NE5210 Philips, NE5210 Datasheet - Page 11

• Manufacturer Part Number: NE5210
• Description: Transimpedance amplifier 280MHz
• Manufacturer: Philips

Philips Semiconductors
THEORY OF OPERATION
Transimpedance amplifiers have been widely used as the
preamplifier in fiber-optic receivers. The NE5210 is a wide
bandwidth (typically 280MHz) 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 240 A. The NE5210 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 1. 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 3.6k .
The simplified schematic in Figure 2 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
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
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.
1995 Apr 26
R
B2
12
11
T
Transimpedance amplifier (280MHz)
. The differential outputs are derived from emitter followers Q
– Q
which are biased by constant current sources. The collectors of
V
12
OUT
are bonded to an external pin, V
I
IN
(diff)
F
=3.6k . The gain from the second stage (A2)
2R
F
1
. The transistor Q1 provides most of the
. The transistor Q
F
3
is approximately the value of the
PHOTODIODE
VOL
2(3.6K)
70. The emitter follower Q
4
INPUT
7.2k
CC2
, resistor R
15
, in order to reduce
– Q
GND
T
16
is
Figure 2. Transimpedance Amplifier
Q
1
R
7
differential
1
1
R
, and V
5
Q
2
R
2
2
Q
B1
3
R
R
11
3
4
–
Q
4
+
R
V
7
11
CC1
Q
15
R
BANDWIDTH CALCULATIONS
The input stage, shown in Figure 3, 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
parallel with the source, I
C
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 60 .
Thus C
Assuming typical values for R
R
12
14
S
R
=0 where C
IN
INPUT
f
IN
GND
3dB
R
V
and R
I
15
IN
Q
2
IN
f
VB2
16
A1
S
3dB
IN
R
Figure 1. NE5210 – Block Diagram
IN
2 7.5pF 60
R
is the external source capacitance.
, in
13
F
1
will form the dominant pole of the entire amplifier;
+
1
R
Q
2 R
11
A
F
S
VOL
, is approximately 7.5pF, assuming that
Q
1
IN
12
IN
, to the corresponding input current, I
F
C
V
OUT–
OUT+
= 3.6k , R
IN
A2
CC2
3.6K
354MHz
71
IN
51
IN
= 60 , C
, is the ratio of the
Product specification
A3
A4
NE5210
IN
= 7.5pF
OUTPUT –
OUTPUT +
SD00328
SD00327
IN