Low Noise, High Speed Current or Voltage Amplifier

1. Field of the Invention

This invention relates to an amplifier system for pre-amplifying a current or voltage waveform. More particularly, it relates to such an amplifier system for amplifying such waveforms of the type that are received from optical sensors, such as photodiodes or avalanche photodiodes. Most especially, it relates to such an amplifier system for pre-amplifying such a current waveform to form a voltage waveform, i.e. a transimpedance amplifier system.

2. Description of the Prior Art

In many optoelectronic applications, junction field effect transfer (JFET) into bipolar cascode amplifiers have become somewhat of a standard for low noise/high speed front ends, primarily because their immediate voltage gain desensitizes them to later stage noise voltage contributions. Other amplifier configurations are also used for these applications. Examples of prior art amplifier systems for such applications are disclosed in, for example, D.R. Smith et al., "High Performance Digital Optical Receivers with PIN Diodes," Proceedings of 1979 ISCAS, pp. 511-513 and M. J. N. Sibley et al., "A Monolithic Common-Collector Front-End Optical Preamplifier," Journal of Lightwave Technology, Vol. LT-3, No. 1, February 1985, pp. 13-15. To minimize noise over a wide bandwidth, input capacitance squared divided by the JFET transconductance (C.sub.T.sup.2 /g.sub.m) is kept small, as are photodiode and JFET leakage currents. Feedback resistor impedance (R.sub.L) is selected large so that its thermal noise current is small. Unfortunately, the bandwidth is a function of C.sub.T and R.sub.L and can become quite small for large R.sub.L and difficult to deal with in closed loop designs. Another disadvantage of the cascode front end is that recovery from amplifier saturation is hindered because some of the peak-to-peak voltage swing to remove charge from C.sub.T through R.sub.L is taken up biasing the second transistor. This is important in applications, such as bar code scanning, where the detector is exposed to signals with a wide dynamic range and ambient light.

Accordingly, it is an object of this invention to provide an improved low noise, high speed amplifier system suitable for use in light detection applications.

It is another object of the invention to provide such a low noise, high speed amplifier system suitable for use in scanning applications.

It is still another object of the invention to provide a low noise, high speed amplifier system having a reduced input capacitance compared to previous such amplifier systems and which is capable of a quick recovery from saturation.

It is a further object of the invention to provide such a low noise, high speed amplifier system which is capable of overcoming the above limitations of cascode amplifiers without sacrificing noise or bandwidth performance.

The attainment of these and related objects may be achieved through use of the novel low noise, high speed amplifier system herein disclosed. A low noise, high speed amplifier system in accordance with this invention has a first amplifier, for example, a front end transistor with a control electrode, an input current flow electrode and an output current flow electrode and an input signal source connected to the control electrode of the front end transistor. The output current flow electrode of the front end transistor is alternating current coupled to at least one contributor, preferably all or most of the contributors, of an effective input capacitance of the amplifier system to reduce the effective input capacitance of the amplifier system. As used herein, the terms "alternating current coupled" and "coupled" refer to direct connections, connections through non-unity voltage gain amplifiers, subsequent stage amplifiers, and near unity gain passive components, such as capacitors. A second amplifier, for example, a second stage transistor having a control electrode, an input current flow electrode and an output current flow electrode has its control electrode connected to the output current flow electrode of the front end transistor. The output current flow electrode of the second stage transistor is alternating current coupled to the input current flow electrode of the front end transistor. This bootstrapping, in conjunction with other bootstrapping described herein, reduces the effective input capacitance at the front end of the amplifier system by a factor of up to and exceeding, for example, about 100, thus reducing signal noise component in the subsequent stage or stages to negligible compared to front end noise.

The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention, taken together with the drawings, in which:

FIG. 1 is a schematic representation of a prior art amplifier for optoelectronic applications.

FIG. 2 is a block diagram and schematic representation of a low noise, high speed transimpedance amplifier system in accordance with the invention.

FIG. 3 is a key showing placement of FIGS. 3A and 3B.

FIGS. 3A and 3B are a more detailed schematic diagram of the low noise, high speed transimpedance amplifier system of FIG. 2.

FIG. 4 is a plot of gain against frequency for the amplifier system of FIGS. 3A-3B.

FIGS. 5, 6 and 7 are waveform diagrams useful for understanding operation of the low noise, high speed transimpedance amplifier system of FIGS. 2-3B.

FIG. 8 is a block diagram of another embodiment of an amplifier system in accordance with the invention.

FIG. 9 is a schematic diagram of the amplifier system shown in FIG. 8.

In FIG. 1, there is shown a prior art JFET front end, cascode amplifier 10 commonly used in optoelectronic applications. Impedance Rf provides a feedback loop on line 12 between a collector of transistor Q2 and a gate of the JFET Q1 in the cascode pair amplifier 10. At the front end is an effective input capacitance Cineff of about 6 picofarads (pf), including C.sub.gs +C.sub.gd +C.sub.pd. Photodiode D1 provides an input signal to be amplified on line 14 to the amplifier 10. Using a source-follower, or common drain, in the front end of the amplifier 10, a known technique, allows the gate-source capacitance C.sub.gs to be effectively eliminated. However, because it is necessary to use a relatively high resistance for Rf, such as about 20 megohm, in order to reduce thermal noise in the resistor, one would also have to reduce the gate-drain and photodiode capacitances C.sub.gd and C.sub.pd in order to significantly increase the high frequency gain of the amplifier 10.

