OPA
®
OPA660
660
OPA
660
Wide Bandwidth
OPERATIONAL TRANSCONDUCTANCE
AMPLIFIER AND BUFFER
FEATURES
APPLICATIONS
● WIDE BANDWIDTH: 850MHz
● BASE LINE RESTORE CIRCUITS
● HIGH SLEW RATE: 3000V/µs
● LOW DIFFERENTIAL GAIN/PHASE
ERROR: 0.06%/0.02°
● VERSATILE CIRCUIT FUNCTION
● EXTERNAL IQ-CONTROL
● VIDEO/BROADCAST EQUIPMENT
● COMMUNICATIONS EQUIPMENT
● HIGH-SPEED DATA ACQUISITION
● WIDEBAND LED DRIVER
● AGC-MULTIPLIER
DESCRIPTION
The OPA660 is a versatile monolithic component
designed for wide-bandwidth systems including high
performance video, RF and IF circuitry. It includes a
wideband, bipolar integrated voltage-controlled current source and voltage buffer amplifier.
● NS-PULSE INTEGRATOR
● CONTROL LOOP AMPLIFIER
● 400MHz DIFFERENTIAL INPUT
AMPLIFIER
200Ω
100Ω
The voltage-controlled current source or Operational
Transconductance Amplifier (OTA) can be viewed as
an “ideal transistor.” Like a transistor, it has three
terminals—a high-impedance input (base), a lowimpedance input/output (emitter), and the current
output (collector). The OTA, however, is self-biased
and bipolar. The output current is zero-for-zero differential input voltage. AC inputs centered about zero
produce an output current which is bipolar and centered about zero. The transconductance of the OTA
can be adjusted with an external resistor, allowing
bandwidth, quiescent current and gain trade-offs to
be optimized.
3 B
R1
6
+1
VO
R3
390Ω
OTA
E
2
IQ = 20mA
G=1+
RP
82Ω
CP
6.4pF
R5
100Ω
R3
=3
2R5
XE
OPA660 DIRECT-FEEDBACK FREQUENCY RESPONSE
20
Output Voltage (dB)
The open-loop buffer amplifier provides 850MHz
bandwidth and 3000V/µs slew rate. Used as a basic
building block, the OPA660 simplifies the design of
AGC amplifiers, LED driver circuits for Fiber Optic
Transmission, integrators for fast pulses, fast control
loop amplifiers, and control amplifiers for capacitive
sensors and active filters.
VI
5
8
C
The OPA660 is packaged in SO-8 surface-mount,
and 8-pin plastic DIP, specified from –40°C to +85°C.
15
5Vp-p
10
2.8Vp-p
5
1.4Vp-p
0
0.6Vp-p
–5
–10
0.2Vp-p
–15
–20
–25
–30
100k
1M
10M
100M
1G
Frequency (Hz)
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 • Twx: 910-952-1111
Internet: http://www.burr-brown.com/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
© 1990 Burr-Brown Corporation
PDS-1072F
Printed in U.S.A. April, 1995
SPECIFICATIONS
Typical at IQ = 20mA, VS = ±5V, TA = +25°C, and RL = 500Ω, unless otherwise specified.
OPA660AP, AU
PARAMETER
OTA TRANSCONDUCTANCE
Transconductance
OTA INPUT OFFSET VOLTAGE
Initial
vs Temperature
vs Supply (tracking)
vs Supply (non-tracking)
vs Supply (non-tracking)
OTA B-INPUT BIAS CURRENT
Initial
vs Temperature
vs Supply (tracking)
vs Supply (non-tracking)
vs Supply (non-tracking)
OTA OUTPUT BIAS CURRENT
Output Bias Current
vs Temperature
vs Supply (tracking)
vs Supply (non-tracking)
vs Supply (non-tracking)
OTA OUTPUT
Output Current
Output Voltage Compliance
Output Impedance
Open-Loop Gain
BUFFER OFFSET VOLTAGE
Initial
vs Temperature
vs Supply (tracking)
vs Supply (non-tracking)
vs Supply (non-tracking)
BUFFER INPUT BIAS CURRENT
Initial
vs Temperature
vs Supply (tracking)
vs Supply (non-tracking)
vs Supply (non-tracking)
CONDITIONS
MIN
TYP
MAX
UNITS
VC = 0V
75
125
200
mA/V
±30
55
40
40
+10
50
60
45
48
mV
µV/°C
dB
dB
dB
–2.1
5
±5
µA
nA/°C
nA/V
nA/V
nA/V
VB = 0
VS = ±4.5V to ±5.5V
V+ = 4.5V to 5.5V
V– = –4.5V to –5.5V
VS = ±4.5V to ±5.5V
V+ = 4.5V to 5.5V
V– = –4.5V to –5.5V
±750
±1500
±500
±10
500
±10
±10
±10
VB = 0, VC = 0V
VS = ±4.5V to ±5.5V
V+ = 4.5V to 5.5V
V– = –4.5V to –5.5V
±10
±4.0
f = 1kHz
±30
VS = ±4.5V to ±5.5V
