CA2025817A1 - Switched wide dynamic range - Google Patents

Abstract

SWITCHED WIDE DYNAMIC RANGE AMPLIFIER

Abstract of the Disclosure There is disclosed an amplifier switchable between transimpedance modes and a follower mode of improved dynamic range for use in a receiver of an optical electric circuit. The amplifier circuit includes a photodiode, an amplifier stage, and first and second feedback paths. The amplifier stage has an input connected to the transducer to amplify the electrical signals and has an output. The first, resistive, feedback path is coupled between the output and the input of the amplifier. The second feedback path is responsive to the gain of the electrical signals at the output of the amplifier stage and is selectively coupled and uncoupled across the input and output of the amplifier stage. The second feedback path has a resistance less than the resistance of the first feedback path and has a capacitance coupled to ground from at least a portion of the resistance to determine a mode cross-over frequency.
The amplifier switches its feedback from the first feedback path to the second feedback path to improve the dynamic range of the amplifier.

Description

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SWITC~ED WIDE DYNAMIC RANGE AMPLIFIER
The present invention relates to an amplifier for use in a receiver of an optical electric circuit. More specifically, the present invention relates to an amplifier switchable between transimpedance modes and a follower mode for improving the dynamic range of the amplifier.

BACRGRO~ND OF THE INVENTION
Transimpedance amplifiers are commonly used in the amplification of electrical signals produced by photodiodes. The bandwidth of the amplifier is inversely proportional to the input capacitance and the load resistance of the transducer. It is desirable to reduce the effective input capacitance as well as the load resistance to improve the operating bandwidth of the ampli~ier. However, at low power signals the load resistance should be as high as possible to match the photodiode low noise.
U.S. Patent No. 4,764,732 issued August 16, 1988 to Bruno Dion discloses a switched mode amplifier that operates in a first mode as a transimpedance amplifier 2 ~ 2 c~ ~ ~ J

having good sensitivity over broad bandwidth operation.
This patent teaches switching the mode of operation by adding transducer load impedance to alter the DC bias of the amplifier. This causes the amplifier to switch from 05 a transimpedance mode to a follower mode of operationO
The amplifier disclosed operates well to amplify low power signals detected by the photodiode and does not saturate for high power signals since the amplifier is switched into a follower mode. In the follower mode the sensitivity of the amplifier is reduced and the power level required to saturate the amplifier is increased.
U.S. Patent No. 4,564,818 issued January 14, 1986 to Timothy R. Jones discloses a transimpedance amplifier having a transimpedance feedback path that includes a first and second resistor. This patent discloses a capacitor having one terminal connected between the first and second resistors, and having its other terminal connected to a further resistor which in turn is connected to ground. The patent teaches that the purpose of the capacitor and further resistor is to compensate for parasitic capacitances across the first and second feedback paths that would otherwise effect the gain bandwidth product.
U.S. Patent No. 4,574,249 issued March 4, 1986 to Gareth F. Williams discloses several circuits adapted to control the dynamic range of an amplifier. In particular, Figure 39 illustrates using an FET as a variable shunt where the FET is controlled by an automatic gain control. A feedback current source is also employed.

S~MMARY OF THE INVEN~ION
It is therefor an object of the present invention to provide an amplifier suitable for use with a photodiode or transducer wherein the dynamic range of the amplifier is improved by switching the amplifier between s~ F~

a first transimpedance mode, a second transimpedance mode and a follower mode of operation.
In accordance with a broad aspect of the present invention there is provided an amplifier circuit, switchable between transimpedance modes and a follower mode of operation, for changing the dynamic range of the amplifier. The amplifier circuit includes a transducer, an amplifier stage, and first and second feedback paths.
The transducer is adapted to convert wave energy signals to electrical signals. The amplifier stage has an input connected to the transducer to amplify the electrical signals and an output. The first, resistive, feedback path is coupled between the output and the input of the amplifier. The second feedback path is responsive to the transimpedance gain of the electrical signals at the output of the amplifier stage and is selectively coupled and uncoupled across the input and output of the amplifier stage. The second feedback path has a resistance less than the resistance of the first feedback path and has a capacitance coupled to ground from at least a portion of the resistance to determine a mode cross-over frequency. When the second feedback path is uncoupled, the amplifier operates in a first transimpedance mode where the transducer load impedance is that of the first resistance feedback path. When the second feedback path is coupled in circuit, the transducer load impedance becomes that of the second feedback path and the amplifier operates in its second transimpedance mode for electrical signals having a frequency below the mode cross-over frequency and in a follower mode for electrical signals having a frequency above the mode cross-over frequency.
The amplifier of the present invention operates to switch the transimpedance between a first relatively higher resistance and a second relatively lower .r ~

