Theory Of Operation - Philips SA5211 Product Specification

Philips Semiconductors
Fig 10. Typical performance characteristics. (cont.)

11. Theory of operation

9397 750 07427
Product specification
Output Step Response
Transimpedance amplifiers have been widely used as the preamplifier in fiber-optic
receivers. The SA5211 is a wide bandwidth (typically 180 MHz) 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
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 is approximately the value of the feedback resistor, R
3
gain from the second stage (A2) and emitter followers (A3 and A4) is about two.
Therefore, the differential transresistance of the entire amplifier, R
V diff
OUT
R = ---------------------------- - = 2 R
T
I
IN
The single-ended transresistance of the amplifier is typically 14.4 k .
The simplified schematic in
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
loading on Q . The transistor Q
1
interface with the Q – Q
15
an internal reference, V
Rev. 03 — 07 October 1998
Transimpedance amplifier (180 MHz)
= 2 14.4 K = 28.8 k
F
Figure 12 shows how an input current is converted to a
70. The emitter follower Q
VOL
, resistor R , and V
4 7
differential pair of the second stage which is biased with
16
. The differential outputs are derived from emitter followers
B2
SA5211
°
= 14.4 k . The
F
is
T
. The transistor Q1 provides most
F
minimizes
2
provide level shifting and
B1
© Philips Electronics N.V. 2001. All rights reserved.
Figure 11.
(1)
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