OPA861
www.ti.com
SBOS338–AUGUST 2005
The forward amplifier shown in Figure 30 and Fig-
ure 31 corresponds to one of the basic circuits used
V+
to
characterize
the
OPA861.
Extended
characterization of this topology appears in the Typi-
cal Characteristics section of this datasheet.
RS
RL
VO
VO
VI
Inverting Gain
VOS = Several Volts
8
R1
RC
C
Ω
160
Ω
500
RS
RE
3
B
OPA861
VI
E
2
−
V
G = 5V/V
IQ = 5.4mA
RE
(a) Transistor Common−Emitter Amplifier
Transconductance varies over temperature.
Ω
78
8
VO
Figure 30. Forward Amplifier Configuration and
Test Circuit
C
Ω
100
3
B
RL
OPA861
VI
E
2
RL1
Noninverting Gain
VOS = 0V
RE
VO
Network
Analyzer
8
3
(b) OTA Common−E Amplifier
Transconductance remains constant over temperature.
RIN
OPA861
Ω
50
R1
100
RL2
rE
Ω
Figure 29. Common-Emitter vs Common-E
Amplifier
2
VI
RL = RL1 + RL2 || RIN
RE
The transconductance of the OTA with degeneration
can be calculated by Equation 2:
1
gm_deg
+
1
g
RL
RE ) rE
1
gm
m ) RE
G +
rE +
(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 Fig-
ure 29a is inverting. Figure 29b 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
output current.
1
At IQ + 5.4mA
rE +
+ 10.5W
95mAńV
RL
G +
at IQ + 5.4mA
RE ) 10.5W
Figure 31. Forward Amplifier Design Equations
14
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