Abstract:
The logic circuit disclosed exhibits push-pull output characteristic by employing a saturated feedback technique. This approach allows for emitter follower like up level drive and transient low impedance down level drive. The disclosed saturated feedback technique improves capacitive drive capability, reduces both load and circuit delay and reduces circuit power dissipation.

Description:
BACKGROUND OF THE INVENTION AND PRIOR ART 
     This invention relates to logic circuits and more particularly to transistor switching circuits having low speed/power product. 
     In the development of data processing equipment, extensive effort is being directed to integrated circuits having improved speed capabilities and speed/power factor. 
     In particular, as the logic circuit density and chip size increases for VLSI technologies, the requirement exists for logic circuits having the capability to drive high capacitance on chip loads at acceptable speed/power factors. 
     Prior art bipolar logic circuits such as DTL, TTL and ECL must operate at high power levels if low delay is required at high load capacitance. This relationship is due to the fact that active drive is available for only one output transition, while the other transition depends on the time constant of the output load resistance and capacitive load. 
     A technique to achieve good capacitive drive is to incorporate a push-pull circuit in the output. The push-pull circuit is well known to the art and is used extensively in off chip drivers and also in complementary NPN/PNP bipolar circuits. The logic circuit in accordance with the invention exhibits push-pull output characteristic by employing saturated feedback technique. This approach allows for emitter follower like up level drive and transient low impedance down level drive. The saturated feedback technique improves capacitive drive capability, reduces both load and circuit delay and reduces circuit power dissipation. 
     As with other circuits containing push-pull outputs, the logical dot function is avoided to eliminate a circuit contention problem. 
     There is a very sizeable number of logic circuits known to the art. It is to be appreciated with reference to the subject invention, that, the following art is not submitted to be the only prior art, the best prior art, or the most pertinent prior art. 
     BACKGROUND ART 
     U.S. Pat. Nos. 
     3,183,370 entitled &#34;Transistor Logic In Circuits Operable Through Feedback Circuitry In Nonsaturating Manner&#34; granted May 11, 1965 to K. M. Trampel 
     3,518,449 entitled &#34;Integrated Logic Network&#34; granted June 30, 1970 to D. H. Chung 
     3,699,355 entitled &#34;Gate Circuit&#34; granted Oct. 17, 1972 to C. F. Madrazo et al 
     4,237,388 entitled &#34;Inverter Circuit&#34; granted Dec. 2, 1980 to J. Nokubo et al 
     4,321,490 entitled &#34;Transistor Logic Output For Reduced Power Consumption And Increased Speed During Low To High Transition&#34; granted Mar. 23, 1982 to R. W. Bechdolt 
     SUMMARY OF THE INVENTION 
     VLSI logic circuits are often required to drive high output load capacitance which is comprised of the capacitance due to circuit fan out and also the capacitance of the interconnection metalization which can be substantial. A logic circuit capable of driving high on chip capacitance at reduced speed power product is disclosed. This desirable circuit feature is accomplished by optimizing the logic signal swing (approx. 600 mV) and providing active drive for both charging and discharging the load capacitance. By utilization of an output feedback path containing a saturated transistor, circuit rising and falling delays are enhanced and power dissipation is reduced. 
     The primary object of the invention is to provide an improved logic circuit. 
     A further primary object of the invention is to provide a logic circuit having a low speed/power product. 
     A still further primary object of the invention is to provide a logic circuit having enhanced ability to drive a highly capacitive load. 
     A still further object of the invention is to provide a logic circuit for driving a highly capacitive load where said logic circuit has an emitter follower like up level drive and transient low impedance down level drive. 
     These and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1, in accordance with the invention, discloses the saturated feedback logic circuit and the input and output steady state signal characteristics. 
     FIG. 2 discloses the rising input and falling output waveforms and the equivalent circuit of FIG. 1. FIG. 2 also discloses load current path for discharging the load capacitance (CL). 
     FIG. 3 discloses the falling input waveform and input current (IP) discharge, enhanced by input undershoot of duration τ, and the corresponding output waveform. 
     FIG. 4 discloses the equivalent circuit of FIG. 1 and DC current flow for Vin =1.25 V. FIG. 4 also discloses the corresponding output waveform illustrating the threshold (0.85 V) of the next logic circuit when the input voltage changes from a logical 1 to 0. 
     FIG. 5 discloses a multi-input version of the saturated feedback logic circuit in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The internal logic gate illustrated in FIG. 1 is designed as an inverting push pull type with speed depending on the load capacitance. Power dissipation is preserved due to the changing emitter impedance of the input device as the output voltage changes. 
     The inclusion of T3, R1 and R2 reduces the emitter load of transistor T1 during the time period A is at the 1 level and transversing to the zero level. This results in very low impedance from A to ground when A rises to the 1 level causing A to fall very fast. Once the 0 level is at A, T3 must turn off therefore the emitter load of T1 changes to RE (approximately 750 ohms) and preserves the power dissipation. Due to the low impedance exhibited by the circuit output, excellent capacitive load driving capability is realized. 
     Circuit Operation 
     Discussion of circuit operation is limited to input A and assumes input B is at +0.65 V. Assume input A (FIG. 1) is at a logical 0 (+0.65 V), T1 cannot conduct causing D1, RF and T2 to conduct, therefore A is at the logical 1 level (+1.25 V). With A up, resistor divider R1 and R2 is chosen to cause T3 to conduct, its low output impedance (T3 is operated in saturation) in parallel with RE results in total emitter load to T1 of: 
     
