NE5210 データシートの表示（PDF） - Philips Electronics
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NE5210 Datasheet PDF : 14 Pages
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Philips Semiconductors
Transimpedance amplifier (280MHz)
Product specification
NE5210
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 240µA. 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 Q3 is approximately the value of the
feedback resistor, RF=3.6kΩ. The gain from the second stage (A2)
and emitter followers (A3 and A4) is about two. Therefore, the
differential transresistance of the entire amplifier, RT is
RT
+
VOUT(diff)
IIN
+ 2RF
+ 2(3.6K)
+ 7.2kW
The single-ended transresistance of the amplifier is typically 3.6kΩ.
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 RF. The transistor Q1 provides most of the
open loop gain of the circuit, AVOL≈70. The emitter follower Q2
minimizes loading on Q1. The transistor Q4, resistor R7, and VB1
provide level shifting and interface with the Q15 – Q16 differential
pair of the second stage which is biased with an internal reference,
VB2. The differential outputs are derived from emitter followers Q11 –
Q12 which are biased by constant current sources. The collectors of
Q11 – Q12 are bonded to an external pin, VCC2, in order to reduce
the feedback to the input stage. The output impedance is about 17Ω
single-ended. For ease of performance evaluation, a 33Ω resistor is
used in series with each output to match to a 50Ω test system.
OUTPUT +
A3
INPUT
A1
A2
RF
A4
Figure 1. NE5210 – Block Diagram
OUTPUT –
SD00327
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, CIN, in
parallel with the source, IS, is approximately 7.5pF, assuming that
CS=0 where CS is the external source capacitance.
Since the input is driven by a current source the input must have a
low input resistance. The input resistance, RIN, is the ratio of the
incremental input voltage, VIN, to the corresponding input current, IIN
and can be calculated as:
RIN
+
VIN
IIN
+
1
RF
) AVOL
+
3.6K
71
+ 51W
More exact calculations would yield a higher value of 60Ω.
Thus CIN and RIN will form the dominant pole of the entire amplifier;
f*3dB + 2p
1
RIN CIN
Assuming typical values for RF = 3.6kΩ, RIN = 60Ω, CIN = 7.5pF
f*3dB + 2p
1
7.5pF 60
+ 354MHz
VCC1
R1
R3
R12
VCC2
R13
INPUT
Q2
Q4
Q1
Q3
+
PHOTODIODE
R2
GND1
Q15
R14
R7
R5
R4
Q16
R15
VB2
GND2
Q11
Q12
OUT–
+
OUT+
Figure 2. Transimpedance Amplifier
SD00328
1995 Apr 26
11
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