Abstract:
A comparator has a simple high frequency signal path from input to output formed by two differentially connected transistors with an output transistor connected to each. A current control circuit maintains a constant total current flowing through the differentially connected transistors. The ratio of currents flowing through the differentially connected transistors is initially set by a control circuit, so that the output of the comparator is predictable when powered up. The control circuit then gradually releases control, so that there is a smooth transfer to control by a reference signal input to the comparator.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to providing a comparator circuit for use in high frequency applications and, more particularly, to a comparator with a smooth start-up for use in a phase-locked loop or level-locked loop, for example in a time-division multiple access (TDMA) telephony system. 
     2. Description of the Related Art 
     A phase-locked loop is commonly used in radio communications at a wide range of frequencies, for both analog and digital signals. In some conventional phase-locked loops for carrier recovery, a comparator is used. It is desirable for phase-locked loops to lock quickly, particularly when the entire transmitter or receiver in a TDMA system is powered up and down many times every second for power savings. Since a comparator is a non-linear high gain circuit, if it is powered up without proper controls, it can throw the loop into states from which it cannot recover, or at least cause a delay in circuit start-up. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a comparator suitable for high frequency phase-locked loop applications. 
     It is also an object of the present invention to provide a comparator having a predictable output when power is initially applied. 
     It is another object of the present invention to provide a comparator gradually changing from a predetermined output to an output based on received signals, shortly after power is initially applied. 
     It is an additional object of the present invention to provide a duty ratio adjustment circuit for high frequency oscillating signals, which quickly produces an output signal after gradually changing from a predetermined duty ratio. 
     The above objects can be attained by a comparator formed of a comparison circuit coupled to receive at least two input signals and generating at least one output signal based on the at least two input signals; and a control circuit, coupled to the comparison circuit and to receive at least one start-up signal indicating start-up of the comparator, controlling the comparison circuit to generate the output signal with a predetermined value upon receipt of the start-up signal and gradually releasing control of the comparison circuit. 
     Preferably, also included is a constant current circuit coupling the control circuit to the comparison circuit and maintaining a constant total current through the comparison circuit when the control circuit has completely released control of the comparison circuit. To handle high frequency signals efficiently, the comparison circuit preferably consists of only a pair of differentially connected transistors having collectors respectively and directly connected to the bases of a pair of output transistors. The emitters of the output transistors are preferably directly connected to output signal lines. 
     These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the block diagram symbol of a comparator; 
     FIG. 2A and 2B are graphs of square waves with different duty ratios produced from sinusoidal waves using different reference voltages; 
     FIG. 3 is a simplified circuit diagram of an embodiment of a comparator according to the present invention; 
     FIG. 4 is a simplified circuit diagram of a control circuit for a comparator according to the present invention; 
     FIG. 5 is a schematic circuit diagram of a comparator according to the present invention; and 
     FIG. 6 is a block diagram of a level-locked loop using a comparator according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrated in FIG. 1 is a block diagram symbol of a comparator that can be used for the present invention. As illustrated in FIG. 1, all inputs and outputs are differential, so that all signal connections to the comparator are shown. The symbol illustrated in FIG. 1 may be used for comparators operating on both analog and digital signals. The present invention is particularly useful as a level shifter, or duty ratio adjustment circuit where an oscillating signal is supplied as Vin and a direct current (DC) voltage is supplied as Vref. The resulting output signal Vout is typically a square wave. For example, the signals Vin and Vref illustrated in FIGS. 2A and 2B produce the output signal Vout. 
     A simplified circuit diagram of a comparator according to the present invention is illustrated in FIG. 3. Two features of the comparator which make it suitable for applications at frequencies as high as 1 GHz are illustrated in FIG. 3. First, the path of the high frequency signals from input to output is very simple and direct. The input signal Vin is supplied to the bases of the differentially connected transistors Q1 and Q2. The collectors of transistors Q1 and Q2 are connected directly to the bases of transistors Q8 and Q3, respectively. Constant current sources I4, I5 are coupled to the emitters of each of these transistors. The output signal is produced at the emitters of the output transistors Q3 and Q8. The alternating current (AC) gain at the amplifier is fixed by the ratio of the resistances of the resistors coupled to the collectors and emitters of transistors Q1 and Q2, i.e., the resistance ratio of R15/R3 which is the same as R5/R4. 
