Circuit for generating a trapezoidal current waveform with matched rise and fall times.

Background of the invention.

Field of the invention.

0001This invention relates to a circuit for generating a current signal having a given current wave­form, in particular a trapezoidal current waveform with substantially evenly matched rise and fall times; and to a pulse transmitter circuit such as those in a data terminal transceiver.

0002The invention also relates to a current amplifier circuit and a voltage-to-current converter circuit suitable for use therein.

0003In computer communication systems having a local data terminal which communicates with a local area network (LAN) or a larger global computer network, a transceiver is used between the data terminal and the network. This invention is particularly useful with those networks in which a pulse is transmitted by applying predetermined values of current pulses to the network cable, rather than transmitting voltage pulses of a given voltage level into the LAN cable.

0004In integrated circuit technology, a typical transmitter circuit receives input data in the form of a terminal, and must generate output data in the form of a low voltage differential input signal from the data higher power output signal having a particular waveform to the network. During the transmission of the particular waveform over media such as coaxial cable, noise and interference may distort the signal, causing errors in data transmission. The magnitude of these errors is affected by the shape of the waveform and by the transmitter output impedance relative to the network impedance.

0005To enable accurate discrimination between signal and noise pulses, it may be especially desirable that data pulses be trapezoidal rather than square, and have closely matched rise and fall times. The matching of these times is more critical than their actual value.

Description of the Prior Art.

0006A well-known transmitter circuit transmits data in the form of a voltage signal having a trapezoidal voltage waveform. The transmitter circuit has a differential amplifier, a pair of current sources, an input switching device and an on-chip integrating capacitor. The capacitor is connected between the negative differential amplifier and the differential output. The first current source is connected between the negative input of the differential amplifier to ground. The second current source, having the opposite polarity and twice the magnitude, is connected <u style="single">via</u> the switching device to the negative input of the differential input, in parallel with the first current source.

0007In response to high to low differential transitions applied to the switching device of the transmitter circuit, the switch opens, and the capacitor changes at a constant rate of I/C volts/sec. Likewise, in response to low to high transitions applied to the switching device, the switch closes, and two current sources combine to discharge the capacitor at a constant rate of -I/C volts/sec. Thus the known transmitter circuit generates a voltage signal having a trapezoidal waveform with matched rise and fall times.

0008Such a transmitter circuit is available from National Semiconductor as the DS 3662 Quad High Speed Transceiver.

0009Another circuit for providing a synthesized waveform which is a replica of an input pulse is taught by U.S. Patent 3,898,571. The output pulse is obtained by summing the output voltages of a plurality of gated ampli­fiers. This circuit is of particular value in receivers which introduce pulse distortion <u style="single">via</u> a logarithmic amplifier.

0010A disadvantage of the transmitter circuit described above is that there are no efficient conventional techniques to convert a voltage signal having a trapezoidal voltage waveform to a current signal having a similarly shaped current waveform without distorting the waveform characte­ristic. Such a voltage-to-current converter which produces a precise current waveform characteristic is particularly desirable for use in transmitter circuits, especially those used in transceivers which transmit data over a coaxial line by sinking current and receive data by sensing changes in voltage. More precisely transmitted current waveforms enable increased speed and accuracy of data communication between each terminal in the network.

Summary of the invention.

0011An object of the invention is to provide a trans­mitter circuit which has a current output waveform, efficient­ly generated in response to an input voltage waveform.

0012A transmitter according to the invention is charac­terized in that the circuit comprises a voltage-to current converter having current switching means comprising a plura­lity of current switches having outputs connected directly to a common output node, for providing a low level current signal having a waveform at least stepwise similar to a desired output pulse waveform, in response to receipt of an input voltage signal having the desired output pulse wave­form, means for providing the input voltage signal having said desired pulse waveform to said voltage-to-current converter and in that the circuit further comprises a current amplifier, receiving input current from said common output node, and having an amplifier output connected to said network conductor for sinking current from said conductor.