V+ = 4.5V to 5.5V
V– = –4.5V to –5.5V
+7
50
60
45
48
mV
µV/°C
dB
dB
dB
–2.1
5
±5
µA
nA/°C
nA/V
nA/V
nA/V
55
40
40
VS = ±4.5V to ±5.5V
V+ = 4.5V to 5.5V
V– = –4.5V to –5.5V
VO = ±100mV
VO = ±1.4V
VO = ±2.5V
3.58MHz, at 0.7V
3.58MHz, at 0.7V
f = 10MHz, VO = 0.5Vp-p
5V Step
2V Step
VO = 100mVp-p
5V Step
Group Delay Time
BUFFER RATED OUTPUT
Voltage Output
Current Output
Gain
IO = ±1mA
±3.7
±10
0.96
RL = 500Ω
RL = 5kΩ
Output Impedance
POWER SUPPLY
Voltage, Rated
Derated Performance
Quiescent Current (Programmable, Useful Range)
±4.5
®
OPA660
mA
V
Ω || pF
dB
±750
±1500
±500
BUFFER INPUT NOISE
Voltage Noise Density, f = 100kHz
Differential Gain Error
Differential Phase Error
Harmonic Distortion, 2nd Harmonic
Slew Rate
Settling Time 0.1%
Rise Time (10% to 90%)
±25
±25
±25
µA
nA/°C
µA/V
µA/V
µA/V
±15
±4.7
25k || 4.2
70
IC = ±1mA
BUFFER and OTA INPUT IMPEDANCE
Input Impedance
BUFFER DYNAMIC RESPONSE
Small Signal Bandwidth
Full Power Bandwidth
±20
2
1.0 || 2.1
MΩ || pF
4
nV/√Hz
850
800
570
0.06
0.02
–68
3000
25
1
1.5
250
MHz
MHz
MHz
%
Degrees
dBc
V/µs
ns
ns
ns
ps
±4.2
±15
0.975
0.99
7 || 2
V
mA
V/V
V/V
Ω || pF
±5
±3 to ±26
±5.5
V
V
mA
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
Top View
Power Supply Voltage ......................................................................... ±6V
Input Voltage(1) ........................................................................ ±VS ±0.7V
Operating Temperature ................................................... –40°C to +85°C
Storage Temperature ..................................................... –40°C to +125°C
Junction Temperature .................................................................... +175°C
Lead Temperature (soldering, 10s) ............................................... +300°C
DIP/SO-8
I Q Adjust
1
8
C
E
2
7
V+ = +5V
B
3
6
Out
1
NOTE: (1) Inputs are internally diode-clamped to ±VS.
PACKAGE/ORDERING INFORMATION
V– = –5V
4
5
PRODUCT
PACKAGE
PACKAGE
DRAWING
NUMBER(1)
OPA660AP
OPA660AU
8-Pin Plastic DIP
SO-8 Surface-Mount
006
182
In
TEMPERATURE
RANGE
–25°C to +85°C
–25°C to +85°C
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix C of Burr-Brown IC Data Book.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation
to complete device failure. Precision integrated circuits may
be more susceptible to damage because very small parametric
changes could cause the device not to meet its published
specifications.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
®
3
OPA660
TYPICAL PERFORMANCE CURVES
IQ = 20mA, TA = +25°C, and VS = ±5V unless otherwise noted.
TOTAL QUIESCENT CURRENT vs RQ
TOTAL QUIESCENT CURRENT vs TEMPERATURE
1.5
Total Quiescent Current (Normalized)
Total Quiescent Current (mA)
100
30
Nominal
Device
High IQ
Device
10
3.0
Low IQ
Device
1.0
100
300
1.0k
3.0k
10k
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
R Q — Resistor Value ( Ω)
–25
25
50
Temperature (°C)
BUFFER AND OTA B-INPUT BIAS CURRENT
vs TEMPERATURE
OTA C-OUTPUT BIAS CURRENT vs TEMPERATURE
0
75
100
5 Representative
Units
OTA C-Output Bias Current (µA)
Input Bias Current (µA)
0.0
–1.0
–2.0
–3.0
–4.0
Trim Point
–40
–5.0
–20
–0
20
40
60
100
80
–20
–0
20
Temperature (°C)
OTA C-OUTPUT RESISTANCE
vs TOTAL QUIESCENT CURRENT (IQ)
60
80
100
OTA TRANSFER CHARACTERISTICS
60
10
50
OTA Output Current (mA)
OTA Output Resistance (k Ω)
40
Temperature (°C)
40
30
20
10
5
IQ = 5mA
0
IQ = 10mA
–5
IQ = 20mA
0
–10
4
6
8
10
12
14
16
18
20
–60
Total Quiescent Current — IQ (mA)
–20
0
20
OTA Input Voltage (mV)
®
OPA660
–40
4
40
60
TYPICAL PERFORMANCE CURVES (CONT)
IQ = 20mA, TA = +25°C, and VS = ±5V unless otherwise noted.
