TZA3023 View Datasheet(PDF) - Philips Electronics

Part Name

Description

Manufacturer

TZA3023

SDH/SONET STM4/OC12 transimpedance amplifier

Philips Electronics 

Philips Semiconductors

SDH/SONET STM4/OC12

transimpedance amplifier

Product specification

TZA3023

AGC

TZA3023 transimpedance amplifier can handle input

currents from 0.5 µA to 1.5 mA. This means a dynamic

range of 72 dB. At low input currents, the transimpedance

must be high to get enough output voltage, and the noise

should be low enough to guaranty minimum bit error rate.

At high input currents however, the transimpedance

should be low to avoid pulse width distortion. This means

that the gain of the amplifier has to vary depending on the

input signal level to handle such a wide dynamic range.

This is achieved in the TZA3023 by implementing an

Automatic Gain Control (AGC) loop.

The AGC loop consists of a peak detector, a hold capacitor

and a gain control circuit. The peak amplitude of the signal

is detected by the peak detector and it is stored on the hold

capacitor. The voltage over the hold capacitor is compared

to a threshold level. The threshold level is set to

10 µA (p-p) input current. AGC becomes active only for

input signals larger than the threshold level.

It is disabled for smaller signals. The transimpedance is

then at its maximum value (21 kΩ differential).

When the AGC is active, the feedback resistor of the

transimpedance amplifier is reduced to keep the output

voltage constant. The transimpedance is regulated from

21 kΩ at low currents (I < 10 µA) to 800 Ω at high currents

(I < 500 µA). Above 500 µA the transimpedance is at its

minimum and can not be reduced further but the front-end

remains linear until input currents of 1.5 mA.

The upper part of Fig.7 shows the output voltages of the

TZA3023 (OUT and OUTQ) as a function of the DC input

current. In the lower part, the difference of both voltages is

shown. It can be seen from the figure that the output

changes linearly up to 10 µA input current where AGC

becomes active. From this point on, AGC tries to keep the

differential output voltage constant around 200 mV for

medium range input currents (input currents <200 µA).

The AGC can not regulate any more above 600 µA input

current, and the output voltage rises again with the input

current.

handboVoko, 1fu.8ll pagewidth

(V)

1.6

1.4

1.2

1

600

Vo(dif)

(mV)

400

200

0

1

Vo(dif) = VOUT − VOUTQ.

(1) VCC = 3 V.

(2) VCC = 3.3 V.

(3) VCC = 5 V.

2000 Mar 29

VOUT

MCD914

VCC = 3 V

VOUTQ

(1)

(2)

(3)

10

102

103

104

Ii (µA)

Fig.7 AGC characteristics.

6

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