0013To provide a smooth waveform, a transmitter according to the invention may include a smoothing filter in the current amplifier, so that the appearance of the current waveform ob­tained from the voltage-to-current converter is smoothed. Preferably the smoothing filter is a capacitor used as a compensating capacitor in the current amplifier. For this reason the current amplifier is further characterized in that said current amplifier comprises filter means for smoothing said stepwise similar signal to a current pulse having said desired pulse waveform. The means for providing the input voltage signal to the voltage-to-current converter are characterized in that said means for providing said input voltage signal further comprise input means for re­ceiving a differential input signal having an arbitrary high and low voltage level and a given pulse width, and converting said differential input signal into the single-­ended voltage signal and that the means for providing said input voltage signal comprise a single-ended voltage signal and in response thereto producing a trapezoidal voltage signal proportional to said single-ended voltage signal, and that in response to said voltage signal, said current switching means produce said current signal having a trapezoidal current waveform stepwise substantially similar to said trapezoidal voltage waveform.

0014The current switching means in the voltage-to-­current converter characterized in that said current switching means is a differential means having a plurality of first and second differential inputs, a current switching differential output, and a corres­ponding plurality of tail current sources each having a constant tail current value of I, each of said second differential inputs being connected to a voltage thres­hold tap of a voltage divider connected to a reference source and each of said first differential inputs being connected together for receiving said voltage signal in common wherein in response to said voltage signal with respect to each of said plurality of voltage threshold taps, said differential means controls the current flow through said current switching differential output by each of said plurality of tail current sources to produce a current signal having a current waveform shaped step­wise similar to said voltage signal.

0015The voltage-to-current converter circuit includes a plurality of differential current switches, connected so that as the voltage rises the switches are turned on consecutively, and as it falls they are turned off in reverse order. The switches are preferably identical, and each have a first and second input, an output, and a constant current source. All the first inputs are connected together directly to a common input node for receiving a voltage signal which is to be converted into a current signal. The outputs of the switches are connected to a common output node, at which a staircase current waveform is produced, stepwise similar to the input voltage waveform. Preferably, the second inputs of each of the switches are connected to respec­tive taps of a voltage divider having equally spaced taps.

0016In an embodiment where the peak value of the current pulse is to be controlled closely, the voltage level at the highest voltage tap is made sufficiently less than the maximum voltage of the voltage signal so that all current switches are fully turned on at the peak of the output trapezoid. It may also be desirable that the lowest tap provide a threshold voltage high enough so that, when the input voltage is at its minimum value between pulses, all of the current switches are fully turned off; that is, they do not provide any significant current into the common output node.

0017This converter circuit has the advantage that the scale factor of voltage-to-current can very easily be changed by varying the voltage applied to, or the current passed through, the voltage divider or by changing the reference level for the set of constant current sources being switched. The number of current switches and divider taps is selected according to a compromise between component simplicity which calls for a small number of switches, and closeness of staircase fit to the desired waveform, which calls for a large number of switches. The fact that many such switches exhibit a "soft switching" characte­ ristic, 60 millivolt offset typically providing a 10%-­90% current switching, means that a relatively straight line characteristic can be obtained if the switch thresholds are 50 to 100 millivolts apart. At the same time it may be desirable to provide a different threshold voltage for the first tap of the divider, so that there is not significant current flow in the face of a zero input voltage.

0018The particular advantages of this converter circuit are that it does not exhibit any saturation effect at high current levels, and that the average slope of a rising or decaying input voltage is very accurately dupli­cated in the current waveform.

0019The integration of the single-ended voltage is done in a circuit characterized in that said integra­tion means comprises differential means having a first and second differential input, a tail current source with a constant current value 2I, and a single-ended output, said first differential input for receiving said single-­ended voltage signal, current source means having a con­stant current value I, said current source means being connected to said differential output, a capacitor means also being connected to said differential output and having a predetermined capacitive value C, negative feedback means for connecting said single-ended output to said second differential input, in response to said single-­ended voltage signal on said first differential input with respect to said second differential input, said diffe­rential means and current source means combining to control current flow charging or discharging said capacitor means at a constant value of I/C or -I/C respectively, to produce an integrated voltage signal on said second differential input proportional to said single-ended voltage signal with respect to said given pulse width, said integrated voltage signal having a particular voltage waveform. This stage provides closely matched charging and discharging currents to a capacitor, the one or the other being applied depending on the high or low voltage state of the single ended voltage signal. Where, as is usually the case, a trapezoidal output waveform is desired, this stage includes a clamping circuit which clamps the rising voltage on the capacitor at a given value such that there is always a time delay between charging and discharging; and clamps the capacitor voltage at a minimum value such that there is always some time delay between the falling slope of the trapezoid and the commencement of rise of the next trapezoidal pulse.

