Abstract:
A differential amplifier includes first and second amplifying circuits for providing a differential output. Circuitry is provided to prevent forward-biasing of the transistors of the differential amplifiers.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to integrated circuits, and more particularly to a differential amplifier. 
     BACKGROUND OF THE INVENTION 
     Differential amplifiers are used in many applications. One such application is to translate voltage levels from one logic type to another. For example, differential amplifiers may be used to translate from an ECL voltage level to a CMOS or TTL voltage level. Currently, many integrated circuits are designed to make use of different technologies, such as ECL, TTL, CMOS and BiCMOS, throughout a circuit. Hence, it is important to be able to quickly and accurately translate levels between circuits having different technologies. 
     Most differential amplifiers include a pair of transistors driven by respective input signals. If the transistors are NPN transistors, for example, the collectors of the transistors will be connected to V CC  via a load, the bases will be connected to the respective input signals and the emitters will be connected to a common current source. The higher input signal will drive the respective transistor to draw more current, and hence, will reduce the voltage at its collector, as more current is drawn through the respective load. Often, a resistor is used as a load; however, a resistor will not provide a full voltage swing between the voltage rails. On the other hand, active load may be used to provide a full voltage swing, but these loads may result in the forward-biasing of the differential NPN transistors, which will slow the circuit. 
     Therefore, a need has arisen in the industry to provide a high speed differential amplifier which may be used in level translation, among other applications. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a differential amplifier is provided which substantially eliminates the disadvantages associated with prior differential amplifiers. 
     In the present invention, a differential amplifier comprises first and second differential amplifiers. The first differential amplifier generates first and second differential output signals responsive to first and second input signals. The second differential amplifier generates third and fourth differential output signals responsive to the first and second differential output signals generated by the first differential amplifier. 
     In one aspect of the present invention, the first differential amplifier comprises an MOS differential amplifier and the second differential amplifier comprises a bipolar differential amplifier. Circuitry is provided to prevent forward-biasing of the bipolar differential amplifiers transistors. Hence, the present invention provides a technical advantage over the prior art in that an active load may be used to provide the high voltage gain necessary to perform level translation, without the danger of the bipolar transistors entering saturation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic representation of the differential amplifier of the present invention; and 
     FIG. 2 illustrates a ECL-to-BiCMOS level translator using the differential amplifier of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention is best understood by referring to FIGS. 1-2 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 illustrates a schematic representation of a differential amplifier 10 according to the present invention. A preamplifier circuit comprises an NMOS transistor 12 having a gate connected to the IN signal, a source connected to current source 14 and a drain connected to the source of N-channel transistor 16. N-channel transistor 18 has a source connected to current source 14, a gate connected to the IN and a drain connected to the source of N-channel transistor 20. An inner differential amplifier comprises NPN transistors 22 and 24 having emitters connected to a current source 26 and collectors connected to the drains of N-channel transistors 16 and 20 and to resistors 28 and 30. The opposite ends of resistors 28 and 30 are connected to V CC . The OUT signal is connected to the collector of transistor 22 and in the OUT signal is connected to the collector of transistor 24. 
     In the preferred embodiment, loads 28 and 30 are preferably active loads. An implementation of an active load is illustrated in connection with FIG. 2. 
     In operation, the differential amplifier 10 performs as follows. The IN and IN signals will be supplied by a circuit external to the differential amplifier 10. Typically, the IN and IN signals are complements of one another; however, they need not be related. For example, the IN signal could be a voltage reference signal against which the logic level of the IN signal is compared. The OUT and OUT signals are amplified versions of the IN and IN signals, respectively. 
     If the IN signal has a voltage greater than the IN signal, then transistor 12 will be turned on harder than transistor 18, thereby drawing a larger proportion of the current through current source 14. Transistors 16 and 20 act as loads for transistors 12 and 18, respectively. Hence, the voltage at the base of transistor 22 will be less than the voltage at the base of transistor 24. The signals at the base of transistors 22 and 24 can therefore be thought of as the outputs of the preamplifier section. 
     In the above example, the higher voltage at the base of the transistor 24 will drive more current than will the voltage at the base of transistor 22. Hence, the voltage at the collector of transistor 24 (and hence at the OUT signal) will be less than the voltage on the collector of transistor 22 (the OUT signal). 