Another disadvantage of the cascode amplifier 10 in FIG. 1 is that part of the output voltage swing for removing charge from the effective input capacitance when the amplifier is saturated is taken up to bias the second transistor Q2 in the amplifier 10. This means that recovery of the amplifier 10 from saturation takes longer than would be the case with a wider voltage swing.

FIG. 2 shows an amplifier system 20 of this invention in which effective input capacitance is bootstrapped to reduce it to a negligible value. In comparison with the amplifier 10 of FIG. 1, effective input capacitance Cineff is reduced from about 6 pf to about 0.1 pf, including the capacitance of the feedback impedance Z.sub.f. In the amplifier system 20, three emitter follower transistor stages are represented by amplifiers 22, 24 and 26 with unit gain. A bias voltage source 25 is connected to the cathode of photodiode D1 by line 27. Because the bias end of the photodiode is ac bootstrapped with output 28 of the JFET Q1 amplifier 22, photodiode capacitance C.sub.pd appears as shown from the gate to the source of the JFET Q1. Similarly, the drain of the JFET Q1 is ac bootstrapped with output 30 of transistor Q2 amplifier 24, so that gate to drain capacitance C.sub.gd appears as shown. This ac bootstrapping effectively reduces all noise voltage contributions beyond the second stage amplifier 24 to be negligible, by putting the noise voltage contributions across the effective bootstrapped input capacitance (.about.C.sub.t / 100), instead of C.sub.t.

Notice that the input noise voltage e.sub.n1 of the first stage 22 appears across C.sub.pd, C.sub.gs and C.sub.gd, while the input noise voltage e.sub.n2 of the second stage 24 appears only across C.sub.gd. The currents generated from these sources are: ##EQU1##

Therefore, further stage e.sub.n noise components are reduced to negligible when compared to that of the first stage.

FIGS. 3A and 3B show details of an implementation of the amplifier system 20. Front end JFET Q1 and a feedback chip resistor R9, which may be between about 0.8 and 100 megohms, preferably about 20 megohms as shown, are enclosed with the photodiode D1 in a windowed hybrid package, as indicated at 40, so that parasitic capacitances are minimized and summing junction 42 is protected from radiated EMI. The chip resistor R9 is preferably provided in non-serpentine form, selected for high ohms per square and low shunt capacitance and mounted un-trimmed on a substrate to minimize and control shunt capacitance. Trimming such a resistor increases its capacitance by thinning it structure. The JFET Q1 is selected and biased to provide minimum C.sub.t.sup.2 /g.sub.m and low leakage. The photodiode is selected and biased for maximum responsivity and minimum capacitance and leakage for the particular application.

The second stage transistor Q2 has its base connected to a source of the JFET Q1 by line 46. The second stage transistor Q2 is selected for low noise voltage (e.sub.n), low base spreading resistance (r.sub.b), and low input capacitance. To lower r.sub.b further, transistor Q4 can be connected in parallel with transfer Q2 by lines 44, 48 and 52. Transistor Q3 has its base connected to the emitters of transistors Q2 and Q4 by lines 52 and 54. The third stage transistor Q3 is selected for low input capacitance to minimize Miller effect loading on the second stage transistors Q2 and Q4.

The AC bootstrapping in accordance with this invention is provided for the photodiode D1 by a 0.01 .mu.f capacitor C2 connected between the source of the JFET Q1 and the cathode of the photodiode D1 by line 56. Line 58 also provides AC bootstrapping to the case of the hybrid module 40 through capacitor C2. Similarly, the source resistor R3 and drain of JFET Q1 are AC bootstrapped through the 1.0 .mu.f capacitors C1 and C4 to the emitters of the transistors Q2 and Q4 by lines 60 and 62. The collectors of the second stage transistors Q2 and Q4 are bootstrapped to the third stage transistor Q3 emitter through capacitor C8 by line 64.

Inductors L1, L2 and L3 are employed with the first stage JFET Q1 to increase the bootstrap efficiency, yet allow high JFET bias current for maximum JFET g.sub.m. They also act to reduce the thermal noise of the adjacent resistors R2, R3 and R5.

Open loop compensation is carried out with the third stage transistor Q3 by compensation circuit 70. Diode D2 improves negative slew rate.

Discrete stage output swing can be from the positive supply rail (Vcc) to ground, if the dark level is designed to be near Vcc. This allows the amplifier system 20 to remove charge from a saturated front end C.sub.t at a rate approaching Vcc/R.sub.f, where Z.sub.f is a resistor. Video clamps 76 provide clamping of the dark level. Bandpass filtering to reject out of band noise is done elsewhere in the system.