0020A suitable voltage-to-current converter cir­cuit is characterized in that it comprises means for receiving a voltage signal, a reference voltage source, a voltage divider connected to said reference voltage source, having a plurality of taps providing threshold voltages, and a plurality of differential current switches, each current switch having a respective first and second input, a constant current source, and an output, said first inputs all being connected directly to a common input node, said outputs all being connected directly to a common output node, and said second inputs being connected respectively to said taps on said divider, whereby current through said output node has a waveform which is stepwise substantial similar to said voltage signal.

0021A suitable current amplifier circuit comprising a current input terminal, a first current output terminal and an amplifier arrangement having a non-­inverting input, an inverting input and an output, which is coupled to the inverting input is characterized in that the current input terminal is coupled to the non-­inverting input and that the first current output terminal is coupled to the current input terminal <u style="single">via</u> a first impedance and to the output of the amplifier arrangement <u style="single">via</u> a second impedance.

0022The invention will be described in detail hereinafter with reference to embodiments shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWING.

0023<ul id="ul0001" list-style="none"><li>Fig. 1 is a block diagram of a pulse receiver and transmitter circuit in accordance with the invention.</li><li>Fig. 2 is a pulse timing diagram showing input or output signals from certain blocks of Fig. 1,</li><li>Fig. 3 is a schematic diagram of the voltage waveform shaping stages of the circuit of Fig. 1,</li><li>Fig. 4 is a schematic diagram of the voltage-to-current switching stage of the circuit of Fig. 1,</li><li>Fig. 5 is a schematic diagram of a current amplification stage of the circuit of Fig. 1, and</li><li>Fig. 6 is a schematic diagram of a circuit that generates reference currents used in the circuit of Fig. 1.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS.

0024Figure 1 shows a block diagram of a trans­ceiver which is the interface between a data terminal having a balanced output voltage, and a local area network which has a conductor 10 which carries data pulses. The receiver section 20 and the transmitter section may pre­ferably be portions of one integrated circuit, or may be separate.

0025The transmitter circuit includes an input stage A₁ which receives serial data pulses at a pre­determined frequency of pulse width, typically in the form of a balanced voltage differential signal V_diff having square pulses as shown in Fig. 2. The voltage differential signal has an arbitrary low voltage level V_L and high voltage level V_H unique to the circuitry of the data terminal. Input stage A₁ converts the voltage differential signal into a single-ended voltage signal V_A1 having square pulses, with a defined low voltage level V₁ and high voltage level V₂ as shown in Fig. 2.

0026A voltage wave-shaping stage A₂ including an integrating capacitor receives the single-ended voltage signal having a given voltage pulse width and produces a trapezoidal voltage signal V_INT having a pulse width determined by that of the single-ended voltage signal. Preferably the trapezoidal voltage waveform has evenly matched rise and fall times. Alternatively, an isosceles-like triangular voltage waveform is also a suitable voltage waveform.

0027The voltage waveform V_INT produced in stage A₂ is applied to the current-switching stage 100 of the voltage-to-current converter. Switching stage 100 produces a low level current signal having a waveform I_T which is stepwise similar to the voltage waveform, and will thus have substantially equal current rise and fall times. The output current amplifier A_O amplifies and filters the pre­amplified current waveform to generate the current I_CX drawn from the network conductor, which causes the trape­zoidal pulses to be transmitted.

0028The reference current source 2 provides the current sinking source needed to drive the differential transistor pairs of the current switches, and the amplifier circuits above.