BUFFER AND OTA B-INPUT OFFSET VOLTAGE
vs TEMPERATURE
BUFFER AND OTA B-INPUT RESISTANCE
vs TOTAL QUIESCENT CURRENT (IQ)
Buffer and OTA B-Input Resistance (MΩ)
20
Offset Voltage (mV)
15
10
5
0
–5
–10
–15
–20
0
25
50
75
RINOTA
3
RINBUF
2
1
0
–1
100
4
8
10
12
14
16
18
Total Quiescent Current — IQ (mA)
BUFFER OUTPUT AND OTA E-OUTPUT RESISTANCE
vs TOTAL QUIESCENT CURRENT (IQ)
BUFFER SLEW RATE
vs TOTAL QUIESCENT CURRENT (IQ)
20
4000
40
3800
3600
30
20
ROUTOTA
ROUTBUF
10
Rising Edge
3400
3200
3000
2800
Falling Edge
2600
2400
2200
2000
0
4
6
8
10
12
14
16
18
4
20
6
8
10
12
14
16
18
20
Total Quiescent Current—IQ (mA)
Total Quiescent Current—IQ (mA)
OTA TRANSCONDUCTANCE
vs TOTAL QUIESCENT CURRENT (IQ)
OTA TRANSCONDUCTANCE vs FREQUENCY
1000
OTA Transconductance (mA/V)
150
OTA Transconductance (mA/V)
6
Temperature (°C)
Slew Rate (V/µs)
Buffer Output and OTA E-Output Resistance (Ω)
–25
4
100
50
RL = 50Ω
IQ = 20mA
106mA/V
100
IQ = 10mA
IQ = 5mA
66mA/V
40mA/V
10
0
0
2
4
6
8
10
12
14
16
18
1M
20
10M
100M
1G
Frequency (Hz)
Total Quiescent Current—IQ (mA)
®
5
OPA660
TYPICAL PERFORMANCE CURVES (CONT)
IQ = 20mA, TA = +25°C, and VS = ±5V unless otherwise noted.
BUFFER FREQUENCY RESPONSE
BUFFER VOLTAGE NOISE SPECTRAL DENSITY
100
20
–3dB Point
2.8Vp-p
10
Output Voltage (dB)
Voltage Noise (nV/ Hz)
15
10
5
1.4Vp-p
0
0.6Vp-p
–5
–10
0.2Vp-p
–15
–20
–25
1
dB
100
1k
10k
100k
1M
10M
100M
200k
1M
10M
100M
1G
Frequency (Hz)
Frequency (Hz)
IQ = 20mA RIN = 160Ω RL = 100Ω
BUFFER MAX OUTPUT VOLTAGE vs FREQUENCY
TRANSCONDUCTANCE vs INPUT VOLTAGE
160
Transconductance (mA/V)
0
RQ = 250Ω
120
RQ = 500Ω
80
RQ = 1kΩ
RQ = 2kΩ
40
0
0.1
1M
10M
100M
1G
–40
–30
–20
–10
0
10
OTA PULSE RESPONSE
30
40
OTA PULSE RESPONSE
+2.5V
VO (V)
+0.625V
0V
0V
–2.5V
–0.625V
Input Voltage = 1.25Vp-p, tR = tF = 1ns, Gain = 4
Output Voltage = 5Vp-p
®
OPA660
20
Input Voltage (mV)
Frequency (Hz)
VO (V)
Buffer Output Voltage (Vp-p)
10
6
TYPICAL PERFORMANCE CURVES
(CONT)
IQ = 20mA, TA = +25°C, and VS = ±5V unless otherwise noted.
