OPA660 Burr-Brown, OPA660 Datasheet - Page 11

• Manufacturer Part Number: OPA660
• Description: Wide Bandwidth OPERATIONAL TRANSCONDUCTANCE AMPLIFIER AND BUFFER
• Manufacturer: Burr-Brown

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-refer-
enced 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.
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 ca-
pacitor to maintain high AC gain. Other circumstances may
suggest a smaller value capacitor used to extend or optimize
high-frequency performance.
The transconductance of the OTA with degeneration can be
calculated by—
Figure 6b shows the OTA connected as an E-follower—a
voltage buffer. The buffer formed by this connection per-
forms virtually the same as the buffer section of the OPA660
(the actual signal path is identical).
FIGURE 8. Common-E Amplifier Performance.
100
R
–10
–15
–20
–25
–30
1
20
15
10
–5
V
5
0
300k
I
3
I
Q
= 20mA R
V
1M
O
8
OTA
2
1
g =
R
r
= 100
E
m
E
2.8Vp-p
10M
R
Frequency (Hz)
L1
g
1
G =
G =
At I = 20mA r =
R
m
E
1
= 51
Q
+ R
R + 8
R + r
E
E
R
R
200mVp-p
L
L
E
R
600mVp-p
100M
R
L2
E
R
1.4Vp-p
L
L
, r
= R
at I = 20mA
= 100
E
E
Q
L1
=
125mA/V
+ R
gm
1
–3dB Point
Gain = 2
1
L2
Network
Analyzer
R
50
|| R
1G
IN
IN
= 8
3G
11
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 25
Figure 7 shows the Common-B amplifier. This configura-
tion 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.
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
• Make short, wide interconnection traces to minimize
• Use a large ground plane to assure that a low impedance
• Do not extend the ground plane under high impedance
• Sockets are not recommended because they add signifi-
a combination between tantalum capacitors (approxi-
mately 2.2 F) and polyester capacitors. Surface-mount
types are best because they provide lowest inductance.
series inductance.
ground is available throughout the layout.
nodes sensitive to stray capacitance.
cant inductance.
–10
–15
–20
–25
–30
–10
–15
–20
–25
–30
20
15
10
–5
20
15
10
–5
5
0
5
0
300k
100k
to 200
I
Q
I
Q
= 20mA R
1M
= 20mA R
1M
are typical.
1
1
= 100
2.8Vp-p
= 100
10M
Frequency (Hz)
Frequency (Hz)
5Vp-p
R
R
E
1.4Vp-p
10M
2.8Vp-p
E
= 51
= 51
OPA660
600mVp-p
200mVp-p
200mVp-p
100M
600mVp-p
R
1.4Vp-p
R
L
L
= 500
= 50
100M
–3dB Point
–3dB Point
Gain = 10
Gain = 1
1G
3G
1G
®