TZA3036 Ver la hoja de datos (PDF) - Philips Electronics
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TZA3036 Datasheet PDF : 15 Pages
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Philips Semiconductors
TZA3036
SDH/SONET STM1/OC3 transimpedance amplifier
7.2 Automatic gain control
The TZA3036 transimpedance amplifier can handle input currents from 0.18 µA to 1.5 mA
which is equivalent to a dynamic range of 78 dB (electrical equivalent with 39 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 TZA3036 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 in 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
001aad079
3.5
VAGC
(V)
2.5
001aad080
1
1.5
10−1
10−1
1
10
102
103
104
IPIN (µA)
Fig 5. Transimpedance as function of the PIN diode
current
0.5
10−1
1
10
102
103
104
IPIN (µA)
Fig 6. AGC voltage as function of the PIN diode
current
TZA3036_1
Product data sheet
Rev. 01 — 24 March 2006
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
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