SA5211D PHILIPS [NXP Semiconductors], SA5211D Datasheet - Page 13

• Manufacturer Part Number: SA5211D
• Description: Transimpedance amplifier 180MHz
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
TYPICAL PERFORMANCE CHARACTERISTICS
THEORY OF OPERATION
Transimpedance amplifiers have been widely used as the
preamplifier in fiber-optic receivers. The SA5211 is a wide bandwidth
(typically 180MHz) transimpedance amplifier designed primarily for
input currents requiring a large dynamic range, such as those
produced by a laser diode. The maximum input current before
output stage clipping occurs at typically 50 A. The SA5211 is a
bipolar transimpedance amplifier which is current driven at the input
and generates a differential voltage signal at the outputs. The
forward transfer function is therefore a ratio of the differential output
voltage to a given input current with the dimensions of ohms. The
main feature of this amplifier is a wideband, low-noise input stage
which is desensitized to photodiode capacitance variations. When
connected to a photodiode of a few picoFarads, the frequency
response will not be degraded significantly. Except for the input
stage, the entire signal path is differential to provide improved
power-supply rejection and ease of interface to ECL type circuitry. A
block diagram of the circuit is shown in Figure 11. The input stage
(A1) employs shunt-series feedback to stabilize the current gain of
the amplifier. The transresistance of the amplifier from the current
source to the emitter of Q
feedback resistor, R
and emitter followers (A3 and A4) is about two. Therefore, the
differential transresistance of the entire amplifier, R
The single-ended transresistance of the amplifier is typically 14.4k .
The simplified schematic in Figure 12 shows how an input current is
converted to a differential output voltage. The amplifier has a
single input for current which is referenced to Ground 1. An input
current from a laser diode, for example, will be converted into a
voltage by the feedback resistor R
of the open loop gain of the circuit, A
minimizes loading on Q
provide level shifting and interface with the Q
pair of the second stage which is biased with an internal reference,
V
Q
1998 Oct 07
R
B2
12
T
Transimpedance amplifier (180MHz)
. The differential outputs are derived from emitter followers Q
+
which are biased by constant current sources. The collectors of
V
OUT
I
IN
(diff)
+ 2R
F
=14.4k . The gain from the second stage (A2)
1
. The transistor Q
F
3
is approximately the value of the
0
+ 2(14.4K) + 28.8kW
F
. The transistor Q1 provides most
2
VOL
70. The emitter follower Q
4
Figure 10. Typical Performance Characteristics (cont.)
, resistor R
4
15
– Q
T
16
is
6
7
differential
, and V
Output Step Response
8
B1
(Continued)
11
–
2
(ns)
10
13
Q
the feedback to the input stage. The output impedance is about 17
single-ended. For ease of performance evaluation, a 33 resistor is
used in series with each output to match to a 50 test system.
BANDWIDTH CALCULATIONS
The input stage, shown in Figure 13, employs shunt-series feedback
to stabilize the current gain of the amplifier. A simplified analysis can
determine the performance of the amplifier. The equivalent input
capacitance, C
7.5pF, assuming that C
capacitance.
Since the input is driven by a current source the input must have a
low input resistance. The input resistance, R
incremental input voltage, V
and can be calculated as:
More exact calculations would yield a higher value of 200 .
Thus C
Assuming typical values for R
The operating point of Q1, Figure 12, has been optimized for the
lowest current noise without introducing a second dominant pole in
the pass-band. All poles associated with subsequent stages have
been kept at sufficiently high enough frequencies to yield an overall
single pole response. Although wider bandwidths have been
achieved by using a cascade input stage configuration, the present
solution has the advantage of a very uniform, highly desensitized
frequency response because the Miller effect dominates over the
external photodiode and stray capacitances. For example, assuming
a source capacitance of 1pF, input stage voltage gain of 70, R
R
11
IN
12
– Q
+
f
*3dB
IN
12
V
I
IN
and R
IN
are bonded to an external pin, V
14
f
+
*3dB
+
IN
IN
2p 4pF 200W
1 ) A
, in parallel with the source, I
will form the dominant pole of the entire amplifier;
+
16
R
2p R
1
F
S
VOL
=0 where C
V
T
20mV/Div
A
CC
1
IN
= 25 C
IN
= 5V
+ 14.4K
18
, to the corresponding input current, I
F
C
= 14.4k , R
+ 200MHz
SD00334
IN
71
S
20
is the external source
+ 203W
CC2
IN
IN
S
= 200 , C
, is the ratio of the
, in order to reduce
, is approximately
Product specification
SA5211
IN
= 4pF
IN
=
IN