BUFFER LARGE SIGNAL PULSE RESPONSE
VO (V)
VO (V)
BUFFER LARGE SIGNAL PULSE RESPONSE
tR = tF = 3ns, VO = 5Vp-p
(HDTV Signal Pulse) tR = tF = 10ns, VO = 5Vp-p
160Ω
50Ω
5
VI
+1
Network
50Ω Analyzer
R6
6
VO
50Ω
RIN = 50Ω
50Ω
50Ω
R7
RL = R6 + R7||RIN = 100Ω
tR = tF = 3ns, VO = 0.2Vp-p
Test Circuit Buffer Pulse and Frequency Response
BUFFER DIFFERENTIAL GAIN ERROR
vs TOTAL QUIESCENT CURRENT (IQ)
BUFFER DIFFERENTIAL PHASE ERROR
vs TOTAL QUIESCENT CURRENT (IQ)
0.10
Differential Phase Error (Degrees)
Differential Gain Error (%)
0.25
0.20
RL = 500Ω
VO = 0.7Vp-p
f = 3.58MHz
0.15
0.10
0.05
0
4
6
8
10
12
14
16
18
20
RL = 500Ω
VO = 0.7Vp-p
f = 3.58MHz
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
4
Total Quiescent Current —IQ (mA)
6
8
10
12
14
16
18
20
Total Quiescent Current—IQ (mA)
®
7
OPA660
TYPICAL PERFORMANCE CURVES (CONT)
IQ = 20mA, TA = +25°C, and VS = ±5V unless otherwise noted.
HARMONIC DISTORTION vs FREQUENCY
HARMONIC DISTORTION vs FREQUENCY
–40
–30
RL = 150Ω
VO = 0.5Vp-p
IQ = 20mA
–50
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
–30
2f
–60
3f
–70
–40
RL = 500Ω
IQ = 20mA
3f
2Vp-p
–50
3f
0.5Vp-p
2f
2Vp-p
–60
2f
0.5Vp-p
–70
Measurement Limit
Measurement Limit
–80
–80
10M
20M
40M
60M
100M
10M
20M
Frequency (Hz)
40M
60M
100M
Frequency (Hz)
APPLICATIONS INFORMATION
The OPA660 operates from ±5V power supplies (±6V
maximum). Do not attempt to operate with larger power
supply voltages or permanent damage may occur.
The buffer output is not current-limited or protected. If the
output is shorted to ground, currents up to 60mA could flow.
Momentary shorts to ground (a few seconds) should be
avoided, but are unlikely to cause permanent damage. The
same cautions apply to the OTA section when connected as
a buffer (see Basic Applications Circuits, Figure 6b).
Inputs of the OPA660 are protected with internal diode
clamps as shown in the simplified schematic, Figure 1. These
protection diodes can safely conduct 10mA, continuously
(30mA peak). If input voltages can exceed the power supply
voltages by 0.7V, the input signal current must be limited.
(7)
+VCC = +5V
Bias
Circuitry
VI
VO
B
E
C
(5)
(6)
(3)
(2)
(8)
BUFFER
OTA
100Ω
50kΩ
–VCC = –5V
I Q Adj.
(1)
R Q (ext.)
(4)
FIGURE 1. Simplified Circuit Diagram.
®
OPA660
8
BUFFER SECTION—AN OVERVIEW
The buffer section of the OPA660 is an open-loop buffer
consisting of complementary emitter-followers. It uses no
feedback, so its low frequency gain is slightly less than unity
and somewhat dependent on loading. It is designed primarily for interstage buffering. It is not designed for driving
long cables or low impedance loads (although with small
signals, it may be satisfactory for these applications).
QUIESCENT CURRENT CONTROL PIN
The quiescent current of the OPA660 is set with a resistor,
RQ, connected from pin 1 to V–. It affects the operating
currents of both the buffer and OTA sections. This controls
the bandwidth and AC behavior as well as the
transconductance of the OTA section.
RQ = 250Ω sets approximately 20mA total quiescent current at
25°C. With a fixed 250Ω resistor, process variations could
cause this current to vary from approximately 16mA to 26mA.
It may be appropriate in some applications to trim this resistor
to achieve the desired quiescent current or AC performance.
TRANSCONDUCTANCE
(OTA) SECTION—AN OVERVIEW
The symbol for the OTA section is similar to a transistor.
Applications circuits for the OTA look and operate much
like transistor circuits—the transistor, too, is a voltagecontrolled current source. Not only does this simplify the
understanding of applications circuits, but it aids the circuit
optimization process. Many of the same intuitive techniques
used with transistor designs apply to OTA circuits as well.
Applications circuits generally do not show resistor, RQ,
but it is required for proper operation.
With a fixed RQ resistor, quiescent current increases with
temperature (see typical performance curve, Quiescent Current
vs Temperature). This variation of current with temperature
holds the transconductance, gm, of the OTA relatively constant with temperature (another advantage over a transistor).
The three terminals of the OTA are labeled B, E, and C. This
calls attention to its similarity to a transistor, yet draws
distinction for clarity.
It is also possible to vary the quiescent current with a control
signal. The control loop in Figure 3 shows a 1/2 of a REF200
current source used to develop 100mV on R1. The loop
forces 100mV to appear on R2. Total quiescent current of the
OPA660 is approximately 85 • I1, where I1 is the current
made to flow out of pin 1.
