Amplifier Circuit With Non-Linear Load Resistance Which Increases Amplifier Forward Gain at High Frequency

This invention relates to amplifiers and in particular but not exclusively to receiver amplifiers for optical transmission systems.

In optical transmission systems, and especially in submarine systems where access for maintenance is often difficult and costly, it is necessary to utilise reliable life tested components. It is also a general aim to optimise bandwidths and minimise noise for a given bit rate, and in general these factors are limited by the performance of the amplifier in the receiver. The bandwidths can be extended by reducing the feedback resistor in the amplifier but this increases the noise spectral density which leads to a reduction in signal to noise ratio. It is desirable to achieve the bandwidth extension but without increasing the noise spectral density.

Also, having established a particular operation bandwidth it is desirable to maintain as closely as possible the frequency response of the amplifier despite fluctuations in supply voltage, supply rail inductance and temperature.

The present invention is directed towards overcoming the problem of extending the bandwidth for minimum increase in noise and towards maintaining the frequency response.

Accordingly the present invention provides a transimpedance amplifier comprising an input stage constituted by at least one bipolar transistor having a load impedance, the amplifier having means for increasing the cut off frequency of the amplifier, said means comprising a compensating active element having a frequency dependent impedance and connected to increase the load impedance as the current gain of the input stage decreases.

A preferred embodiment of the invention comprises a transimpedance, d.c. coupled amplifier comprising an input stage constituted by at least one bipolar transistor connected in common emitter configuration with a collector load resistor, and a transistor in series with the load resistor having a frequency response selected to increase the forward gain of the amplifier at the upper end of the bandwidth of the amplifier.

The invention is now described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a first embodiment of a transimpedance preamplifier for an optical receiver;

FIG. 2 illustrates a second embodiment similar to that of FIG. 1.

FIG. 3 illustrates a third embodiment of the invention with a further modification for improving frequency response,

FIG. 4 shows a fourth embodiment of the invention in which induced supply rail ripple is reduced.

FIG. 1 illustrates a transimpedance amplifier which is primarily intended for use as a preamplifier stage of an optical receiver. Normally, a photodiode, which would be optically coupled to an optical transmission line, would be connected to an input terminal 1 of the preamplifier. The numerals without lead lines refer to exemplary resistor values.

In this embodiment the amplifier comprises a cascode stage 2, constituted by bipolar transistors 3 and 4, and a common collector, or emitter follower transistor 5. The transistor 5 has an emitter load resistor 6; a resistive feedback network 7 provides shunt voltage feedback to the input terminal 1. The emitter follower stage is followed by an output buffer stage 8 having output terminals 9 and 10 connected to the emitter and collector respectively of the bipolar transistor 11, which has an emitter resistor 12 connected to a negative rail 13, at minus 6 volts, and a collector resistor 14 connected to a zero volts rail 15. Outputs may be taken from either output terminal or from both, and when outputs are taken from both output terminals they may either be used independently or in balanced mode.

A bias network 17 for the cascode stage comprises a chain of diodes, in this example four diodes constituted by transistors which are similar to those in the cascode and common emitter stages and of which each has a shorting connection between base and collector terminals. Thus each diode 18 provides an emitter base voltage comparable with those of transistors 3, 23, 5 and 21 and therefore compensates thermal drift in those transistors. In addition, the cascode stage voltage is derived from the emitter of transistor 21 via the diodes 18 making the supply less dependent on the supply rail voltage. The chain of diodes is in series with resistors 19 and 20, to the junction of which is connected the base of a transistor 21 of which the collector emitter path is in series between the zero volts rail 15 and the collector load resistor 16 of the cascode stage. An additional resistor 22 may be interposed between resistor 20 and the rail 15; otherwise resistor 20 may be directly connected to the rail 15. On a chip resistors 20 and 22 will be provided with a facility to bond out resistor 22 which gives the facility for on chip tuning of the frequency response.

Bias for the common collector stage 5 is provided by the transistors of the cascode stage. The diode connected transistor 23 in the emitter circuit of transistor 5 is a dc shifting diode to enhance the dynamic range of the circuit.

