THS4601 Datasheet, PDF (16/27 Pages) Texas Instruments – WIDEBAND, FET-INPUT OPERATIONAL AMPLIFIER

THS4601
SLOS388B – OCTOBER 2001 – REVISED JUNE 2002
APPLICATION INFORMATION
measuring transimpedance bandwidth (continued)
Network Analyzer
50 Ω
VS
50 Ω
RS
IO
C2
C1
ǒ Ǔ ^IO
VS
+
2R_S
1
1
)
C₁
C₂
(above the pole frequency)
NOTE: This interface network creates a capacitive, constant current source from a network analyzer and properly terminates the network analyzer
at high frequencies.
Figure 32. Emulating a Capacitive Current Source With a Network Analyzer
The transconductance transfer function of the interface circuit is
s
ǒ Ǔ _IO
Vs
(s)
+
2R_S
1)^C1
C₂
.
s)
1
2R_SǒC₁ ) C₂Ǔ
This transfer
function
contains
a
zero
at
DC
and
a
pole
at
s+
1
2R_S^ǒC₁ )
C₂^{Ǔ .}
The
transconductance
is
1
ǒ Ǔ constant at
2R_S
1
)
C₁
C₂
, above the pole frequency, providing a controllable AC current source. This circuit
also properly terminates the network analyzer with 50 Ω at high frequencies. The second requirement for this
current source is to provide the desired output impedance, emulating the output impedance of a photodiode or
other current source. The output impedance of this circuit is given by
ǒ Ǔ ^{ȧȧȧȱȲ ȳȴȧȧȧ ZO(s)}
+
C₁ ) C₂
C₁C₂
s
)
1
2R_sǒC₁
)
C₂Ǔ
s
s)
1
2R_sC₁
.
Assuming
C₁
>>
C₂,
the
equation
reduces
to
Z_O
[
1
sC₂
,
giving
the
appearance
of
a
capacitive
source
at
higher
frequency.
Capacitor values should be chosen to satisfy two requirements. First, C₂ should represent the anticipated
capacitance of the true source. C₁ should then be chosen such that the corner frequency of the
transconductance network is much less than the transimpedance bandwidth of the circuit. Choosing this corner
frequency properly leads to more accurate measurements of the transimpedance bandwidth. If the interface
circuit’s corner frequency is too close to the bandwidth of the circuit, determining the power level in the flatband
is difficult. A decade or more of flat bandwidth provides a good basis for determining the proper transimpedance
bandwidth.
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