While it is similar to a transistor, one essential difference is
the sense of the C output current. It flows out the C terminal
for positive B-to-E input voltage and in the C terminal for
negative B-to-E input voltage. The OTA offers many advantages over a discrete transistor. The OTA is self-biased,
simplifying the design process and reducing component
count. The OTA is far more linear than a transistor.
Transconductance of the OTA is constant over a wide range
of collector currents—this implies a fundamental improvement of linearity.
Internal
Current Source
Circuitry
OPA660
V+
BASIC CONNECTIONS
Figure 2 shows basic connections required for operation.
These connections are not shown in subsequent circuit
diagrams. Power supply bypass capacitors should be located
as close as possible to the device pins. Solid tantalum
capacitors are generally best. See “Circuit Layout” at the end
of the applications discussion and Figure 26 for further
suggestions on layout.
1/2 REF200
100µA 1kΩ
R1
RQ
250Ω
8
2
3
–5V(1)
1
4
FIGURE 3. Optional Control Loop for Setting Quiescent
Current.
10nF
With this control loop, quiescent current will be nearly
constant with temperature. Since this differs from the temperature-dependent behavior of the internal current source,
other temperature-dependent behavior may differ from that
shown in typical performance curves.
2.2µF
6
5
RB
(25Ω to 200Ω)
470pF
2.2µF
Solid
Tantalum
IQ ≈ 85 • I1
R1
= 85 • (100µA)
R2
= 20mA
NOTE: (1) Requires input common-mode range and
output swing close to V–, thus the choice of OPA1013.
Solid
Tantalum
+
–VCC
425Ω
R2
+5V (1)
470pF
+
10nF
4
I1
7
RB
(25Ω to
200Ω)
50kΩ
1
1/2
(1)
OPA1013
RQ = 250Ω sets roughly
IQ ≈ 20mA
1
100Ω
The circuit of Figure 3 will control the IQ of the OPA660
somewhat more accurately than with a fixed external resistor, RQ. Otherwise, there is no fundamental advantage to
NOTE: (1) VS = ±6V absolute max.
FIGURE 2. Basic Connections.
®
9
OPA660
using this more complex biasing circuitry. It does, however,
demonstrate the possibility of signal-controlled quiescent
current. This may suggest other possibilities such as AGC,
dynamic control of AC behavior, or VCO.
+5V
4.7kΩ
Figure 4 shows logic control of pin 1 used to disable the
OPA660. Zero/5V logic levels are converted to a 1mA/0mA
current connected to pin 1. The 1mA current flowing in RQ
increases the voltage at pin 1 to approximately 1V above the
–5V rail. This will reduce IQ to near zero, disabling the
OPA660.
Internal
Current Source
Circuitry
0/5V
Logic In
5V: OPA660 On
2N2907
OPA660
100Ω
50kΩ
BASIC APPLICATIONS CIRCUITS
Most applications circuits for the OTA section consist of a
few basic types which are best understood by analogy to a
transistor. Just as the transistor has three basic operating
modes—common emitter, common base, and common collector—the OTA has three equivalent operating modes common-E, common-B, and common-C. See Figures 5, 6, and 7.
IC
1
4
RQ
250Ω
IC = 0: OPA660 On
IC ≈ 1mA: OPA660 Off
–5V
FIGURE 4. Logic-Controlled Disable Circuit.
V+
RB
RL
VO
100Ω
VI
Inverting Gain
VOS ≈ several volts
VI
RB
VO
8
C
3 B
OTA
Non-Inverting Gain
VOS ≈ 0
RL
E
2
RE
RE
V–
(b) Common-E Amplifier
(a) Common-Emitter Amplifier
Transconductance varies over temperature.
Transconductance remains constant over temperature.
FIGURE 5. Common-Emitter vs Common-E Amplifier.
V+
8
C
V+
100Ω
VI
3 B
G=–
RL
OTA
G≈1
VOS ≈ 0
Non-Inverting Gain
VOS ≈ several volts
VO
VO
G≈1
VOS ≈ 0.7V
RE
RE
100Ω
RE
(b) Common-C Amplifier
(Buffer)
3 B
V–
1+
(a) Common-Collector Amplifier
(Emitter Follower)
RO =
1
gm ¥ R E
≈1
(a) Common-Base
Amplifier
RL
RE
8
C
OTA
VO
Inverting Gain
VOS ≈ 0
RL
RE
VI
1
gm
(b) Common-B Amplifier
FIGURE 6. Common-Collector vs Common-C Amplifier.
FIGURE 7. Common-Base vs Common-B Amplifier.