     In the preferred embodiment, loads 28 and 30 are active loads, which provide a full voltage swing between the power rails (V CC  and ground). However, using active loads, it is possible that the voltage at the collector of one of the NPN transistors 22 or 24 could be greater than the voltage on the respective base. Thus, both the collector-base and base-emitter voltages would be forward-biased, and the transistor would be in saturation. This is undesirable because of the significantly lower speed of a transistor in saturation. 
     In the preferred embodiment illustrated in FIG. 1, however, the collector-base junction of either bipolar transistor cannot become forward-biased, because of the MOS transistors 16 and 20. If the IN signal is high, causing a low voltage on the collector of the bipolar transistor 24, the N-channel transistor 20 will also be enabled, thereby lowering the base potential until an equilibrium is reached. Similarly, if the IN signal is high, N-channel transistor 16 will be enabled, thereby lowering the base voltage of the NPN transistor 22. Consequently, N-channel transistor 20 act not only as a load for the differential preamplifier transistors 12 and 18, but also prevent forward-biasing of the NPN transistors 22 and 24. 
     FIG. 2 illustrates an embodiment of the present invention used in an input buffer/level translator which inputs standard ECL signals and converts to BiCMOS signals. A P-channel transistor 32 has its source connected to V CC , its gate connected to the gate and drain of P-channel transistor 34 and its drain connected to the source of N-channel transistor 16 and to the collector of NPN transistor 22. P-channel transistor 34 has its source connected V CC  and its drain connected to the collector of NPN channel transistor 36. NPN transistor 36 has an emitter connected to current source 38 and a base connected to a current source 40 and to the cathode of a diode 42. The IN signal is connected to the base of NPN transistor 44 which has an emitter connected to the anode of diode 42 and to the gates of N-channel transistors 12 and 20. The collector of NPN transistor 44 is connected to V CC . NPN transistor 46 has a collector connected to V CC , an emitter connected to current source 48 and to the OUT signal, and a base connected to the collector of transistor 22 and to the drain of P-channel transistor 32. 
     P-channel transistor 50 has a source connected to V CC , a drain connected to the source of N-channel transistor 20 and to the collector of NPN transistor 24. The gate of transistor 50 is connected to the gate and drain of P-channel transistor 52. The source of P-channel transistor 52 is connected to V CC . The drain of P-channel transistor 52 is also connected to the collector of NPN transistor 54. The emitter of transistor 54 is connected to current source 38, and the base of transistor 54 is connected to current source 56 and to the cathode of diode 58. The V ref  signal is connected to the base of NPN transistor 60. The emitter of NPN transistor 60 is connected to the anode of diode 58, the gate of transistor 18 and the gate of transistor 16. The collector of NPN transistor 60 is connected to V CC . NPN transistor 62 has a base connected to the collector of NPN transistor 24, a collector connected to V CC , and an emitter connected to current source 64. The OUT signal is taken from the emitter of transistor 62. 
     In operation, the P-channel transistors 32 and 34 form a current mirror which acts as an active load for NPN transistor 22. Transistor 36 serves as the push-pull load for the PMOS transistors. Similarly, P-channel transistors 50 and 52, along with NPN transistor 54, act as an active load for NPN transistor 24. Transistors 46 and 62 are emitter-followers for driving the output signals OUT and OUT. Transistors 44 and 60 are emitter-follower transistors providing an input buffer. 
     The V ref  signal is the threshold with which determines whether the IN signal is a logic high or logic low. If the IN signal is higher than the V ref  signal, transistor 12 will driven harder than transistor 18, resulting in the voltage at the base of transistor 22 being lower than the voltage at the base of transistor 24. Consequently, the voltage at the collector of transistor 24 will be lower than the voltage at the collector of transistor 22 and the OUT signal will be lower than the OUT signal. Typically, the OUT signal would, in this example, be approximately five volts and the OUT signal would be approximately zero volts. 
     Conversely, if the IN signal is less than the V ref  signal, the voltage at the collector at the NPN transistor 24 will be less than the voltage at the collector at transistor 22. Consequently, the OUT signal will be at approximately zero volts and the OUT signal will be at approximately five volts. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.