0029Figure 3 is a schematic diagram of wave-­shaping stage A₂, which converts an arbitrary square waveform V_A1 into an internally defined waveform V_INT. Transistors Q₁ and Q₂ combine to form a differential pair having their emitters tied together and connected to a tail current source 2I_R1. In the preferred embodiment current source 2I_R1 equals 2 mA. Capacitor C_INT and buffer/­feedback stage A₃ integrate and clamp the internally defined square waveform V_A1 into a trapezoidal voltage waveform V_INT. Transistors Q₁ and Q₂ combine to form a differential pair having their emitters tied together and a tail current source 2I_R1 connected to the tied emitters. The collector of transistor Q₁ is grounded, while the collector of transistor Q₂ is connected to capacitor C_INT, to the base of emitter follower transistor Q₃, and to the collector of transistor Q₅. As will be shown below, in response to the single-ended voltage signal at voltage levels V₂ and V₁, capacitor C_INT charges and discharges at a substantially constant rate of I_R1/C_INT.

0030Transistors Q₄, Q₅, Q₆ and Q₇, and resis­tors R_E3 and R_E4, are configured to form a super Wilson mirror. A constant current source I_R1 isconnected in series with the interconnected collector and base of transistors Q₄. Because of the current mirroring an identical current I_R1 flows through transistors Q₅ and Q₇. This is one half the value of the tail current 2I_R1 of the differential pair. Thus by alternatively switching Q₁ and Q₂ on and off, a current I_R1 will either charge or discharge capacitor C_INT at the same accurate and precise rate of I_R1/C_INT.

0031In response to V_A1 = V₂, transistor Q₁ is turned on, transistor Q₂ is turned off, and current I_R1 flows through transistors Q₇ and Q₅ to charge capacitor C_INT at a rate of I_R1/C_INT. The voltage at the collector of transistor Q₁₂ rises as C_INT charges, while at the same time the stage output voltage V_INT rises. Because of connec­tion of the emitter of Q₃ to the base of Q₂, when V_INT rises to the value V₂ or V_A1, transistor Q₂ also turns on and carries a current I_R1 so that charging of C_INT stops.

0032When V_A1 drops to V₁, transistor Q₁ is turned off, transistor Q₂ is turned on, and current flows through transistors Q₂ at a constant value of 2I_R1. Since only a current I_R1 flows through transistors Q₅ and Q₇ of the Wil­son mirror, a current I_R1 flows from the capacitor C_INT, and the capacitor now discharges at a constant rate of I_R1/C_INT. Thus the voltage at the collector of transistor Q₁₂ decreas­es at a steady rate back to voltage V₁. Again, transistor Q₃ acts as a unity gain buffer so that the stage output voltage V_INT at the emitter of transistor Q₃ follows the voltage at the collector of transistor Q₂, down to a voltage level V₁ where the differential pair of Q₁ and Q₂ again are balanced at a current I_R1 each, and V_INT stops falling.

0033Since the capacitor C_INT charges and dis­charges at substantially the same rate I_R1/C_INT, a particular voltage waveform with a very evenly matched rise and fall time results.

0034Figure 4 is a schematic of the current switching stage which will convert the voltage signal V_INT into a low level current signal I_T having a step­wise similarly shaped current waveform shown idealistically in Figure 2. Transistor pairs Q₂₁, Q₂₂; Q₂₃, Q₂₄; Q₂₅, Q₂₆; Q₂₇, Q₂₈; and Q₂₉ each form a differential pair with a tail current source I_R2. The collectors of each transistor Q₂₁, Q₂₃, Q₂₅, Q₂₇ and Q₂₉ are all tied together at a common output node drawing the current I_T, while the collectors of transistors Q₂₂, Q₂₄, Q₂₆, Q₂₈ and Q₃₀ are connected to ground. Cascode transistors (not shown) may be connected respectively in series with the tail current source I_R2 to reduce spikes at the output caused by feedback form base to collector of the current sources.

0035A voltage divider is formed by five resistors a current source and a reference voltage source V_REF. Threshold voltage taps V_T1, V_T2, V_T3, V_T4, and V_T5 have constant voltage outputs at fixed threshold values, and are connected respectively to the bases of transistors Q₂₁, Q₂₃, Q₂₅, Q₂₇ and Q₂₉. The bases of transistors Q₂₂, Q₂₄, Q₂₆, Q₂₈ and Q₃₀ are connected to a common stage input node receiving the voltage signal V_INT from the wave-shaping stage A₂. Resistors R_D between the taps are identical in value, while resistor R_F between the voltage source V_TEF and tap V_T5 may be selected for convenience in scaling.

