Philips SA5211 11. Theory of operation

Philips Semiconductors SA5211

Transimpedance ampliﬁer (180 MHz)

Product speciﬁcation Rev. 03 — 07 October 1998 13 of 28

11. Theory of operation

Transimpedance ampliﬁers have been widely used as the preampliﬁer in ﬁber-optic

receivers. The SA5211 is a wide bandwidth (typically 180 MHz) 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, RF= 14.4 kΩ. The

gain from the second stage (A2) and emitter followers (A3 and A4) is about two.

Therefore, the differential transresistance of the entire ampliﬁer, RT is

(1)

The single-ended transresistance of the ampliﬁer is typically 14.4 kΩ.

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 RF. The transistor Q1 provides most

of the open loop gain of the circuit, AVOL≈70. The emitter follower Q2 minimizes

loading on Q1. The transistor Q4, resistor R7, and VB1 provide level shifting and

interface with the Q15 – Q16 differential pair of the second stage which is biased with

an internal reference, VB2. The differential outputs are derived from emitter followers

Fig 10. Typical performance characteristics. (cont.)

VOUT diff()

IIN

----------------------------- 2RF2 14.4 K()28.8 kΩ====