     The variable current sources I1 and I2 respectively connected to the emitters of transistors Q1 and Q2 define the direct current (DC) offset level. A current control circuit represented by comparator symbol 10 receives the DC reference voltage Vref to control the current sources I1 and I2. Inside the current control circuit 10, first zero volts is applied instead of the DC reference voltage Vref. This ensures that currents produced by variable current sources I1 and I2 are equal at the beginning of the power-up cycle. Then Vref is gradually applied by applying αVref, where α is linearly increased from zero to one. This guarantees a smooth and predictable transition of Vout from fully balanced to the final state controlled by Vref. 
     Details of the current control circuit 10 and a control circuit for smooth start-up are illustrated in FIG. 4. As illustrated in the upper left-hand corner of FIG. 4, a pair of differentially coupled transistors Q12, Q13 have bases which receive the DC reference voltage Vref. 
     The variable current sources I1 and I2 (FIG. 3) in the current control circuit are provided by current control transistors Q21, Q22 in FIG. 4. The bases of current control transistors Q21, Q22 are connected to the bases of diode-connected transistors Q15, Q14, respectively, to mirror the current flowing through transistors Q15, Q14. This is one of the design features of the present invention which enable the signal path of the high frequency signals to be as simple and direct as possible, while the signal processing for Vref is done in a more complex circuit without affecting the high frequency path. 
     As illustrated in FIG. 4, a control circuit is preferably included in a comparator according to the present invention to provide a smooth start-up. The control circuit includes a shunt circuit formed by transistors Q19, Q20, Q28 and Q29. As illustrated in FIG. 4, transistors Q19 and Q20 are supply transistors that supply current to transistors Q12, Q13 from the power supply line VCC. Shunt transistors Q28, Q29 provide a shunt path around the differentially connected transistors Q12, Q13 in the current control circuit. When transistors Q19 and Q20 are turned off and transistors Q28 and Q29 are turned on, identical currents flow through diode-connected transistors Q14, Q15 and thus through current control transistors Q21, Q22. 
     The shunt circuit is controlled by the remainder of the control circuit illustrated on the right side of FIG. 4. Diode-connected transistor Q18 is connected to the bases of the supply transistors Q19, Q20, while diode-connected transistor Q27 is connected to the bases of the shunt transistors Q28, Q29. Current flow through the diode-connected transistors Q18, Q27 is controlled by a third pair of differentially connected transistors Q24, Q25. These control transistors have bases connected to receive external control signals represented by switches X1 (power up) and X2 (delay). The base of the first control transistor Q24 is also connected to a capacitor C3. 
     In the preferred embodiment, the power-up signal is generated first and is represented in the simplified embodiment illustrated in FIG. 4 by closing switch X1. A delay signal is generated later, opening switch X2. As a result, the second control transistor Q25 is turned on first and the first control transistor Q24 is kept turned off until the delay signal is received and switch X2 is opened. Prior to receipt of the delay signal, current control transistors Q28 and Q29 will be on and supply transistors Q19, Q20 will be off, thereby shunting or bypassing the second differentially connected transistors Q12, Q13 in the current control circuit. Therefore, the currents I1 and I2 will be equal and a predetermined value will be used in generating the output of the comparator. 
     When the delay signal is received, switch X2 is opened permitting the capacitor C3 to charge up gradually and turn on transistor Q24. The diode-connected transistors Q30, Q32, coupled to the base of the second control transistor Q25, maintain a constant voltage. As the voltage at the base of the first control transistor Q24 builds up, the supply transistors Q19, Q20 gradually turn on and as the voltage at the base of the first control transistor Q24 exceeds the fixed voltage at the base of transistor Q25, shunt transistors Q28, Q29 gradually turn off. Thus, a short period of time after the delay signal is received, the currents I1, I2 are controlled by the value of Vref supplied to the bases of the second pair of differentially connected transistors Q12, Q13 in the current control circuit. 
     The difference between a comparator according to the present invention and a conventional comparator can be expressed mathematically. The output of a conventional comparator can be represented by equations (1) and (2), where the Sgn function extracts the algebraic sign of VinP-VinN. 
     
