Philips SA5211 Manuel D’Utilisation

9397 750 07427

© Philips Electronics N.V. 2001. All rights reserved.

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

µ

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

3

 is approximately the value of the feedback resistor, R

F

= 14.4 k

Ω

. The

gain from the second stage (A2) and emitter followers (A3 and A4) is about two.
Therefore, the differential transresistance of the entire amplifier, R

T

 is

(1)

The single-ended transresistance of the amplifier is typically 14.4 k

Ω

.

The simplified schematic in

 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

F

. The transistor Q1 provides most

of the open loop gain of the circuit, A

VOL

≈

70. The emitter follower Q

2

 minimizes

loading on Q

1

. The transistor Q

4

, resistor R

7

, and V

B1

 provide level shifting and

interface with the Q

15

 – Q

16

 differential pair of the second stage which is biased with

an internal reference, V

B2

. The differential outputs are derived from emitter followers

0

2

4

6

8

10

12

14

16

18

20

(ns)

V

CC

= 5V

T

A

= 25°C

20mV/Div

Output Step Response

R

T

V

OUT

diff

(

)

I

IN

-----------------------------

2 R

F

2 14.4 K

(

)

28.8 k

Ω

=

=

=

=