®
OPA660
≈–
E
2
VI
1
G=
1
RE +
gm
VO
E
2
VI
RL
10
A positive voltage at the B, pin 3, causes a positive current
to flow out of the C, pin 8. Figure 5b shows an amplifier
connection of the OTA, the equivalent of a common-emitter
transistor amplifier. Input and output can be ground-referenced without any biasing. Due to the sense of the output
current, the amplifier is non-inverting. Figure 8 shows the
amplifier with various gains and output voltages using this
configuration.
It is recommended to use a low value resistor in series with
the B OTA and buffer inputs. This reduces any tendency to
oscillate and controls frequency response peaking. Values
from 25Ω to 200Ω are typical.
Figure 7 shows the Common-B amplifier. This configuration produces an inverting gain, and a low impedance input.
This low impedance can be converted to a high impedance
by inserting the buffer amplifier in series.
Just as transistor circuits often use emitter degeneration,
OTA circuits may also use degeneration. This can be used to
reduce the effect that offset voltage and offset current might
otherwise have on the DC operating point of the OTA. The
E-degeneration resistor may be bypassed with a large capacitor to maintain high AC gain. Other circumstances may
suggest a smaller value capacitor used to extend or optimize
high-frequency performance.
CIRCUIT LAYOUT
The high frequency performance of the OPA660 can be
greatly affected by the physical layout of the circuit. The
following tips are offered as suggestions, not dogma.
• Bypass power supplies very close to the device pins. Use
a combination between tantalum capacitors (approximately 2.2µF) and polyester capacitors. Surface-mount
types are best because they provide lowest inductance.
The transconductance of the OTA with degeneration can be
calculated by—
1
gm =
1
+ RE
gm
• Make short, wide interconnection traces to minimize
series inductance.
• Use a large ground plane to assure that a low impedance
ground is available throughout the layout.
Figure 6b shows the OTA connected as an E-follower—a
voltage buffer. The buffer formed by this connection performs virtually the same as the buffer section of the OPA660
(the actual signal path is identical).
• Do not extend the ground plane under high impedance
nodes sensitive to stray capacitance.
• Sockets are not recommended because they add significant inductance.
RL1
20
VO
OTA
100Ω
R1
RIN
50Ω
RL2
rE
RL = RL1 + RL2 || RIN
VI
2
G=
RE
RL
RE + r E
, rE =
G=
RE + 8 Ω
1.4Vp-p
0
600mVp-p
–5
–10
200mVp-p
–15
–25
–30
300k
1M
10M
100M
1G
3G
Frequency (Hz)
at I Q = 20mA
IQ = 20mA R1 = 100Ω RE = 51Ω RL = 50Ω Gain = 1
20
20
15
–3dB Point
10
Output Voltage (dB)
5
1.4Vp-p
0
–5
600mVp-p
–10
–15
200mVp-p
–5
–15
–25
100M
1G
–30
100k
3G
Frequency (Hz)
600mVp-p
–10
–25
10M
1.4Vp-p
0
–20
1M
2.8Vp-p
5
–20
–30
300k
–3dB Point
5Vp-p
15
2.8Vp-p
10
Output Voltage (dB)
5
–20
1
gm
1
At IQ = 20mA r E =
= 8Ω
125mA/V
RL
–3dB Point
2.8Vp-p
10
Output Voltage (dB)
3
15
Network
Analyzer
8
200mVp-p
1M
10M
100M
1G
Frequency (Hz)
IQ = 20mA R1 = 100Ω RE = 51Ω RL = 100Ω Gain = 2
IQ = 20mA R1 = 100Ω RE = 51Ω RL = 500Ω Gain = 10
FIGURE 8. Common-E Amplifier Performance.
®
11
OPA660
• Use low-inductance components. Some film resistors are
trimmed with spiral cuts which increase inductance.
• A resistor (25Ω to 200Ω) in series with the buffer and/or
B input may help reduce oscillations and peaking.
• Use surface-mount components—they generally provide
the lowest inductance.
• Use series resistors in the supply lines to decouple multiple devices.
OPA660 CURRENT-FEEDBACK
C1
20
5
6
+1
VO
Output Voltage (dB)
56Ω
R2
8
C
3 B
OTA
E
2
200Ω
R1
47Ω
VI
G=1+
R4
R5
5Vp-p
10
2.8Vp-p
5
1.4Vp-p
0
–5
0.6Vp-p
–10
0.2Vp-p
–15
–3dB Point
–20
R4
R5
22Ω
15
–25
–30
100k
≈ 10
1M
10M
100M
1G
Frequency (Hz)
R Q = 250Ω (IQ ≈ 20mA)
IQ = 20mA R1 = 47Ω R2 = 56Ω R4 = 200Ω R5 = 22Ω Gain = 10
FIGURE 9. Current-Feedback Amplifier.
20Ω
VIN
FIGURE 10. Current-Feedback Amplifier Frequency
Response, G = 10.