         Vout.sup.+ =+A·Sgn(VinP-Vin)                      (1) 
    
     
         Vout.sup.- =-A·Sgn(VinP-Vin)                      (2) 
    
     In a comparator according to the present invention, the output can be represented by equations (3)-(7). 
     
         Vout.sup.+ =+A·Sgn(VinP-Vin)+αVref          (3) 
    
     
         Vout.sup.- =-A·Sgn(VinP-Vin)-αVref          (4) 
    
     where ##EQU1## 
     A detailed circuit diagram of a comparator according to the present invention is provided in FIG. 5. The same reference numerals are used to identify the circuit components illustrated in FIGS. 3 and 4. Illustrated in FIG. 5 are biasing circuits, some of which will be described later, and switching circuits replacing the simplified switches X1, X2 illustrated in FIG. 4. The POWER UP signal controls switching transistors M5-M7 to supply power to the control circuit. The delay signal is formed by differential signals DELAYN and DELAYP. When DELAYN is high, the switching circuit formed by transistors M8-M11 ground the base of first control transistor Q24 and the capacitor C3. When DELAYN goes low and DELAYP goes high, the connection to ground is broken and current flows through the Schottky diode D10, diode-connected transistor Q31 and capacitor C3, gradually turning on the first control transistor Q24, as described above. 
     Two signals are received for biasing purposes. At the input, a signal with a current IBUF maintains a constant DC bias for the inputs IN, INB by turning on transistors Q4, Q5, Q10 and Q11, so that current flows through Schottky diodes D1, D2, resistors R1A, R2A, R9, R10, R12 and R13. In addition to turning on transistors Q10, Q11, transistors Q17, Q23 in the control circuit and transistors Q7, Q9 in the output circuit are also turned on. 
     A bias signal VBIAS is also supplied via resistors R20, R27 to the bases of the second pair of differentially connected transistors Q12, Q13 in the current control circuit. This voltage provides an operating bias on top of which the reference voltage Vref is applied. In the embodiment illustrated in FIG. 5, the reference voltage Vref is not illustrated as a differential voltage, because it is supplied relative to ground. 
     As described above, a comparator according to the present invention can be used as a level shifter, or duty ratio adjustment circuit generating a square wave at low frequencies or a quasi-square wave at high frequencies, having a duty ratio which depends upon the reference signal Vref. A comparator circuit according to the present invention is particularly well suited for high-frequency applications, such as the level-locked loop circuit disclosed in U.S. patent application Ser. No. 08/366,550, filed Dec. 30, 1994, commonly assigned to U.S. Philips, and incorporated herein by reference. For example, in the level-locked loop circuit illustrated in FIG. 6, the comparator 20 may be a comparator constructed according to the present invention. 
     The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 
     REFERENCE NUMBER LIST 
     C1A-C4 capacitors 
     D1-D7 Diodes 
     I1-I5 Current sources 
     IN, INB Input signal lines 
     M1-M11 Transistors (FETs used for switching) 
     OUT, OUTB Output signal lines 
     R1-R35 Resistors 
     Q1-Q32 Transistors 
     VCC Power supply voltage 
     Vin Input voltage 
     Vout Output voltage 
     Vref Reference voltage