Datasheet

G=
AtI =5.4mA
Q
G=atI =5.4mA
Q
V
I
V
O
3
2
8
R
E
r
E
R
L2
R
1
100W
R
IN
50W
R =R +R ||R
L L1 L2 IN
OTA
R
L1
Network
Analyzer
R
L
R +r
E E
R
L
R +10.5W
E
r ==10.5W
E
1
95mA/V
1
g
m
r =
E
g
m_deg
+
1
1
g
m
) R
E
100
Ω
V
I
V+
V
−
V
I
V
O
3 B
2
E
C
8
R
S
R
S
R
L
R
E
V
O
R
E
R
L
Inverting Gain
V
OS
= Several Volts
Noninverting Gain
V
OS
= 0V
(a) Transistor Common−Emitter Amplifier
Transconductance varies over temperature.
(b) OTA Common−E Amplifier
Transconductance remains constant over temperature.
OPA861
R
1
160
Ω
V
I
V
O
3 B
2
E
C
8
R
E
78
Ω
R
C
500
Ω
G = 5V/V
I
Q
= 5.4mA
OPA861
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013 www.ti.com
The forward amplifier shown in Figure 33 and
Figure 34 corresponds to one of the basic circuits
used to characterize the OPA861. Extended
characterization of this topology appears in the
Typical Characteristics section of this datasheet.
Figure 33. Forward Amplifier Configuration and
Test Circuit
Figure 32. Common-Emitter vs Common-E
Amplifier
The transconductance of the OTA with degeneration
can be calculated by Equation 2:
(2)
A positive voltage at the B-input, pin 3, causes a
positive current to flow out of the C-input, pin 8. This
gives a noninverting gain where the circuit of
Figure 32a is inverting. Figure 32b shows an amplifier
connection of the OPA861, the equivalent of a
common-emitter transistor amplifier. Input and output
can be ground-referenced without any biasing. The
amplifier is non-inverting because of the sense of the
Figure 34. Forward Amplifier Design Equations
output current.
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