Datasheet
65
70
75
80
85
10 k 100 k 1 M 10 M 1 G
f − Frequency − Hz
Transimpedance Gain − dB
V
S
= ±15 V
R
L
= 1 k
R
F
= 10 k
C
S
= 100 PF
C
F
= 3 PF
C
S
= 18 PF
C
F
= 2 PF
C
S
= 47 PF
C
F
= 2.2 PF
50 W 50 W
R
S
V
S
C1
C2
I
O
Network Analizer
I
O
V
S
(s) +
1
2R
S
ǒ
1 )
C1
C2
Ǔ
(Above the Pole Frequency)
I
O
V
S
(s) +
s
2R
S
ǒ
1)
C1
C2
Ǔ
s )
1
2 R
S
ǒ
C1)C2
Ǔ
85
90
95
100
105
10 k 100 k 1 M 10 M 1 G
f − Frequency − Hz
Transimpedance Gain − dB
V
S
= ±15 V
R
L
= 1 k
R
F
= 100 k
C
S
= 47 PF
C
F
= 0.7 PF
C
S
= 100 PF
C
F
= 1 PF
C
S
= 18 PF
C
F
= 0.5 PF
1
2 R
S
(
C1 ) C2
)
1
2 R
S
ǒ
1 )
C1
C2
Ǔ
95
100
110
120
10 k 100 k 1 M 10 M
f − Frequency − Hz
Transimpedance Gain − dB
125
V
S
= ±15 V
R
L
= 1 k
R
F
= 1 M
115
105
C
S
= 18 PF
C
F
= 0 PF
C
S
= 47 PF
C
F
= 0.2 PF
C
S
= 100 PF
C
F
= 0.2 PF
Z
O
(s) +
C1 ) C2
C1 C2
ȧ
ȱ
Ȳ
s )
1
2R
S
ǒ
C1)C2
Ǔ
s
ǒ
s )
1
2 R
S
C1
Ǔ
ȧ
ȳ
ȴ
Z
O
[
1
sC2
THS4631
www.ti.com
SLOS451B –DECEMBER 2004– REVISED AUGUST 2011
Table 1. Transimpedance Performance Summary is difficult to measure the frequency response with
for Various Configurations (continued) traditional laboratory equipment because the circuit
requires a current as an input rather than a voltage.
10-kΩ TRANSIMPEDANCE RESPONSES
Also, the capacitance of the current source has a
direct effect on the frequency response. A simple
interface circuit can be used to emulate a capacitive
current source with a network analyzer. With this
circuit, trans- impedance bandwidth measurements
are simplified, making amplifier evaluation easier and
faster.
Figure 35.
A. The interface network creates a capacitive,
100-kΩ TRANSIMPEDANCE RESPONSES
constant current source from a network
analyzer and properly terminates the
network analyzer at high frequencies.
Figure 38. Emulating a Capacitive Current Source
With a Network Analyzer
The transconductance transfer function of the
interface circuit is:
(3)
The transfer function contains a zero at dc and a pole
Figure 36.
1-MΩ TRANSIMPEDANCE RESPONSES
at: . The transconductance is constant
at: , 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:
Figure 37.
(4)
Assuming C1 >> C2, the equation reduces to:
MEASURING TRANSIMPEDANCE
BANDWIDTH
, giving the appearance of a capacitive
While there is no substitute for measuring the
source at a higher frequency.
performance of a particular circuit under the exact
conditions that are used in the application, the
Capacitor values should be chosen to satisfy two
complete system environment often makes
requirements. First, C2 represents the anticipated
measurements harder. For transimpedance circuits, it
capacitance of the true source. Second C1 is chosen
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