The first stage gain is given by load impedance divided by effective emitter impedance, where effective emitter impedance equals actual emitter impedance plus base impedance divided by .beta. (the common emitter current gain), and with increasing frequency the effective emitter impedance of transistor 3 increases which would normally cause the open loop gain to fall off with increasing frequency.

In order to compensate for the increase in effective emitter impedance of transistor 3 load is provided with a component (transistor 21) selected to have a frequency response that causes the load impedance to increase at the same time as the increase in effective emitter impedance of transistor 3. With transistor 21 providing a specific, compensating frequency dependent component of the load the total load is given by load resistance 16+ base impedance divided by .beta. of transistor 21 and the second part of this expression increases as .beta. decreases at higher frequencies. A further resistor may be placed in series with the base of transistor 21 in order to enhance the effect of the decreasing .beta. (this is shown with resistor 45 in FIG. 4). In this way it is possible to maintain the overall first stage gain into the range of frequencies where fall off would otherwise occur due to the increasing effective emitter impedance of the input transistor 3. The transistor 21 also compensates for phase changes in transistor 3 at higher frequencies by virtue of undergoing similar phase changes.

The values of the resistors for the circuit depends on the parameters of the transistors and other components including parasitic capacitances, and for accurate component value selection computer modelling techniques will usually be required. Fine tuning of the frequency response is provided as mentioned above by bonding out selected resistors.

FIG. 2 illustrates a circuit which is generally similar to that of FIG. 1 but the impedance of the feedback loop is reduced to approximately 4k ohms, and the bias resistors 19, 20 and 22 have different values. For a given set of transistors the circuit of FIG. 2 will operate at a higher bit rate than the circuit of FIG. 1.

FIG. 3 illustrates a circuit which has a cascode stage and an emitter follower stage similar to that shown in FIG. 1, but also includes emitter follower buffers to lower the output impedance. The output channels are 9 and 34, transistor 27 becoming essentially a phase inverter with emitter load resistor 28 and collector load resistor 29, and a common collector stage 30 comprising two parallel transistors 31 and 32 with an emitter load resistor 33. The output terminal 34 is connected between the load resistor 33 and the conjoint emitters of transistors 31 and 32. In the circuit of FIG. 3 the buffer stage 26 includes a transistor 35 in series with the collector load; the base of this transistor is biased by a resistor 36 connected to the base of transistor 21. Transistor 35 is an active load and compensates high frequency variations in transistor 27 in a way analogous to the compensation of variations in transistor 3 by transistor 21. Resistor 36 enhances this high frequency compensation by virtue of increasing the effective emitter impedance at high frequencies.

The output buffer stage 8 of FIG. 3 also has two additional capacitance compensation terminals 24 and 37. Signal dependent supply current fluctuation will induce voltage ripple on the supply rails 13 and 15 due to inductance, and this can induce ripple into the amplifier input via various stray capacitances, for example by the capacitance between rail 15 and the input pad 1. Any such ripple will be amplified along with the signal and causes distortion. The negative gain of the amplifier causes this amplified ripple at node 40 to be in antiphase to the supply rail ripple. The Miller capacitance of transistor 11 also produces ripple at node 40 from the supply rail ripple and the amplitude of this component can be adjusted by varying the load resistor of transistor 11 to cancel out the amplified antiphase ripple from the input. Fine tuning of the amplitude of the ripple can be provided by bonding out either resistor 25 or resistor 14. It will be noted that in the circuits of FIGS. 1 and 2 transistor 11 will also produce compensating ripple that will go some way towards cancelling the amplified ripple in those circuits, however the arrangements in FIGS. 1 and 2 do not permit tuning of the amplitude of the compensating ripple.

FIG. 4 illustrates a further embodiment of the invention in which signal induced supply rail ripple is substantially prevented by connecting transistors 46,47, 48 and 49 as constant current sources. This circuit may be regarded as a circuit similar to that of FIG. 1 floated within a circuit that pins the supply currents.

Although the invention has been described with reference to a d.c. coupled amplifier, an analogous a.c. coupled amplifier is contemplated.