0036In the preferred embodiment V₂ = -4.9 volts, V₁ = -6.3 volts and the threshold voltages are spaced 200 mv apart. For preferred voltage threshold values of V_T1 = -6.0 v, V_T2 = -5.8 v, V_T3 = -5.6 v, V_T4 = -5.4 v and V_T5 = -5.2 v, turn-off of transistors Q₂₂, Q₂₄...Q₃₀ is assured when V_INT = V₁; and full turn-on of all these transistors is also assured for V_INT = V₂ despite shifts in V_INT due to variations or drift in component values.

0037As the voltage V_INT rises from V₁ to V₂, and it sequentially exceeds the voltage thresholds V_T1, V_T2, V_T3, V_T4 and V_T5, and transistors Q₂₂, Q₂₄, Q₂₆, Q₂₈, Q₃₀ are progressively turned on. Similarly, when the voltage on the capacitor C_INT falls from V₂ to V₁, then transistors Q₂₂, Q₂₄, Q₂₆, Q₂₈ and Q₃₀ are progressively turned off. This provides a polarity reversal, by which the current I₂ falls as the voltage V_INT rises, to accommodate a corresponding reversal in the following amplifier stage.

0038Those of ordinary skill in the art will recognize that the graph of I_T in Fig. 2 is idealized. In a practical integrated circuit using bipolar technology, with a 200 mv differential voltage between the bases of a differential pair, one will be essentially fully off. With a 60 mv differential, current sharing between the pair will be about a 1-to-10 ratio, while equal base voltages produce roughly equal currents. Thus in the preferred embodiment with 5 switches at 200 mv spacing, one switch is practically fully switched when the next has started to switch.

0039By using a greater number of switches, with more closely spaced divider tap voltages, a smoother rising and falling ratio of I_T vs. time may be obtained. However, making the lowest step (V_TI-V_I) smaller raises the possibility that I_T ≠ 5I_R2 between pulses; while making V₂ - V₅ smaller raises the possibility that the zero value of the trapezoid will vary.

0040The advantage of having more switching pairs for an increasingly smoother output current waveform also leads to a design comprise, because of the disadvantage of increasing the area on the silicon chip to accommodate more switching pairs, and causing a decrease in the yield and reliability of the overall circuit.

0041A very precise and repeatable current wave­form results from the circuit according to the invention. The output current amplitude of the current waveform may be adjusted simply by setting the reference for the current sources I_R2 in the converter circuit of Figure 4. The rise and fall time of the current waveform are adjusted simply by either adjusting the reference voltage V_REF or current I_R. Matching of the rise and fall times is controlled by adjusting the ratio of current sources I_R1 and 2I_TI that charge and discharge the capacitor C_INT in the stage A2. An accurate and precise current waveform with closely matched rise and fall times thereby results because resistors and current sources can be very accura­tely and precisely produced when the circuit is embodied in a silicon chip. A further advantage of using switched current sources in a transmitter circuit of a transceiver device is that the output pulse amplitudes at both the maximum and minimum current levels are extremely well-­defined. This is a very important quality in the transmit­ter circuit of an transceiver.

0042The current amplifier circuit A_O of Fig. 5 provides an accurately defined linear gain to amplify with minimum distortion the particular pre-shaped current wave­form outputted as I_T.

0043Transistors Q₅₄ and Q₅₅ combine to form a differential pair having their emitters tied together and a tail current source 2I_cs connected to the tied emitters. The collector of transistor Q₅₄ is grounded, and the collec­tor of transistor Q₅₅ is connected to a constant current source I_cs. Transistors Q₅₆ and Q₅₇ combine to form a Darlington pair which functions as a very accurate unity gain feedback buffer, having the base of transistor Q₅₆ connected to the collector of transistor Q₅₅, the collec­tors of transistors Q₅₆ and Q₅₇ connected to ground, the emitter of Q₅₆ and base of Q₅₇ connected to a current source I_B, and the emitter of transistor Q₅₇ connected to the base of transistor Q₅₅ and to the driving stage for the output mirror through transistor Q₅₈. The Darlington pair Q₅₆ and Q₅₇ minimize error caused by large base currents in other unity gain feedback buffers.