Operational amplifier stages 72 and 74 provide for gain and frequency compensation to cancel the single discrete closed loop pole due to R.sub.f in parallel with its shunt capacitance. FIG. 4 is a plot of gain versus frequency in the preamplifier system 20 and shows the frequency compensation of the system. Curve 82 shows the open loop gain of the discrete stages up to test point (TP) 2. Line 84 shows the closed loop gain at TP 2 after connecting feedback resistor R9. Curve 80 shows lead compensation due to network 71 in FIG. 3B. Curve 86 shows the final compensated bandwidth of the preamplifier system.

FIG. 5 is a plot of voltage against time for the preamplifier system 20 during scanning of a test bar code label with a Datachecker BDI Scanner having the preamplifier system 20 installed. Curve 88 shows the voltage at TP 2, prior to amplification by operational amplifier 72. Curve 90 shows the voltage at TP 7, after amplification by the operational amplifier 72. These curves show that the amplifier system 20 produces clear signals in response to scanning a bar code.

FIG. 6 is a plot of voltage against time when the Datachecker scanner with the preamplifier system 20 installed scans a label with a 13/4 inch white border right after a 100 watt incandescent lamp 2 ft. away from the scanner surface. Such ambient light levels are sometimes encountered in scanner installations. Curve 92 shows the voltage at TP2, curve 94 the voltage at TP7, and curve 96, as the final filtered video output. Region 98 on each curve represents the signal at saturation of the amplifiers due to the incandescent light. These curves show that the amplifier system 20 has recovered from saturation by the time that the scanner reaches the bar code on the label. Curve 100 in FIG. 7 is a similar plot showing the voltage at R11, i.e., at the emitter output of the third stage transistor Q3. Curve 101 shows the voltage at the collector of transistor Q3.

FIG. 8 shows an FET probe voltage amplifier system 110. A voltage source 112 is connected to a control electrode of unity gain amplifier 114 by line 116. The signal output of the amplifier 114 is AC coupled by capacitor C10 on line 118 to a bias voltage input to the amplifier 114 on line 119. The signal output of amplifier 114 is connected to a control electrode of amplifier 120 having a gain of, for example, 10 by line 122. The amplifiers 114 and 120 are packaged in a case 124. The signal output of amplifier 114 is also AC coupled by capacitor C10 to the case on line 126 and optionally to case 128 of the voltage source 112 by line 130. By reducing the effective input capacitance Cineff of the amplifier 110 in this manner, a low noise, high speed amplifier system 110 suitable for FET probe applications is obtained.

FIG. 9 shows a specific circuit implementation of the amplifier system 110. The input voltage on line 116 is supplied to the gate of a front end JFET Q1. As a source follower, the output at the source of the JFET Q1 is connected to the base of bipolar transistor Q2. The emitter of bipolar transistor Q2 is ac coupled by capacitor C10 on line 118 to the drain of the JFET Q1, which receives a bias voltage on line 132. Bias resistor R1 for the source of the JFET Q1 is bootstrapped to the emitter of transistor Q2 on line 133. Line 126 connects the capacitor C10 to the case 124 of the amplifier system 110. Line 134 connects the source of JFET Q1 to the case 136. The emitter of the bipolar transistor Q2 is similarly connected to the base of bipolar transistor Q3, and the emitter of the transistor Q3 is ac coupled to the collector of transistor Q2 by capacitor C11 on line 138. The output of the amplifier system 110 is at the collector of transistor Q3. In addition to its use as a FET probe, the amplifier system 110 is useful for other applications of a low noise, high speed voltage amplifier.

Various modifications can be made in the amplifier systems 20 and 110. Other types of transistors, such as MESFETs, gallium arsenide FETs, MOSFETs or bipolar transistors, could be substituted for the front end JFET Q1 or the bipolar transistors Q2, Q3 or Q4. Part or all of the remainder of the amplifier system 20 could be enclosed in a larger hybrid package than the module 40, with the modification or deletion of circuit elements that do not hybridize well, such as inductors and capacitors. Similarly, part or all of the amplifier system 20 could be implemented in integrated circuit form, such as a combined bipolar-FET (BiFET) integrated circuit, or an all bipolar integrated circuit, or a gallium arsenide FET integrated circuit, with the modification or deletion of circuit elements, such as inductors or capacitors, which are not compatible with the monolithic process. In addition to the scanning application and the particular transducers used, the amplifier system 20 is equally useful in other applications and with other transducers where low capacitance is needed, such as in a low capacitance FET probe, current probe, or an RF detector. It could also be used to make an instrumentation amplifier, high speed buffer operational amplifier, or used as the front end of an operational amplifier.

It should now be readily apparent to those skilled in the art that a novel low noise, high speed current or voltage amplifier system capable of achieving the stated objects of the invention has been provided. The low noise, high speed amplifier system of this invention is suitable for a variety of light detection applications, such as scanners for bar codes or other signal inputs. The amplifier system is responsive to a relatively high frequency varying input, and recovers quickly from saturation. The amplifier system is therefore capable of overcoming limitations of prior art cascode amplifiers used in scanner applications.

It should further be apparent to those skilled in the art that various changes in form and details of the invention as shown and described may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto.