Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03K—PULSE TECHNIQUE
  • H03K19/00—Logic circuits, i.e. having at least two inputs acting on one output; Inverting circuits
  • H03K19/003—Modifications for increasing the reliability for protection
  • H03K19/00369—Modifications for compensating variations of temperature, supply voltage or other physical parameters
  • H03K19/00376—Modifications for compensating variations of temperature, supply voltage or other physical parameters in bipolar transistor circuits

Definitions

• the present claimed invention relates to the field of DC level compensating circuits. More particularly, the present claimed invention relates to DC level compensating circuits for transistors. BACKGROUND ART
• Modern integrated circuit (IC) chips typically contain millions of transistors on a single chip. With the speed of some IC chips operating at over a gigahertz (GHz), one of the persistent problems facing IC chip design is the adverse effects of temperature on the performance of the devices (e.g., transistors, resistors, etc.) embedded in the chip.
• the devices in the IC chips typically produce heat during operation of the circuit.
• the heat generated from the devices can increase the overall temperature of the chip in the order of several hundred degrees.
• the temperature increase in an IC chip environment adversely affects the performance of the circuit. For example, a temperature increase typically increases switching time of transistors in an IC chip because electrical conductivity is inversely proportional to temperature.
• integrated circuits may also suffer from process variations.
• One of the main causes of the process variation is non-uniform base-emitter voltages of bipolar junction transistors.
• non-uniform doping concentrations may produce bipolar junction transistors that have varying base-emitter voltages. Higher base-emitter voltages speed up and improve conductivity of the transistors while lower threshold voltages slow down and reduce conductivity of the transistors.
• the variation in the process therefore introduces variation in speed and conductivity of transistors in integrated circuits.
• IC chips are more sensitive to variations in temperature and process. Since the invention of the IC devices, the speed of IC chips has been steadily increasing by implementing faster switching circuits.
• ECL emitter-coupled logic
• One of the fastest IC is emitter-coupled logic (ECL) circuit with propagation delay of less than 1 nanosecond and clock rates over 1 GHz.
• ECL circuits are often used in high speed digital logic circuits. To operate at such high speeds, the differential ECL circuit requires low capacitive load at load resistors to avoid parasitic filtering.
• the differential ECL circuit is typically highly sensitive to variations in temperature and process. In particular, the DC output level of the ECL circuit changes with the variation in temperature and process.
• Prior Art Figure 1 illustrates a conventional temperature compensation circuit 102 for compensating temperature variations in an ECL circuit 100 through DC level compensation.
• the ECL circuit 100 comprises transistors Nl, N2, N3, N4, N5, and N6, load resistors 106 and 108, an emitter series feedback resistor 110, and a reference voltage network 104.
• the ECL circuit 100 receives two input signals II and 12 at the base of the transistors Nl and N2, respectively.
• the reference voltage network 104 provides a bias voltage to the bases of the transistors N3 and
• the transistors N5 and N6 form output follower transistors for providing outputs Ol and O2 at the respective emitters.
• the temperature compensation circuit 102 comprises a pair of diodes Dl and D2 coupled in parallel and a resistor 112.
• the resistor is coupled in series to the parallel coupled diodes Dl and D2.
• the diodes Dl and D2 operate to allow current in one direction at a time between junctions 116 and 118.
• the compensation circuit 102 provides DC level compensation to the output transistors N5 and N6 by adjusting current flow through the load resistors 106 and
• the temperature compensation circuit 102 presents several drawbacks.
• the compensation circuit also affects the AC operations of the ECL circuit 100 with a resulting adverse effect on the high speed signal.
• the compensation circuit 102 causes a large capacitive load due to a large current that can flow through the diodes Dl and D2.
• the compensation circuit 102 is not operative to start compensation at a specified temperature and is generally not process independent since the circuit is not symmetrical.
• a DC level compensating circuit that provides a substantially constant DC voltage level at the output transistor of a differential ECL circuit by compensating for variations in temperature and process without affecting AC operation.
• a compensation circuit having low capacitive load to allow high frequency applications in ECL circuits.
• the present invention fills these needs by providing a circuit and method for compensating DC output voltage level for a transistor. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
• the present invention provides a DC output level compensating differential emitter-coupled logic circuit.
• the DC output level compensating differential emitter- coupled logic circuit includes a differential pair, an output stage, and a compensation circuit.
• the differential pair is configured to receive a differential signal and is operative to generate a differential signal.
• the output stage is coupled to the differential pair to receive the differential signal and is operative to generate a differential output signal at a DC output voltage level.
• the compensation circuit is coupled to the differential pair and the output stage and is operative to develop a compensating voltage drop in the differential pair so as to compensate for a change in the DC output voltage level when the temperature varies such that the output stage outputs the differential output signal at the DC output voltage level.
• the present invention provides a method for compensating output DC voltage level variations in a transistor.
• the method includes: (a) generating a first constant reference voltage at a node coupled to the transistor, the first constant reference voltage being independent of temperature; (b) developing a compensating current through a resistor, which is coupled to the transistor at the node; and (c) developing a compensating voltage drop across the resistor that is coupled to the transistor at the node, wherein the compensating voltage drop compensates for a change in the DC output voltage level due to a change in temperature such that the DC output voltage level of the transistor remains substantially constant.
• the present invention provides a circuit for compensating a change in base-emitter voltage of an output transistor, which has a base, an emitter, and a collector.
• the output transistor is coupled to a load resistor and receives an input signal at the base for output at the emitter.
• the circuit includes a voltage generating circuit, an emitter series resistor, and a compensating transistor.
• the voltage generating circuit is coupled to a temperature independent constant voltage rail and is operative to develop a temperature independent constant voltage.
• the compensating transistor is coupled to the load resistor and the base of the output transistor.
• the compensating transistor is further coupled to the emitter series resistor and is coupled to receive the temperature independent constant voltage.
• the compensating transistor is operative to develop a compensating current to cause a compensating voltage drop in the load resistor.
• the compensating voltage drop compensates for the change in the base-emitter voltage of the output transistor such that the output transistor outputs the input signal at a substantially constant DC output voltage level.
• the present invention provides a circuit for compensating a change in base-emitter voltage of a transistor having a base, an emitter, and a collector.
• the transistor is coupled to a first resistor at a node and receives an input signal at the base for output at the emitter.
• the circuit includes voltage generating means and compensating means.
• the voltage generating means generates a temperature independent constant voltage.
• the compensating means coupled to the first resistor and the transistor at the node.
• the compensating means receives the temperature independent constant voltage for developing a compensating current to cause a compensating voltage drop in the first resistor.
• the compensating voltage drop compensates for the change in the base-emitter voltage of the transistor such that the transistor outputs the input signal at a substantially constant DC output voltage level.
• the present invention provides a substantially constant DC output voltage level by compensating for variations in temperature and/or process in the output transistor.
• the present invention also allows decoupling of the compensation circuit to provide a constant DC output voltage level over wide temperature range and process variation.
• the compensation circuit of the present invention provides low capacitive load to allow high frequency applications in ECL circuits.
• Prior Art Figure 1 illustrates a conventional temperature compensation circuit for compensating temperature variations in an ECL circuit through DC level compensation.
• Figure 2 illustrates a differential ECL circuit including a DC level compensation circuit in accordance with one embodiment of the present invention.
• Figure 3 illustrates the differential ECL circuit including a cascode stage for decoupling the compensation circuit in accordance with one embodiment of the present invention.
• FIG. 2 illustrates a differential ECL circuit 200 including a DC level compensation circuit 210 in accordance with one embodiment of the present invention.
• the differential ECL circuit 200 includes an input stage, an output stage, and the compensation circuit 210.
• the input stage includes a differential amplifier, a pair of load resistors Rl and R2, and a DC current source II.
• the differential amplifier includes a pair of transistors Ql and Q2, which are preferably bipolar junction transistors having an emitter, a collector, and a base.
• the emitters of the transistors Ql and Q2 are coupled to each other to form an emitter-coupled transistor pair.
• the bases of the transistors Ql and Q2 are operative to receive a differential signal Vin.
• the transistor Ql receives an input signal Vin, at its base and the transistor Q2 receives an input signal Vin 2 at its base.
• the transistors Ql and Q2 are matching transistors.
• the present invention employs npn transistors, it may also utilize pnp transistors.
• the constant current source II is coupled between a supply voltage rail Vee and emitters, i.e., common-emitters, of the transistors Ql and Q2.
• the current source II is a temperature dependent DC current source and is operative to sink a constant DC current through either transistors Ql or Q2.
• the DC current source is temperature dependent so as to compensate for a change in resistance over temperature to provide a constant current. This leads to constant voltage drops across the load resistors Rl and R2.
• the constant DC current is switched between the transistors Ql and Q2 of the differential amplifier in response to the differential signal Vin. For example, when the transistors Ql is turned on and the transistor Q2 is turned off by the differential signal Vin, the current source II draws current through the transistor Ql .
• the resistors Rl and R2 are coupled between the collectors of the transistors Ql and Q2, respectively, and a supply voltage rail Vcc.
• the resistors Rl and R2 are matching resistors characterized by substantially identical resistance values.
• the emitter coupled transistor pair, Ql and Q2 receives the input signals Vinl and Vin2 and is operative to provide currents, Ic, and Ic 2 , through the load resistors Rl and R2, respectively.
• the currents Ic, and Ic cause voltage drops across the resistors Rl and R2 and thus generate voltages at nodes 202 and 204, respectively.
• the differential amplifier generates a differential ECL output voltage swing at the nodes 202 and 204.
• the output stage includes a pair of output buffers operative to shift the DC voltage levels of the nodes 202 and 204 and output the differential ECL output voltage swing of nodes 202 and 204 as output signal Vout.
• the output buffers are implemented as an emitter- follower pair, which includes a pair of transistors Q3 and Q4.
• the transistors Q3 and Q4 are preferably matching transistors and are bipolar junction transistors having an emitter, a collector, and a base.
• the collectors of the transistors Q3 and Q4 are coupled to the supply voltage rail Vcc.
• the transistors Q3 and Q4 are npn transistors.
• the output stage also includes a pair of resistors R7 and R8, which are coupled between a supply voltage rail Vee' and the emitters of the transistors Q3 and Q4, respectively.
• the resistors R7 and R8 are matching resistors having substantially identical resistance values.
• the resistors R7 and R8 are both 50 ⁇ resistors and the voltage rail Vee' is 2 volt below the supply voltage rail Vcc.
• the bases of the transistors Q3 and Q4 are coupled to receive the voltage signals at the nodes 202 and 204, respectively, for driving the voltage signals for output.
• the transistors Q3 and Q4 generates output signals Voutl and Vout2, respectively, at output nodes 206 and 208.
• the differential ECL voltage as the output signal Vout which is defined as the difference between the output signals Vout, and Vout 2 .
• the compensation circuit 210 is coupled to the nodes 202 and 204 and is operative to draw current from the currents I R , and 1 ⁇ through the load resistors Rl and R2, respectively, to compensate for DC level changes caused by temperature and/or process variations in the output buffers (e.g., transistors Q3 and Q4). Specifically, the compensation circuit 210 compensates for the temperature and process variation of emitter-base voltages (V BE ) of the emitter- follower transistors Q3 and Q4.
• the compensation circuit 210 includes a pair of common-base transistors Q5 and Q6, a pair of emitter-series feedback resistors R5 and R6, and a pair of voltage divider resistors R3 and R4.
• the transistors Q5 and Q6 are bipolar transistors having an emitter, a collector, and a base.
• the transistors Q5 and Q6 are preferably matching transistors having substantially matching emitter areas.
• the transistors Q5 and Q6 are both npn transistors. Even though the present invention employs such transistors, it may also utilize other suitable transistors such as pnp transistors.
• the common-base transistors Q5 and Q6 are coupled to each other at their bases.
• the resistors R5 and R6 are coupled between the supply voltage rail Vee and the emitters of the transistors Q5 and Q6, respectively.
• the collectors of the transistors Q5 and Q6 are coupled to the nodes 202 and 204, respectively.
• the resistors R5 and R6 are preferably matching resistors having substantially identical resistance values.
• the voltage divider resistors R3 and R4 are coupled in series between a constant reference voltage source Vref and the voltage rail Vee.
• the node at which the resistors R3 and R4 are coupled to each other is defined as a node 212.
• the common bases of the transistors Q5 and Q6 are coupled to the node 212.
• the reference voltage source Vref is a constant voltage source so as to provide a constant voltage independent of temperature and process. In another embodiment, the reference voltage source Vref provides a constant voltage independent of temperature.
• the base currents of all transistors in the circuit 200 are assumed to be negligible so that the emitter and collector currents of the transistors in the circuit 200 are approximately equal.
• the differential pair provides a substantially over temperature constant voltage drop across the load resistors Rl and R2.
• the present invention provides an additional current through the load resistors Rl and R2 of the differential amplifier.
• the voltage divider resistors R3 and R4 shifts the reference voltage Vref down and sets up a constant voltage at the node 212 through a voltage divider effect.
• the reference voltage source Vref is a constant voltage source independent of temperature and process
• the voltage thus generated up at the node (e.g., common-base) across the resistor R4 is also independent of temperature and process.
• the common bases of the transistors Q5 and Q6 are connected to the constant voltage set up at the node 212.
• the base-emitter voltages of the transistors Q5 and Q6 are dependent on variations in temperature and process. Since a collector current typically varies in relation to the base current of a transistor, the collector currents of the transistors Q5 and Q6 are also dependent on the variations in temperature and process of the transistors Q5 and Q6. Hence, the voltage drops across the resistors R5 and R6 depend on the base-emitter voltages (V BE ) of the transistors Q5 and Q6, respectively.
• the temperature and process dependent collector currents of the transistors Q5 and Q6 provides the compensation currents through the load resistors Rl and R2. Specifically, the collector currents of the transistors Q5 and Q6, which are approximately equal to the emitter currents, are converted to a compensation voltage through the load resistors Rl and R2. The compensation voltages across the load resistors Rl and R2 are then superimposed with the differential voltage caused by the collector output currents Icl and Ic2 of the differential pair transistors Ql and Q2, respectively. The changes in the base-emitter voltages V BE3 and V BE4 are compensated by the temperature dependent voltage drop or rise across the load resistors R2 and Rl, respectively.
• the V BE3 rises in response to a rise in temperature
• the voltage across the load resistor R2 drops to compensate for the increased DC level of the V BE3 caused by the temperature rise so that the sum of the voltage drop across the resistor R2 and the V BE3 is a constant value.
• the V BE3 decreases with a drop in temperature
• the voltage across the load resistor R2 rises to compensate for the decrease in the V BE3 so that the total voltage drop across the resistor R2 and the V BE3 remains a constant value.
• the variations in the base- emitter voltages of the transistors Q3 and Q4 due to the variations in temperature and process are compensated by the changes in the voltages at the nodes 202 and 204.
• the base-emitter voltage V BE3 of the transistor Q3 changes with temperature
• the base-emitter voltage V BE5 of the compensating transistor Q5 also changes over the temperature.
• the transistor Q5 provides a compensating current Ic5 through the resistor R2.
• This compensating current Ic5 causes a compensating voltage drop across the load resistor R2, thereby compensating for the change in the base-emitter voltage V BE3 of the transistor Q3.
• the resistors R2 and R5 are selected to provide an optimum base- emitter voltage compensation as mentioned below.
• the differential ECL circuit 200 includes two symmetrical branches.
• the first branch of the circuit 200 includes the transistors Ql, Q4, and Q6, and resistors Rl, R8, and R6.
• the second branch of the circuit 200 is symmetrical to the first branch and includes the transistors Q2, Q3, and Q5, and resistors R2, R7, and R5.
• the current source II and the voltage divider resistors coupled to both the first and second branches.
• the temperature and process compensation of the base-emitter voltages of the output buffer transistors Q3 and Q4 operates for each of the individual branches.
• the compensation circuit 210 also includes two symmetrical branches.
• the first branch of the compensation circuit 210 includes the transistor Q6 and the resistor R6.
• the second branch of the compensation circuit 210 includes the transistor Q5 and the resistor R5. Both branches may share the voltage divider resistors R3 and R4. It should be appreciated that the compensation scheme of the present invention may also work in a single-ended circuit.
• the first branch of the compensating circuit 210 may work individually with the first branch of the ECL circuit 200 without the presence of the second branches.
• the second branch of the compensating circuit 210 may work with the second branch of the ECL circuit 200 without using the first branches.
• the temperature and process compensation in the base-emitter voltages of the transistors Q3 and Q4 may be explained with reference to one of the branches of the differential
• a temperature coefficient of the DC voltage level at the output port can be describes as:
• Equation (3) shows that the temperature change of V BE3 and the temperature coefficient of V BE5 are of opposite polarity.
• equation (3) indicates that the changes in the base- emitter voltage of the transistor Q3 caused by variations in temperature may be compensated by the changes in the base-emitter voltage of the compensating transistor Q5 together with the appropriate selection of the resistors R2 and R5 to satisfy the ratio R2/R5.
• the product of the common mode gain factor, -R2 R5 with the temperature coefficient of the base- emitter voltage V BE5 equals the temperature coefficient of the base-emitter voltage V BE3 , the DC output level of the transistor Q3 is fully compensated.
• the compensation scheme of the present invention utilizes matching devices as described previously. By using matching devices, the variations in processes are compensated also.
• FIG 3 illustrates a differential ECL circuit 300 including a cascode stage 302 for decoupling the compensation circuit in accordance with one embodiment of the present invention.
• the differential ECL circuit 300 is identical to the ECL circuit 200 of Figure 2 with the exception of the cascode stage 302.
• the temperature and process compensation of the ECL circuit 300 operates as described above in conjunction with Figure 2.
• the cascode stage 302 depicted in Figure 3 is operative to decouple the compensation transistors Q5 and Q6 from the rest of the ECL circuit 300.
• the cascode stage 302 provides constant collector emitter voltages for the compensation transistors Q5 and Q6.
• the constant collector and emitter voltages for the transistors Q5 and Q6 ensure that the compensation current is independent on the output level of the differential amplifier as well as on the power supply voltages.
• the cascode stage 302 includes a pair of matching transistors Q7 and Q8.
• the transistors Q7 and Q8 are preferably bipolar transistors, which include a base, an emitter, and a collector.
• the collectors of the transistors Q7 and Q8 are coupled to the nodes 204 and 202, respectively.
• the bases of the transistors Q7 and Q8 are coupled together in a common-base arrangement to the node 304.
• the emitters of the transistors Q7 and Q8 are coupled to the collectors of the transistors Q5 and Q6, respectively.
• a resistor R9 is coupled between the supply voltage rail Vcc and the node 304.
• a transistor Q9 is coupled to the Vref at its base.
• the transistor Q9 is a bipolar transistor, preferably a pnp transistor, having an emitter, a collector, and a base.
• the emitter of the transistor Q9 is coupled to the common-base node 304 and the collector is coupled to the supply voltage rail Vee.
• the cascode stage 302 decouples the transistors Q5 and Q6 from the rest of the differential ECL circuit 300 so as to insulate the transistors Q5 and Q6 from a transient swing at the output ports.
• the transistor Q9 is a emitter-follower and is preferably a bipolar transistor.
• the transistor Q9 is a pnp emitter follower and the resistor R9 is an emitter-series feedback degeneration resistor, which defines the collector current of the transistor Q9.
• the transistor Q9 shifts the voltage at the node 304 up by its base-emitter voltage, V BE9 , from the reference voltage Vref supplied at its base.
• the transistors Q7 and Q8 shifts the voltage level down by the respective base- emitter voltages V BE7 and V BE8 . Accordingly, the transistors Q7 and Q8 decouples the transistors Q5 and Q6 and provides the reference voltage Vref to the collector nodes of the transistors Q5 and Q6. In addition, by decoupling the transistors Q5 and Q6, the cascode stage 302 allows the compensation circuit 210 to operate independent of changes in power supply voltage. For example, when the supply voltages (e.g., Vcc, Vee, Vee') change, the voltages at the collectors of the transistors Q5 and Q6 also change in response. The variation of voltages at the collectors are due to undesirable early voltage effect typically associated with a transistor.
• the supply voltages e.g., Vcc, Vee, Vee'
• the cascode stage 302 including the emitter follower transistor Q9 effectively eliminates the undesirable early voltage effects by maintaining the voltages at the collectors of the transistors Q5 and Q6 at a proper operating voltage. Furthermore, the cascode stage 302 also enables use of large devices by decoupling parasitic capacitance of the large device transistors.
• the present invention provides a substantially constant DC output voltage level by compensating for variations in temperature and/or process in the output transistor.
• the cascode stage also allows decoupling of the compensation circuit to provide a substantially constant DC output voltage level over a wide temperature range and process variation.
• the compensation circuit of the present invention provides low capacitive load to allow high frequency applications in ECL circuits.

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• 1998
  • 1998-08-27 US US09/141,334 patent/US6104232A/en not_active Expired - Fee Related
• 1999
  • 1999-08-26 JP JP2000568185A patent/JP2002524903A/ja active Pending
  • 1999-08-26 WO PCT/US1999/019545 patent/WO2000013317A1/en not_active Ceased
  • 1999-08-26 EP EP99945223A patent/EP1110319A4/en not_active Withdrawn

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