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TZA3046_1 © Koninklijke Philips Electronics N.V. 2006. All rights reserved.
Product data sheet Rev. 01 — 19 May 2006 6 of 15
Philips Semiconductors
TZA3046
Fiber Channel/Gigabit Ethernet transimpedance amplifier
7.2 Automatic gain control
The TZA3046 transimpedance amplifier can handle input currents from 2.5 µA to 1.7 mA
which is equivalent to a dynamic range of 56 dB (electrical equivalent with 28 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 TZA3046 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.
Fig 5. Transimpedance as function of the PIN diode
current
Fig 6. AGC voltage as function of the PIN diode
current
001aae515
I
PIN
(µA)
110
4
10
3
10 10
2
1
10
transimpedance
(kΩ)
10
−1
001aae516
1.5
2.5
3.5
V
AGC
(V)
0.5
I
PIN
(µA)
110
4
10
3
10 10
2