resistance. The first relatively higher resistance provides a high input transducer load resistance that increases the sensitivity of the amplifier to low power transducer signal levels. When the power level of 5 signals received from the transducer rises above a predetermined power level, the second feedback path dominates the transducer load impedance. By reducing the transducer load impedance to that of the second feedback path, the power range of the amplifier before it saturates is increased thereby increasing the dynamic range of the amplifier. The capacitor functions to open circuit the feedback path for higher frequencies by short circuiting these higher frequencies signals to ground.
This in effect switches the amplifier to a follower mode 15 of operation. Moreover, the capacitor in the s~cond feedback path also functions to stabilize the amplifier by introducing a phase shift to higher frequency signals.
The second feedback path may be coupled in and out of circuit with the first feedback path by means of a switch such as an field effect transistor (FET). The gate of the transistor is connected to an automatic gain control circuit (AGC) which controls the conduction of the FET transistor in response to changes in the transimpedance gain of the amplified signal at the output of the amplifier stage.

BRIEF DE8CRIPTION OF T~E DRAWINGS
For a better understanding of the nature and objects of the present invention reference may be had by way of example to the accompanying diagrammatic drawings in which:
Figure 1 is a schematic circuit drawing of the amplifier circuit of the present invention; and, Figure 2 is a graph of the relative transimpedances of the first and second feedback paths.

Referring to Figure 1, the amplifier circuit of the present invention is shown at 10. The amplifier circuit includes a transducer or photodiode 12 having one 5 terminal connected to a DC biasing potential -V and its other ter~inal connected to the input 14 of the amplifier stage shown within broken box 16.
The amplifier stage comprises an FET transistor 18 having its gate 20 connected to the input 14. The source 22 of the FET is connected to ground while the drain 24 of the FET is connected through biasing resistor 26 to DC
bias potential +V. FET 18 provides a high input impedance.
The drain 24 is connected to the emitter of pnp transistor 28 whose collector is connected to the base of pnp transistor 30. Transistor 28 has its base connected to biasing capacitors 32 and resistors 34 and 36 connected to a +V biasing potential. The gain of the amplifier stage is determined by the transconductance current of FET 18 and the collector resistance 38 of transistor 28. The gain of the amplifier stage lies in the range of 10 to 200 and is preferably about 100.
Transistor 30 is biased by resistor 40 and acts a buffer to provide the output at 42. The output of the amplifier stage is passed through a further buffer transistor 44.
A first feedback path comprising resistor 46 is connected between output 42 and input 14 of the amplifier stage 16. This resistor provides a load impedance to photodiode 12 that effects the bandwidth. This resistor may have a value in the range of 1 kilo-ohm to 10 Mega-ohms and is preferably about 330 kilo-ohms.
A second feedback path shown within broken box 48 is coupled between the output 42 and the input 14 of the amplifier stage 16. The second feedback path 48 includes an FET transistor 50 whose gate 52 is connected to an AGC

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circuit 54. The drain of the FET 58 is connected to the input 14 while the source of the FET is connected to the resistor 60. Resistor 60 is also connected in series with resistor 62 to the output 42 of the amplifier stage.
A capacitor 64 is connected between the resistors 60 and 62 to ground. Resistor 62 has a low value of a few hundred ohms while resistor 60 has a value of a few kilo-ohms. The ratio of resistors 60 and 62 is chosen such that a flat frequency response can be obtained from DC up to the desired amplifier bandwidth (illustrated by F in Figure 2).
It should be understood that the sensitivity and bandwidth requirements of the amplifier are determined by the value of resistor 46 when the second feedback path 48 is uncoupled. The value of resistor 46 (R46) in this preferred embodiment is 330 kilo-ohms which sets the transimpedance gain of the amplifier from dc up to the bandwidth. Referring to Figure 2 this transimpedance is shown by upper line 70 to be approximately the value of R46. In order to obtain a predetermined tran~impedance reduction when the second feedback path 48 is coupled in circuit, a reduction factor of, for example X = -22dB or X = 0.082 is chosen. This results in the second feedback path having a transimpedance chosen to be in the order of 27 kilo-ohms (R46 x 0.082). This reduced transimpedance is shown in Figure 2 by lines 72 and 74. Line 72 is determined by the primarily by the value of resistor 62 (R62) for frequencies below the mode cross-over frequency. (It should be understood that the actual transimpedance will be the sum of r~sistors 62 and 60(R60)in parallel with R46). At frequencies above the mode cross-over frequency, the transimpedance becomes that of R60 times the open gain of the amplifier which is in the order of 100. As a result a relatively flat frequency response along lines 72 and 74 is achieved. In ~ ~3 ! J i ~