0044A compensation capacitor C_c connected to current source I_cs at the connection to the collector of transistor Q₅₅ and base of Q₅₆ also provides filtering, so that the output current is quite smooth. A small value of 2 picofarads has been found effective.

0045The differential pair Q₅₄, Q₅₅ and Darlington pair Q₅₆, Q₅₇ provide large current gain and a unity voltage gain feedback. This is so because the Darlington pair Q₅₆, Q₅₇ forces the voltage at the base of transistor Q₅₅ to be substantially equal to the input voltage at the base of transistor Q₅₄.

0046The base of transistor Q₅₈ is connected to its collector, so the transistor provides a relatively constant voltage drop for the combined voltage drop current I_T passing through resistor N₁R₂, and amplified current N₁I_T passes through resistor R₂, allowing a current (N₁+1)I_T to pass into the next amplification stage. This results in a linear current gain of N₁+1.

0047Transistors Q₅₉ and Q₆₀ combine to form a current mirror, having resistor N₂R₃ connected between the emitter of transistor Q₅₉ and the negative voltage V_EE, and resistor R₃ connected between the emitter of transis­tor Q₆₀ and the negative bus. Labels "x" and "nx" represent the ratio of the emitter areas of each transistor on the silicon chip. Transistor Q₅₈ functions as a level shifting diode. Thus in response to the current (N₁+1)I_T passing through diode Q₅₈, a total current N₂(N₁+1)I_T is drawn as current I_cx through output transistor Q₆₀. In this particular design, for example, a first amplification stage value of N₁ = 8, and a final amplification stage of N₂ = 4, results in a total current gain of 36.

0048Thus it is possible that a low level current signal, having a trapezoidal waveform shape with a well defined rise/fall time generated at an amplitude of 2mA, may then be amplified to 72mA without distorting the current waveform.

0049In this circuit the current gain is determined by the various resistor ratios. Such resistor ratios are well controlled parameters in integrated circuit technology.

0050Figure 6 shows a current reference source that can be a typical building block for the source 2. Transistors Q₇₁ and Q₇₂ form a differential pair having its emitters tied and connected through resistors R_E71, R_E72 to tail current 2I. Transistors Q₇₃ forms a unity gain buffer. Current source I_PTAT allows a current to flow which is proportional to absolute temperatures. Diode Q_D provides a decrease in voltage proportional to temperature, and resistor R₇₁ an increase in voltage proportional to temperature; thus the voltage drops across Q_D and R₇₁ compensate for one another during temperature change. Capacitor C_comp is a compensating capacitor and diode D₇₈ and D₇₉ are level shifting diodes. This circuit results in current flow through resistor R_ref which is substan­tially not effected by changes in ambient temperature.

0051Transistors Q₇₄, Q₇₅, Q₇₆ and Q₇₇ combine to form a super Wilson mirror. Transistors Q₈₀, Q₈₁, Q₈₂... Q_n also combine to form a feedback mirror, producing typical reference currents I_ref.

0052The voltage-to-current stage of Fig. 4 may be used by itself to provide a current wave which is step­wise similar to a voltage wave in applications unrelated to a terminal-to-Lan transciever. In such circumstances it may be desirable that the voltage range of the divider taps exceeds or is at least approximately equal to the peak voltage range of the voltage signal to be converted. Connection of the collectors to ground, and to the common output node, may be interchanged so that the current I_T rises for rising voltage V_INT.

0053The embodiments described above are intended for use with bipolar technology and a negative voltage supply, where the LAN typically has a 50 ohm impedance and a signal conductor at ground potential. Those of ordinary skill will recognize that many alternatives to these embodiments fall within the scope of the inven­tion. For example, the circuits or portions thereof may by MOS types, and may be of any polarity or grounding arrangement. The input stage or wave-shaping stage may not be required, or may have substantially different forms. The output need not be a current mirror. Thus the scope of the invention should be limited only by the appended Claims.