         RE.sup.∇≈ 11.03Kr≈30 ohms 
    
     As input A rises thru a threshold of approximately +0.85 V, T1 begins to conduct a current in the range of 1 to 2 ma depending on the load capacitance. The input and output waveforms and circuit current paths are illustrated in FIG. 2. 
     Inspecting FIG. 2 we see the output voltage has a transient excursion of approximately 1.0 volt. This ΔV is determined by the path consisting of the voltage drop across RF, the VCE of T1 and the VCE of T3 (saturated). This is a desirable condition which enhances the speed of the next internal gate as illustrated in FIG. 3. The transient undershoot of 0.45 V below the +0.65 V level for τ ns causes the charge to be pulled out of the base of T1 (next internal gate load) resulting in its rapid turn off. The time period τ is easily controlled by selecting the level of saturation for T3 via R1/R2 design and/or the T3 base-collector area and/or doping concentration. 
     As previously stated, by this design technique, very fast output down going delays are achieved as a result of saturated feedback (T3). It should also be noted that due to the saturated feedback the circuit input threshold voltage is effected. For a logical input level (0.65 V) T3 is saturated and the input voltage only has to rise approximately 200 mv (0.85 V) to switch T1. This factor also enhances the rising input to falling output circuit delay. 
     Consider input A is now at approximately +1.25 V and output A is down at approximately +0.65 V, T1 is conducting such that RE is chosen to conduct a current level to support the conduction of both D1 and T2 as illustrated in FIG. 4. 
     Since D1 and T2 are designed to conduct in the steady state (output down) the junction capacitances of D1 and T2 are essentially precharged. When input A falls resulting in T1 turning off (very fast), D1 and T2 can rapidly supply the current required to switch the output to a logical 1 level. As previously stated, due to the saturated feedback technique, output A only has to rise to approximately 0.85 V before the threshold of the next stage is achieved which enhances circuit performance. 
     Inspecting the circuit in FIG. 1, chip active area may be preserved by the integration of D1 and T1 within the same collector bed. Resistors R1 and R2 may be ion implant type, RC and RE can be P-type and the feedback resistor RF (100Ω) could be an extension of the collector bed of D1 and T1. 
     While the invention has been particularly described with reference to the preferred embodiment, it will be understood by those skilled in the art that the foregoing and other changes and details may be made therein without departing from the spirit and scope of the invention.