SA5211 Datasheet, PDF (13/20 Pages) NXP Semiconductors – Transimpedance amplifier 180MHz

Philips Semiconductors
Transimpedance amplifier (180MHz)
TYPICAL PERFORMANCE CHARACTERISTICS (Continued)
Output Step Response
Product specification
SA5211
V_CC = 5V
T_A = 25°C
20mV/Div
0
2
4
6
8
10
12
14
16
18
20
(ns)
SD00334
Figure 10. Typical Performance Characteristics (cont.)
THEORY OF OPERATION
Transimpedance amplifiers have been widely used as the
preamplifier in fiber-optic receivers. The SA5211 is a wide bandwidth
(typically 180MHz) 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 50µA. The SA5211 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 11. The input stage
(A1) employs shunt-series feedback to stabilize the current gain of
the amplifier. The transresistance of the amplifier from the current
source to the emitter of Q₃ is approximately the value of the
feedback resistor, R_F=14.4kΩ. The gain from the second stage (A2)
and emitter followers (A3 and A4) is about two. Therefore, the
differential transresistance of the entire amplifier, R_T is
R_T
+
V_OUT(diff)
I_IN
+ 2R_F
+ 2(14.4K) + 28.8kW
The single-ended transresistance of the amplifier is typically 14.4kΩ.
The simplified schematic in Figure 12 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 the feedback resistor R_F. The transistor Q1 provides most
of the open loop gain of the circuit, A_VOL≈70. The emitter follower Q₂
minimizes loading on Q₁. The transistor Q₄, resistor R₇, and V_B1
provide level shifting and interface with the Q₁₅ – Q₁₆ differential
pair of the second stage which is biased with an internal reference,
V_B2. The differential outputs are derived from emitter followers Q₁₁ –
Q₁₂ which are biased by constant current sources. The collectors of
Q₁₁ – Q₁₂ are bonded to an external pin, V_CC2, in order to reduce
the feedback to the input stage. The output impedance is about 17Ω
single-ended. For ease of performance evaluation, a 33Ω resistor is
used in series with each output to match to a 50Ω test system.
BANDWIDTH CALCULATIONS
The input stage, shown in Figure 13, 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, C_IN, in parallel with the source, I_S, is approximately
7.5pF, assuming that C_S=0 where C_S is the external source
capacitance.
Since the input is driven by a current source the input must have a
low input resistance. The input resistance, R_IN, is the ratio of the
incremental input voltage, V_IN, to the corresponding input current, I_IN
and can be calculated as:
R_IN
+
V_IN
I_IN
+
1
R_F
) A_VOL
+
14.4K
71
+ 203W
More exact calculations would yield a higher value of 200Ω.
Thus C_IN and R_IN will form the dominant pole of the entire amplifier;
f_*3dB
+
2p
1
R_IN
C_IN
Assuming typical values for R_F = 14.4kΩ, R_IN = 200Ω, C_IN = 4pF
f_*3dB
+
2p
1
4pF
200W
+ 200MHz
The operating point of Q1, Figure 12, 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
a source capacitance of 1pF, input stage voltage gain of 70, R_IN =
1998 Oct 07
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