5
+1
6
C1
100pF
20Ω
VOUT
OPA650
R2
100kΩ
D1
25Ω
D1, D2 = 1N4148
RQ = 1kΩ
D2
R1
40.2Ω
• The OTA amplifier works as a current conveyor
(CCII) in this circuit, with a current gain of 1.
• R1 and C1 set the DC restoration time constant.
CCII
8
C
2
E
• D1 adds a propagation delay to the DC restoration.
B
• R2 and C1 set the decay time constant.
3
20Ω
FIGURE 11. DC Restorer Circuit.
8
C
3 B
+IN
OTA
IO
VI
150Ω
E
2
3 B
OTA
RL
150Ω
E
2
RE
50Ω
Tuning Coil
Magnetic Head
Driver Transformer
G=
RL
R E + rE
R Q = 250Ω (IQ ≈ 20mA)
FIGURE 13. Cable Amplifier.
3 B
OTA
C
8
FIGURE 12. High Speed Current Driver (bridge combination for increased output voltage capability).
®
OPA660
6
+1
RE
42Ω
2
E
–IN
5
8
C
12
VO
≈ +3
C8
0.5...2.5pF
+5V
R8
27kΩ
R6
47kΩ
Offset
R2
Trim 10kΩ
+5V
–5V
R1
100Ω
VI
R3
100Ω
+5V
7
3
RC5
150Ω
2.2µF
C3
R4
150Ω
8
5
+1
6
OTA
2
4
C3
R2
100Ω
C3
2.2µF
1
4
BUF600
1
5
RQ
250Ω
R5
47Ω
C3
2.2µF
VO
2.2µF
–5V
–5V
Propagation Delay Time = 5ns
Rise Time = 1.5ns
D1
D2
DMF3068A
FIGURE 14. Comparator (Low Jitter).
+5V
22Ω
IO = IO1 + IO2
180Ω
VI
8 IO1
C
3 B
OTA
8 IO1
C
3 B
Q1
+IB
22Ω
Q2
1kΩ
OTA
Diode
E
2
E
2
RE
50Ω
RE
50Ω
180Ω
Q1, Q2: 2N3906
FIGURE 15. High Speed Current Driver.
®
13
OPA660
8
C
180Ω
3 B
VI
200Ω
33pF
OTA
47Ω
E
2
VO
f–3dB
±100mV
±300mV
±700mV
±1.4V
±2.5V
351MHz
374MHz
435MHz
460MHz
443MHz
8
C
780Ω
VI
Network
Analyzer
VO
RE
50Ω
3 B
RIN
50Ω
OTA
820Ω
620Ω
1µF
50kΩ
1
G=
1+
≈ 1; RO =
1
2gm • (RE + RIN)
1
2gm
+5V
FIGURE 16. Voltage Buffer with Doubled-Output Current.
10nF
+5V
7
R6
150Ω
–VI
FIGURE 17. Integrator for ns-pulses.
R9
240Ω
R3
51Ω
R6
150Ω
–5V
+5V
2.2pF
+VI
22pF
8
OPA660
3
5
10nF
R10
150Ω
OTA
+1
1
4 BUF601
8
5
R7
51Ω
4
1
R16
560Ω
10nF
6 R8
43Ω
2
10nF
Rg
G = ––––––––– = 4
R8 + rE
C5
18pF
2.2µF
2.2µF
rE = 1/gm
–5V
–5V
FIGURE 18. 400MHz Differential Amplifier
–10
10
–20
0
without C5
–10
–40
with C5
–50
–20
–60
IQ = 20mA, G = +4V/V
–30
300k
1M
10M
100M
Frequency (Hz)
FIGURE 19. CMRR and Bandwidth of the Differential Amplifier
®
14
–70
1G
3G
CMRR
Gain (dB)
–30
OPA660
+1
27pF
E
2
50Ω
5
R11
51Ω
VO
6
VO
C
3
B
C
E
TRANSFER CHARACTERISTICS
2
B
R3
E
F(p) =
R2
VI
=
R1M
1
R2M
+
s2C1C2R1M R3 + sC1 R2
R1
s2C1C2R1M R2M + sC1 R1M
1
+
R2S
R1S
R3S
C
C
VI
VO
7
1
B
C2
C
E
Lowpass
B
E
6
B
C1
R1
R2M
R2 = R3 = ∞
Highpass
R1 = R2 = ∞
Bandpass
R1 = R3 = ∞
Band Rejection R2 = ∞, R1 = R3
E
Allpass
R1 = R1S, R2 = –R2S, R3 = R3S
R1M
VO
C
8
C
B
C
4
E
5
B
B
E
RB
RB
R3S
E
RB
R1S
R2S
FIGURE 20. High Frequency Universal Active Filter.
