then set the bias to the
desired transconductance.
10 k
1.0 k
100
10
Electronics Fundamentals 8th edition
Floyd/Buchla
1
10
100
Bias current (mA)
1000
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
The bias current also affect the input
and output resistance. If you require
a very high input resistance, you can
choose a smaller bias current.
Output resistance
10 M
Resistance (W)
What is the input resistance
indicated by the plot if the bias
current is 20 mA?
100 M
1.0 M
Input resistance
100k
Reading the graph, the input
resistance is approximately 1.5 MW
Electronics Fundamentals 8th edition
Floyd/Buchla
10k
1
100
10
Bias current (mA)
1000
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
The LM13700 is a representative OTA which has a gm that can be
set over a six decade range. There are many applications1 for this
OTA including modulators, function generators, and voltage
controlled circuits including amplifiers, filters, and resistors.
+15 V
Vin
R1
10 kW
The bias current for the LM13700 is
found from the formula
RBIAS
OTA
I BIAS 
Vout
VBIAS -  -V  - 1.4 V
RBIAS
+
R2
10 kW
RL
20 kW
-15 V
Electronics Fundamentals 8th edition
Floyd/Buchla
1
see: http://cache.national.com/ds/LM/LM13700.pdf
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
What value of bias current will produce a voltage gain of
40 for the OTA? The transconductance curve is shown.
+15 V
Vin
R1
10 kW
RBIAS
OTA
Vout
+
R2
10 kW
RL
20 kW
-15 V
Electronics Fundamentals 8th edition
Floyd/Buchla
Transconductance, gm (mS)
The required gm is Av/RL = 40/20 kW = 2,000 mS.
105
From the graph, IBIAS ≈ 125 mA
104
103
102
10
1
0.1
1
10
100
1000
Bias current (mA)
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Operational Transconductance Amplifier (OTA)
What value of bias resistor will set the bias current at
125 mA for the previous circuit?
I BIAS 
VBIAS -  -V  - 1.4 V
RBIAS
RBIAS 
+15 V

Vin
R1
10 kW
OTA
RBIAS
220 kW
Vout
+
R2
10 kW
RL
20 kW
VBIAS -  -V  - 1.4 V
I BIAS
15 V -  -15 V  - 1.4 V
125 μA
= 228 kW
The nearest standard
5% value is 220 kW.
-15 V
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Clamping circuits
A basic clamping circuit (also called a dc restorer) adds a
dc level to a signal voltage using a diode and capacitor.
Vp(in) - 0.7 V
-
C
+
Vin
0V
Vout
D
RL
0V
Diode conducts
The first negative cycle of the signal biases the diode on and
causes the capacitor to charge to Vp(in) - 0.7 V. The long time
constant keeps the capacitor charged, which adds a dc voltage
to the signal voltage at the output.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Clamping circuits
The 0.7 V diode drop in the basic circuit can
be a problem for low-level signals, which are
common in signal processing applications. 0 V
Vin
0V
Vin
-
C
+
Vout
R1
Active diode circuits use an op-amp
and feedback to closely
approximate the behavior of an
ideal diode. The same clamping
circuit with an active diode has a
nearly ideal response.
Electronics Fundamentals 8th edition
Floyd/Buchla
+
D
RL
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Clamping circuits
Active clamping circuits can produce negative
0V
clamping action by reversing the diode.
Vin
0V
C
Vin
+
-
Vout
R1
The first positive cycle of the
signal charges the capacitor as
shown, adding a negative dc
voltage to the input.
Electronics Fundamentals 8th edition
Floyd/Buchla
+
D
RL
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Clamping circuits
An application for a clamping circuit is shown.
C1
Vs
+15 V
R1
D1
R2
This circuit adds or subtracts a dc
voltage to the output, depending
on the setting of the DC Offset
control.
_
DC Offset
Control
R3
+
Vout
RL
R4
-15 V
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Limiting circuits
Diode limiters (clippers) are circuits that limit voltage
above or below a specified level. A basic limiter
circuit that clips voltages below 0.7 V is shown.
Vin
0V
If the input is above
0.7 V, the diode
conducts, causing the
output to be limited to
this level.
Electronics Fundamentals 8th edition
Floyd/Buchla
Vout
0V
R1
D
0.7 V
RL
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Limiting circuits
Reversing the diode causes the limiter to clip signal
voltages that are below –0.7 V.
