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• Course overview and information
09/16/2010
© 2010 NTUST
Instrumentation Amplifier
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
R3
+
- R
1
-
R2
+
R5
R4
+
R6
Vout =
Acl (Vin2-Vin1)
Instrumentation Amplifier
For R1 = R2 = 10 kW, what value of RG will set
the gain to 40?
Vin1
2R
Acl  1 
RG
- R
1
Solving for RG,
2R
RG 
Acl - 1
RG 
2 10 kW 
 513 W
40 - 1
R3
+
RG
-
R2
Vin2
(510 W is the nearest standard value).
+
R5
R4
+
R6
Vout
Instrumentation Amplifier
The bandwidth of instrumentation amplifiers decreases
when higher gain is selected.
1000
Voltage gain
The frequency response of
the AD633 is shown. If the
highest frequency in the
signal is not greater than
about 3.5 kHz, the
maximum gain of 1000 can
be selected.
100
10
1
0.1
100
1k
Frequency (Hz)
10k
100k 1M
10M
Isolation Amplifiers
An isolation amplifier provides dc isolation for
applications where electrical separation between
input and output is necessary.
+V -V
A block diagram
of a capacitively
coupled isolation
amplifier is
shown. Notice the
separate power
supplies and
grounds for each
stage.
+V -V
Input stage
Op-amp
Modulator
Output stage
Demodulator
Oscillator
Isolation barrier with
capacitive coupling
Op-amp
Isolation Amplifiers
For the capacitively coupled isolation amplifier, the input signal is
modulated. Modulation is the process of modifying the input with
another waveform in order to transmit the signal across the barrier.
+V -V
Original
input
+V -V
Input stage
Op-amp
Modulator
Output stage
Demodulator
AM
Oscillator
PWM
Modulated signal
Op-amp
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
Shielded
cable
3656KG
Input
Com
Output
Heart
monitor
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
Iout = KIBIASVin
Output
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.
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.
105
104
103
102
10
1
0.1
1
10
100
Bias current (mA)
1000
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
1
see: http://cache.national.com/ds/LM/LM13700.pdf
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
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
Bias current (mA)
1000
Operational Transconductance Amplifier (OTA)
What value of bias resistor will set the bias current at
125 mA for the previous circuit?
I BIAS
V -  -V  - 1.4 V
 BIAS
RBIAS
RBIAS
VBIAS -  -V  - 1.4 V

I BIAS
+15 V

Vin
R1
10 kW
OTA
RBIAS
220 kW
Vout
+
R2
10 kW
RL
20 kW
-15 V
15 V -  -15 V  - 1.4 V
125 μA
= 228 kW
The nearest standard
5% value is 220 kW.
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.
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.
+
D
RL
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.
+
D
RL
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.
Vout
0V
R1
D
RL
0.7 V
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
If the input is below -0.7 V,
the diode conducts, causing
the output to be limited
(clipped) for voltages less
than this level.
RL
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.
+
D
A different reference level is
easy to achieve by putting the
desired reference voltage on
the non-inverting input.
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.
Vin
Ri
+
-
R1
D
VOUT
C
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
transconductance the gain is set by a bias current.
amplifier (OTA)
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.
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.
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
-
R2
+
R5
R4
+
R6
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.
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.
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.
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.
Quiz
7. Another name for a clamping circuit is a
a. dc restorer.
b. clipping circuit.
c. limiter.
d. peak detector.
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. C
-
Vin
+
Vout
D
RL
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.
Quiz
10. Assume the circuit has an ac input as shown. The
output will be
a. an amplified sine wave.
b. 0.5 Vdc
Ri
c. 1.0 Vdc
+
+1.0 V
d. 2.0 Vdc
Vin
0V
-
R1
D
VOUT
-1.0 V
C
Quiz
Answers:
1. a
6. b
2. b
7. a
3. c
8. c
4. d
9. a
5. d
10. c