120Ω
150Ω
5
+1
6
VLUMINANCE
8
C
3 B
OTA
E
2
665Ω(1)
200Ω
VRED
340Ω(1)
VGREEN
1820Ω(1)
VBLUE
RQ = 500Ω (IQ ≈ 20mA)
NOTE: (1) Resistors shown are 1% values that
produce 30%/59%/11% R/G/B mix.
FIGURE 21. Video Luminance Matrix.
®
15
OPA660
+VO
290Ω
VO INT
8
3
OTA
10Ω
IN6263
+5V
IN6263
+5V
220Ω
180Ω
8
VI
7
7
1µF
100Ω
5
6
+1
180Ω
–VO
3
15nF
2
220Ω
100Ω
5
+1
6
OTA
1
4
4
1.2kΩ
20kΩ
–5V
12kΩ
–5V
220Ω
+
1.2kΩ
2
390Ω
–
5kΩ
Offset Trim
33pF
FIGURE 22. Signal Envelope Detector (Full-Wave Rectifier).
120Ω
100Ω
VI
8
C
3
B
5
+1
200Ω
6
VO
R2
OTA
IQ = 20mA
R1
E
2
RP
82Ω
R5
100Ω
50Ω
RIN
VO
f–3dB
±100mV
±300mV
±700mV
±1.4V
±2.5V
331MHz
362MHz
520MHz
552MHz
490MHz
R3
+ R5
R3
2
G=
=1+
1
2R5
R5 +
2 • gm
CP
6.4pF
XE
FIGURE 23. Direct-Feedback Amplifier.
®
OPA660
R4
R6
68Ω
R3
390Ω
Network
Analyzer
16
OPA660 DIRECT FEEDBACK
15
5Vp-p
10
2.8Vp-p
5
Gain = 3, tR – tF = 2ns, VI = 100mVp–p
1.4Vp-p
+150mV
0
0.6Vp-p
–5
VO (V)
Output Voltage (dB)
20
–10
0.2Vp-p
–15
0V
–20
–25
–30
–150mV
1M
100k
10M
100M
1G
Frequency (Hz)
0
R1 = 100Ω R2 = 120Ω R3 = 390Ω R4 = 200Ω
R5 = 100Ω R6 = 68Ω IQ = 20mA Rp = 82Ω Cp = 6.4pF
5
10
15
20
30
VO
Gain = 3, VI = 2Vp-p, tR = tF = 2ns
8
C
R1
160Ω
+3V
VI
0V
Network
Analyzer
56Ω
15
20
25
30
35
40
R3
50Ω
RIN
OTA
IQ = 20mA
R4
51Ω
C4P
10
45 50
3 B
R4P
75Ω
–3V
5
40
180Ω
R2
E
2
0
35
FIGURE 25. Direct-Feedback Amplifier Small-Signal Pulse
Response.
FIGURE 24. Frequency Response Direct-Feedback Amplifier.
VO(V)
25
Time (ns)
5.6pF
VO
f–3dB
±100mV
±300mV
±700mV
±1.4V
±2.5V
351MHz
374MHz
435MHz
460MHz
443MHz
FIGURE 27. Forward Amplifier.
45 50
Time (ns)
SPICE MODELS
FIGURE 26. Direct-Feedback Amplifier Large-Signal Pulse
Response.
Computer simulation using SPICE models is often useful
when analyzing the performance of analog circuits and systems. This is particularly true for video and RF amplifier
circuits, where parasitic capacitance and inductance can have
a major effect on circuit performance. SPICE models are
available from Burr-Brown.
OPA660 OTA FORWARD AMPLIFIER
Output Voltage (dB)
20
15
5Vp-p
10
2.8Vp-p
5
1.4Vp-p
0
0.6Vp-p
–5
–10
0.2Vp-p
–15
–20
–25
–30
100k
1M
10M
100M
1G
Frequency (Hz)
IQ = 20mA R1 = 160Ω R4 = 51Ω
R2 = 180Ω R3 = 56Ω R4p = 75Ω C4p = 5.6pF
FIGURE 28. Frequency Response Forward Amplifier.
®
17
OPA660
FIGURE 29. Evaluation Circuit Silk Screen and Board Layouts.
R5
160Ω
BUF In
5
6
+1
R6
470Ω
BUF Out
R7
56Ω
R2
24Ω
OTA Out
R1
100Ω
OTA In
8
C
3 B
R3
51Ω
OTA
–5V
+5V
RQC
820Ω
1
470pF 470pF
E
2
C1
2.2µF
R4
51Ω
C2
3.3nF
10nF
10nF
2.2µF
2.2µF
1N4007
7
FIGURE 30. Evaluation Circuit Diagram.
®
OPA660
18
4