the preferred embodiment R60 is chosen to be 270 ohms for a gain in the amplifier stage of 100. It should be understood that the above teachings may only provide an approximation of the values of resistors R46, R60 and R62 and that final adjustments of the component values may be required.
The mode cross-over frequency (illustrated by FMoDE
in Figure 2) is determined by R60 and C64. Capacitor 64 and resistor 62 determine a pole for frequencies above which the feedback signal will be shorted to ground.
Above these frequencies, the amplifier effectively operates in a follower mode. The introduction of capacitor 64 provides a phase compensation that keeps the amplifier stable when the second feedback path 48 is switched in circuit. If a lower feedback resistance is switched without phase compensation (ie: without C64~, then the amplifier would have a tendency to oscillate.
In operation, the photodiode provides electrical signals of varying frequency to the input 14 of the amplifier stage. These signals are amplified by the amplifier stage 16 and taken at the output of transistor 44. For low power signals, the AGC 54 provides a low bias voltage to the drain of FET 50 maintaining YET 50 "off". As a result, the second feedback path 48 is uncoupled from the feedback circuit and the feedback is that of resistor 46. Since the resistance of resistor 46 is relatively high, the load impedance as seen by the diode 12 and its sensitivity to low power signals is enhanced. As the gain of the electrical signals from photodiode 12 increases in the power, at a given power level, the AGC increases its output bias to the gate of FET 50 resulting in FET 50 conducting and coupling the second feedback path 48 in parallel with resistor 46.
Since the resistance of resistors 60 and 62 are considerably less than that of resistor 46, the DC

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- % - GECAN 3058 feedback current passes through the second feedback path 48. As a result, the load transimpedance (Output Voltage/Input Current) of the diode 12 will be that of resistor 60 seen in series with the parallel combination of resistor 62 and capacitor 64. The load transimpedance will be effectively reduced by a factor of 10 allowing the power to be increased before saturation and thereby improving the dynamic range of the amplifier.
An amplifier as described in the preferred embodiment has been tested to have a sensitivity in the first transimpedance mode of -51 dBm of optical power at 25 Megabits per second. When the amplifier is switched to its second transimpedance and follower modes, the saturation power level becomes -4dBm which results in an overall dynamic range of 47 dB.

Claims (3)

1. An amplifier circuit, switchable between transimpedance modes and a follower mode of operation for changing the dynamic range of the amplifier, comprising:
a transducer adapted to convert wave energy signals to electrical signals;
an amplifier stage having an input connected to the transducer to amplify the electrical signals and having an output;
a first, resistive, feedback path coupled between the output and the input of the amplifier;
a second feedback path responsive to the transimpedance gain of the electrical signals and being selectively coupled and uncoupled across the input and output of the amplifier stage, the second feedback path having a resistance less than the resistance of the first feedback path and having a capacitance coupled to ground from at least a portion of the resistance to determine a mode cross-over frequency, when the second feedback path is uncoupled the amplifier operating in a first transimpedance mode where the transducer load impedance is that of the first resistance feedback path and the amplifier is sensitive to transducer noise, when the second feedback path is coupled in circuit the transducer load impedance being that of the second feedback path improving the dynamic range of the amplifier, the amplifier operating in its second transimpedance mode for electrical signals having a frequency below the mode cross-over frequency and operating in a follower mode for electrical signals having a frequency above the mode cross-over frequency.

2. The amplifier of claim 1 wherein the second feedback path includes a series connected switch means, a first resistor and a second resistor, and a capacitor coupled between the first and second resistors to ground, the first and second resistors having a resistance considerably less than the resistance of the first feedback path, and the second resistor and capacitor chosen to determine the mode cross-over frequency.

3. An amplifier circuit, switchable between transimpedance modes and a follower mode of operation for changing the dynamic range of the amplifier comprising:
a detector adapted to convert wave energy signals to electrical signals;
a first FET transistor having its gate input connected to the transducer and its drain electrode connected to an amplifier for amplifying the electrical signals;
a first feedback resistor connected between the output of the amplifier and the input of the first FET
transistor;
second and third feedback resistors coupled in series from the output of the amplifier to the source of a second FET transistor, the drain of the second FET
transistor being connected to the gate of the first FET
transistor, and the gate of the second FET transistor being responsive to an automatic gain control signal representative of the amplified electrical signals, and a capacitor coupled between the second and third resistors to ground, the second and third resistors having a resistance considerably less than the resistance of the first resistor, and the third resistor and capacitor being chosen to determine a mode cross-over frequency;
and, when the second FET transistor is open, the feedback resistance being that of the first resistor and the amplifier operating in a first transimpedance mode where the transducer load impedance is that of the first resistance feedback path, when the second FET is conducting the feedback resistance effectively being that of the second and third resistors and the amplifier operating in a second transimpedance mode for electrical signals having a frequency below the mode cross-over frequency and operating in a follower mode for electrical signals having a frequency above the mode cross-over frequency.