Great Britain 1 GHZ LOW-NOISE HYBRID PREAMPLIFIER FOR OPTICAL DETECTION

A 1 GHz low- noise preamplifier, especially designed for photodetection purposes is described. The amplifier noise 
which sets a limit to the sensitivity of the photodiode-preamplifier combination is minimized using a hybrid technology 
which allows very low values of the parasitics. Results on a complete photodiode-equalized preamplifier are 
given.


INTRODUCTION
The work reported in this communication originates from the need to characterize components for optical fiber telecommunications and to evaluate parameters of future high bit rate optical systems.Only a few results have become available until now on the optimization of avalanche photodiode (APD) pre- amplifier receivers at such high data rates as low-noise 50 2 preamplifiers are usually used.The present optimization of a large bandwidth preamplifier follows the realization, in our laboratory, of other preamplifiers for 34 and 140 Mbit/s systems, for which the interest of using hybrids was clearly established.: , 3 2. CHARACTERISTICS OF A LOW-NOISE OPTICAL PREAMPLIFIER The sensitivity of an optical receiver is limited by the APD noise and the preamplifier noise.Expressed in terms of currents at the preamplifier input, the signal to noise ratio S/N is given as: SIN (MSP) 2 f AF(2qMZ+XSp + I.)df where M, S and x are the multiplication factor (20 to 100), the sensitivity (0.5 A/W)and the excess noise factor (0.3 to 0.5) of the APD, P is the collected optical power, I is the preamplifier noise current density and AF the required bandwidth.
The preamplifier to be associated with a photodiode must be designed so as to amplify the photocurrent while not impairing the sensitivity of the photodetector by a high I value.This subject has been widely studied in recent years. 4In particular it has been shown that, for the larger bandwidth amplifiers which use bipolar transistors, the base current of the first stage must be chosen according to the required bandwidth (AF) and the input capacitance (Ce) to obtain a minimum noise.It then appears that the optimized amplifier noise roughly varies like Ce.This capacitance must be kept as small as possible.
At the optimum current, the input impedance of the preamplifier is usually quite high (>>50 f2) and the APD photocurrent is integrated by the input impedance.To compensate for this integration, one of the two following structures is usually used.
1) The transimpedance structure in which feedback between output and input lowers the  (b) amplifier-equalizer receiver.Req//Ceq is the equalizing cell.increasing the noise significantly (Rf is a high value resistor).
2) The amplifying-differentiating structure, with an equalizer following the preamplifier (Figure lb).This structure appears well suited for the larger bandwidths.Worth noticing however is that equalization by a single RC cell assumes that the input impedance can also be described by a single RC dipole over the whole bandwidth.
The present work was undertaken to investigate the possibility of realizing a large bandwidth optical receiver of the amplifying-differentiating type.The characteristics chosen for the preamplifier are given in Table I. 3. RECEIVER DESIGN

TheHybridTechnology
The choice of a hybrid technology results from the need to reduce both internal propagation times and parasitics (either capacitances or connection length resulting in radiation pick-up).This technology also allows a remarkable reproducibility of the operating characteristics and a good agreement between these characteristics and the results of computer simulations of the circuit.The technology, in itself, is characterized by the following points.Thick films are printed onto an alumina substrate for both conductors and resistors.Active components and capacitors are in chip form.Transistor dies are bonded by Au-Si eutectic formation.Thermo- compressed Au wires are used for the emitter and base connections.

2 The Preamplifier
The preamplifier schematic appears on Figure 2. The first two stages are common, emitter amplifying stages.RC reaction networks in the emitters allow adjustments of the gain flatness throughout the bandwidth while not increasing the noise significantly.
The bias currents are experimentally chosen so as to get the GHz bandwidth together with a minimum noise.A third emitter follower buffer stage gives a low output impedance.The total power consumption is about 16 mA.The typical frequency response of the preamplifiers, as measured with a 50 2 wobulator appears in Figure 3.The low frequency input impedance consists of a resistance Re of kf2 to 1.3 k2 depending upon the value of the resistor in the emitter of T1, in parallel with a capacitance Ce of 4 pF to 5 pF, depending upon the transistor T1.At increasing frequency, the capacitance Ce remains approximately constant while the resistance Re decreases, because of the.decreasing current gain.

The Receiver
Silicon APDs are presently available with both a good sensitivity S in the visible and near infrared and a fast intrinsic transit time (response time typically less than 300 ps FWHM).Such photodiodes have a small junction capacitance Cj of the order of pF.APD chips were hybrided together with their bias circuit on a substrate, which was positioned above the preamplifier substrate.A direct connection between the two was provided by the input capacitor chip.The equalization network was integrated on the preamplifier substrate.The receiver was mounted in a T08 package.A 50 s2 resistor is connected to the input for the measurement. 4. RECEIVER EXPERIMENTAL RESULTS

Response Time of the Receiver
Because of its high input impedance, the preamplifier integrates (Figure 4b) the signal generated by the photodiode (Figure 4a).The RC product inferred from Figure 4b is about 6.3 ns, in close agreement with the measured values of Re and Ce + Cj.The equalization network (Req, Ceq) allows one to obtain, at the receiver output, a response time " of about 500 ps FWHM, corresponding to the APD and preamplifier bandwidths combination.
Note that the lower cut-off frequency of the receiver is of the order of 30 kHz, much lower than the preamplifier cut-off frequency which is set by the output dipole Cou -50 2.

2 Noise Measurements
The noise density has been measured for amplifiers with and without a 50 2 resistor connected at the input.The measurements were performed at low V m V/div / V 5 mV/div :2oo,/iv I-:oo/iv 7 multiplication factors of the APD, so as to measure the preamplifier noise alone.
For preamplifier without a 50 2 resistor, the noise spectral density slightly increases with frequency.The low frequency value ('-3.10 -23 A2/Hz) is close to the theoretical value (2q IBase).This should be compared to the theoretical value for a 0 dB noise factor preamplifier with a 50 f2 input resistor (4 kT/50 2--" 3.10 .22A2/ttz).At high frequency, the noise density levels tend to become comparable for both types of preamplifiers, because of the lowering input impedances.Sensitivity measurements show a 6 dB noise margin for the high impedance receiver over the 50 f2 receiver.

CONCLUSION
The design and the evaluation of a GHz low-noise optical receiver has shown the need of a hybrid technology.This technology gives reproducible characteristics quite close to the expected ones, and almost negligible parasitics, as evidenced by the agreement we observed between simulated and real characteristics.Presently efforts are continuing to increase the receiver sensitivity by selecting low capacitance transistors.Such low-noise receivers may be of utmost interest in 1.3/m transmission links for which APDs appear to be noisier than silicon ones.
preamplifier input impedance (Figure a) while not

FIGURE
FIGUREReceiver schematic diagram: (a)Transimpedance receiver with feedback resistor Rf.Re//Ce is the input impedance of the preamplifier RL is the load resistor.

FIGURE 3
FIGURE 3 Frequency response of the preamplifier.A 50 s2 resistor is connected to the input for the