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TZA3026 Datasheet PDF : 15 Pages
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Philips Semiconductors
TZA3026
SDH/SONET STM4/OC12 transimpedance amplifier
7.2 Automatic gain control
The TZA3026 transimpedance amplifier can handle input currents from 0.85 µA to 1.5 mA
which is equivalent to a dynamic range of 65 dB (electrical equivalent with 32.5 dB
optical). At low input currents, the transimpedance must be high to obtain enough output
voltage, and the noise should be low enough to guarantee a minimum bit error rate. At
high input currents however, the transimpedance should be low to prevent excessive
distortion at the output stage. To achieve the dynamic range, the gain of the amplifier
depends on the level of the input signal. This is achieved in the TZA3026 by an AGC loop.
The AGC loop comprises a peak detector and a gain control circuit. The peak detector
detects the amplitude of the signal and stores it on a hold capacitor. The hold capacitor
voltage is compared to a threshold voltage. The AGC is only active when the input signal
level is larger than the threshold level and is inactive when the input signal is smaller than
the threshold level.
When the AGC is inactive, the transimpedance is at its maximum. When the AGC is
active, the feedback resistor value of the transimpedance amplifier is reduced, reducing its
transimpedance, to keep the output voltage constant. Figure 5 shows the transimpedance
as function of the input current.
To reduce sensitivity to offsets and output loads, the AGC detector senses the output just
before the output buffer. Figure 6 shows the AGC voltage as function of the input current.
102
transimpedance
(kΩ)
10
1
001aac621
4
VAGC
(V)
3
2
1
001aac622
10−1
1
10
102
103
104
IPD (µA)
Fig 5. Transimpedance as function of the input
current
0
1
10
102
103
104
IPD (µA)
Fig 6. AGC voltage as function of the input current
9397 750 14763
Product data sheet
Rev. 01 — 2 May 2005
© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
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