Vin
0V
Vout
R1
-0.7 V
D
RL
If the input is below -0.7 V,
the diode conducts, causing
the output to be limited
(clipped) for voltages less
than this level.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Limiting circuits
An active limiter uses an op-amp and diode circuit to
form an almost ideal diode. This means the 0.7 V
forward drop of the diode does not affect the output.
Vin
0V
Vout
0V
R
-
This circuit limits the
positive waveform
(clipping voltages above
ground) because the +
input reference is at 0 V.
Electronics Fundamentals 8th edition
Floyd/Buchla
+
D
A different reference level is
easy to achieve by putting the
desired reference voltage on
the non-inverting input.
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Summary
Peak detector
Another useful active diode circuit is the peak detector.
The purpose of the circuit is to store the maximum
positive value of a voltage on a capacitor and hold the
value for a certain time.
The op-amp is set up as a
comparator. If Vin > VC , the
diode is forward biased and
charges to the peak of Vin. For
example if a 1.0 Vpp sine
wave is the input, the output
will be a dc level of +0.5 V.
Electronics Fundamentals 8th edition
Floyd/Buchla
Vin
Ri
+
-
R1
D
VOUT
C
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Selected Key Terms
Instrumentation An amplifier specifically designed for
amplifier amplifying small differential signals and
rejecting large common-mode voltages.
Isolation An amplifier with electrically isolated
amplifier internal stages.
Operational A voltage-to-current amplifier in which the
transconductance gain is set by a bias current.
amplifier (OTA)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Selected Key Terms
Clamper A circuit that adds a dc level to an ac signal;
a dc restorer.
Limiter A circuit that removes part of a waveform
above or below a specified level; a clipper.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
1. Selecting the highest gain for an instrumentation
amplifier means that
a. the bandwidth will be less.
b. the CMRR is higher.
c. both of the above.
d. none of the above.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
2. The block diagram for an instrumentation amplifier is
shown. The resistor shown in the blue box
a. determines the CMRR.
R3
+
b. determines the gain.
c. both of the above.
d. none of the above.
- R
1
RG
+
Electronics Fundamentals 8th edition
Floyd/Buchla
-
R2
-
R5
R4
+
R6
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
3. For an isolation amplifier, the input and output stages
cannot
a. have a common power supply.
b. be connected with a conductive path.
c. both of the above.
d. none of the above.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
4. In an isolation amplifier the purpose of the modulator is
to
a. increase the signal-to-noise ratio.
b. increase the bandwidth.
c. remove high frequency noise from the signal.
d. modify the signal for transmission.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
5. The gain of an operational transconductance amplifier
(OTA), is specified as the ratio of
a. output current to input current.
b. output voltage to input current.
c. output voltage to input voltage.
d. output current to input voltage.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
6. To increase the gain of an operational transconductance
amplifier (OTA), you would
a. change the ratio of the feedback resistors.
b. reduce the size of the bias resistor.
c. increase the size of the gain resistor.
d. increase the size of the load resistor.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
7. Another name for a clamping circuit is a
a. dc restorer.
b. clipping circuit.
c. limiter.
d. peak detector.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
8. The dc voltage you would expect to measure across the
capacitor is equal to 0.7 V less than the
a. peak-to-peak value of the input voltage.
b. rms value of the input voltage.
c. peak value of the input voltage.
d. average value of the input voltage.
Vin
Electronics Fundamentals 8th edition
Floyd/Buchla
C
+
Vout
D
RL
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
9. Reversing the diode in a clipping circuit causes
a. the opposite side of the input to be clipped.
b. a dc level shift in the output.
c. the clipping level to increase.
d. the ground reference to change.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
10. Assume the circuit has an ac input as shown. The
output will be
a. an amplified sine wave.
Ri
b. 0.5 Vdc
+1.0 V
c. 1.0 Vdc
d. 2.0 Vdc
Electronics Fundamentals 8th edition
Floyd/Buchla
Vin
0V
+
-
R1
D
VOUT
-1.0 V
C
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 20
1
Quiz
Answers:
Electronics Fundamentals 8th edition
Floyd/Buchla
1. a
6. b
2. b
7. a
3. c
8. c
4. d
9. a
5. d
10. c
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.