Philips Semiconductors

Product specification

SA5212A

Transimpedance amplifier (140MHz)

1998 Oct 07

11

THEORY OF OPERATION

Transimpedance amplifiers have been widely used as the

preamplifier in fiber-optic receivers. The SA5212A is a wide

bandwidth (typically 140MHz) 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

µA. The SA5212A 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 10. The input stage

(A1) employs shunt-series feedback to stabilize the current gain of

the amplifier. The transresistance of the amplifier from the current

and emitter followers (A3 and A4) is about two. Therefore, the

R

V

I

IN

+ 2R

The single-ended transresistance of the amplifier is typically 7.2k

Ω.

The simplified schematic in Figure 11 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

pair of the second stage which is biased with an internal reference,

the feedback to the input stage. The output impedance is about 17

Ω

single-ended. For ease of performance evaluation, a 33

Ω resistor is

used in series with each output to match to a 50

Ω test system.

BANDWIDTH CALCULATIONS

The input stage, shown in Figure 12, 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.

Since the input is driven by a current source the input must have a

and can be calculated as:

R

V

IN

I

IN

+

R

F

1

) A

VOL

+ 7.2K

70

+ 103W

More exact calculations would yield a higher value of 110

Ω.

1

2

p R

1

2

+ 145MHz

The operating point of Q1, Figure 2, 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

60

lead to only a 12% bandwidth reduction.

INPUT

OUTPUT +

OUTPUT –

A1

A2

A3

A4

RF

SD00327

Figure 10. SA5212A – Block Diagram

NOISE

Most of the currently installed fiber-optic systems use non-coherent

transmission and detect incident optical power. Therefore, receiver

noise performance becomes very important. The input stage

achieves a low input referred noise current (spectral density) of

3.5pA/

√Hz. The transresistance configuration assures that the

external high value bias resistors often required for photodiode

biasing will not contribute to the total noise system noise. The

bandwidth; however, it is not dependent upon the internal

Miller-capacitance. The measured wideband noise was 52nA RMS

in a 200MHz bandwidth.